The Nature and Evolution of Disks around Hot Stars: Abstracts

A workshop to be held 2004 July 7, 8, and 9 at the Carnegie Hotel in Johnson City, TN, and hosted by East Tennessee State University



REVIEW TALK: Modeling the Structure of Hot Star Disks

J. E. Bjorkman

University of Toledo

It is now commonly believed that many kinds of hot stars are surrounded by disks. Examples include the accretion disks of Herbig Ae/Be stars and the outflowing disks of classical Be stars, B[e] stars, and possibly LBV stars. In this talk, I will review what theoretical models tell us about the structure of these disks. To connect the hydrodynamics models to observations, one must determine the detailed physical state of the gas. As an example, I will present results from our newly developed 3-D NLTE Monte Carlo radiation transfer code, which produces a fully self-consistent solution for the temperature, density, level populatons, and ionization structure of the disk. This model also predicts the observational properties of the disk, thereby allowing critical tests of different disk models.


REVIEW TALK: The Observed Properties of Hot Star Disks

Karen S. Bjorkman

University of Toledo

There is observational evidence for disks around hot stars of every type, from the pre-main-sequence through the main-sequence and into the evolved hot stars. This evidence comes from many different wavelength regimes and from many different observational techniques. I will review the status of what we know about the properties of disks around hot stars on the basis of these observations, and try to summarize the similarities and differences between hot star disks at various evolutionary stages.


REVIEW TALK: The Effects of Magnetic Fields in Winds and Disks

J.C. Brown 1, and J.P. Cassinelli 2

1Department of Physics and Astronomy, University of Glasgow

2Department of Astronomy, University of Wisconsin

The problems facing magnetically guided wind models for the generation of stellar disks will be outlined, particularly in relation to hot, and especially Be, stars. Various parametric, analytic and numerical treatments have been published some with and some without rotation, but all considering how dipole like fields can steer winds to create a compressed equatorial region, variously termed MTD (magnetically torqued disk - Cassinelli et al 2002), MRD or MCWSD (magnetically rigidised disk, magnetically confined wind shocked disk - Owocki et al). Similarities and inconsistencies between these treatments will be discussed. The essential issues over the MTD model for Be stard disks are: a) What field and rotation are required to create a MTD dense enough and of large enough radial extent to generate the emission line EWs, scattering polarisation, and IR excess observed, without fall back or break out of the material? b) Can the semi-corotational velocity field of such a disk be reconciled with observed line profiles and with long term V/R variations normally attributed to spiral density waves in a Keplerian disk? c) What limits the lifetime of such a disk? d) How can Keplerian disk models be reconciled with the fact that recently observed B fields are large enough for MTD production? e) Can any other model predict reasonably well, as MTD does, the range of Spectral Types in which disks are observed? f) What critical observations might test the MTD model?


POSTER: The Mid-IR interferometer VLTI/MIDI and the study of hot star disks

O. Chesneau 1, L.B.F.M. Waters {23},, C. Leinert 1, R. van Boekel 2

1Max-Planck-Institut für Astronomie K\"{o}nigstuhl 17, D-69117 Heidelberg

2Astronomical Institute `Anton Pannekoek', University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

3Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200B, B-3001 Heverlee, Belgium

The Mid-IR interferometric instrument MIDI has performed its first scientific observations on the Very Large Telescope Interferometer (VLTI) in June 2003. MIDI allows interferometric observations over the 8 $\mu$m-13 $\mu$m wavelength range, with a spatial resolution ranging from 5 to 20 milliarcsec, a spectral resolution of 30 and 250, and a expected point source sensitivity of N = 4 mag (1 Jy). The instrument is particularly well suited to the study of dust shells and disks. It is able to resolve the location and geometry of the dusty circumstellar environment of bright hot stars, and to study spatially {\it and} spectrally the innermost regions where dust survives. In a few scientific nights, MIDI has already provided beautiful results concerning disks around hot stars, ranging from Herbig AeBe stars to the famous LBV Eta Car. In particular, for the first time studies of the chemical composition of the dust near and far from the central source in Herbig AeBe stars, B[e] stars and LBVs are possible.


FOCUS SESSION: Diagnostic Methods

D. H. Cohen 1, M. Hanson 2

1Swarthmore College

2University of Cincinnati

This focus session will consist of presentations on diagnostic methods used in understanding the nature of disks around hot stars. Methods to be covered include spectral line diagnostics, spectropolarimetry, and high-energy observations. Emphasis will be given to reviewing aspects that exploit upcoming next generation telescopes and instrumentation. Guest speakers will identify questions of current interest and discuss how the different diagnostics can be used to address these issues, including, for example, the distribution of rotational velocities of Be stars, the physical parameters of disks around hot stars, and the role of magnetic fields and winds in hot star disks. Each topic presentation will include a tutorial on the underlying physics of the diagnostics, a discussion of applications to specific open questions of interest to the field, and time for audience participation. Audience members are welcome to share a slide or two, either directly responding to a speaker's presentation or on a related topic. Session organizers will distribute a document in the spring detailing the specific topics of the diagnostics session. If you wish to contribute a slide, please contact D. Cohen (dcohen1@swarthmore.edu) or M. Hanson (hanson@physics.uc.edu). (We will do our best to accomodate impromptu contributions, but only as time permits.)


POSTER: A Small Magellanic Cloud UXOR star: multi-epoch monitoring and deep photometric imaging

W.J. de Wit 1, J.P. Beaulieu 2, S. Brillant 3

1INAF, Osservatorio Astrofisico di Arcetri, Italy

2Institut d'Astrophysique de Paris, France

3ESO, Chile

We present the first results of spectroscopic monitoring over 9 months with VLT/UVES of a UXOR-type star in the Small Magellanic Cloud. This peculiar object has been proposed to be still in its pre-main sequence phase, based on its photometric variability (i.e. CMD "hockey-stick" behaviour) and its minor infra-red excess. In addition, the poster will also present high resolution broad-band and narrow-band imaging of the stellar population surrounding the SMC UXOR using NTT/SUSI, in a search for pre-main sequence stars of lower mass in this particular region.


FOCUS SESSION: Disk Modeling

K. G. Gayley 1, J. Porter 2

1Department of Physics and Astronomy, University of Iowa

2Liverpool John Moores University

This focus session is intended to provide an opportunity for those interested in disk simulations to share ideas about when various physical effects need to be included, and the corresponding technical challenges. The session will be subdivided into two phases, a "phenomena" phase and an "arts" phase. In the phenomena phase, we will guide a discussion about major physical principles that are most relevant to disk modeling, and the interface between expectations and observations. Optimal allocation of modeling resources requires tradeoffs between accuracy and idealizations, prudently tailored to focus on specific physical effects. This discussion will be aimed at sharing ideas about various possible approaches and the interesting physics they include, involving such things as magnetic fields, viscosity, quadrupole and/or relativistic gravity corrections, and radiative transport of energy and momentum. In the arts phase, we will will provide resources for practitioners interested in including additional physics or improving their techniques. Problems around convergence, instability, and proper boundary conditions will be addressed.


POSTER: NLTE models of axi-symmetric winds

L. N. Georgiev 1, J. D. Hillier 2, J. Zsarg\'{o} 2

1Instituto de Astronomia, UNAM, Apartado Postal 70-264, Cuidad Universitaria, 04510 Mexico, D. F. Mexico

2University of Pittsburgh, 3941 O'Hara str., Pittsburgh, PA 15260

We present a 2D NLTE model for a hot star with an axi-symmetric wind. In the calculation the wind is modeled as a radial flow with the equatorial velocity smaller than the polar velocity. Hydrogen, Helium and CNO are treated using the preconditioning technique and short- characteristic radiative transfer. Lines are currently treated using the Sobolev approximation, although we are working towards implementing a short characteristic treatment. The effects of the deviation from the spherical symmetry on the emitted spectrum and their application as diagnostics are discussed.


FOCUS SESSION: Optical/IR/MM Interferometric Studies of Hot Star Disks

D. Gies 1, P. Stee 2

1Georgia State University

2Observatoire de la Cote d'Azur

Time will be equally divided between the topics of interferometry from the optical to millimeter wavelength, the unique challenges and opportunities presented by interferometric studies of circumstellar disks, and a description of current and future instrumentation, facilities, and access. We will start by a short tutorial for people unfamiliar with interferometry but who want to learn about the prospects for new research. The goal of this session will be to highlight how existing problems and questions concerning hot star disks can be addressed with interferometry at optical/IR/mm wavelengths, and to answer questions about available resources and observing opportunities. This session will also give the opportunity to present a few "hot" results through short presentations (3 transparencies maximum). Since place (and time !) will be limited we ask you to contact ASAP Ph. Stee (Philippe.Stee@obs-azur.fr) or D. Gies (gies@chara.gsu.edu) if you wish to give a short presentation.


REVIEW TALK: The Evidence for Disks Around Herbig Ae/Be Stars

C. A. Grady

Eureka Scientific and Goddard Space Flight Center

Protoplanetary disks are where planets form, migrate to their final orbits, and where the pre-biotic materials that can ultimately produce life-bearing worlds are assembled or produced. We need to understand them, how they interact with their central stars, and their evolution both to reconstruct the Solar System's history, to account for the observed diversity of exo-planetary systems, and in particular to understand over what range of stellar masses and star-forming environments planetary systems can be produced. At present, our most complete view of the process is for disks around young, intermediate-mass stars, the Herbig Ae stars. The millimeter and IR data for the presence and inferred spatial distribution of dust are compared with optical and near-IR coronagraphic imagery for the nearby Herbig Ae stars and with direct imaging of proplyds in Orion. UV and X-ray observations allow us to probe the coupling between the disk and the star, and have revealed the accretion shock, infalling gas, and evolution in wind signatures over the first 10 Myr of the Herbig Ae star's lifetime which may be linked to the decay of a remnant dynamo or clearing of the inner disk. The more fragmentary data for Herbig Be stars is reviewed with implications for detection of x-rays from these stars, and for survival of circumstellar disks.


REVIEW TALK: The Physics of Circumstellar Disks

Lee Hartmann

Smithsonian Astrophysical Observatory

I will review some general aspects of the physics of protoplanetary disks, mostly gleaned from studies of T Tauri stars and related pre-main sequence objects. Although significant progress has been made in understanding the mechanisms of angular momentum transport, there is still much to be learned about the relative roles of the magnetorotational and gravitational instabilities. I will consider what observations of both short- and long-term variability in disk accretion rates can tell us, and address the implications of current observational estimates of the dependence of accretion rates on stellar mass.


REVIEW TALK: Magnetism Observed in Massive Stars

H. F. Henrichs

Astronomical Institute, University of Amsterdam, Netherlands

A large range of observational phenomena among massive stars (the OB stars and A supergiants) cannot be explained without the presence of surface magnetic fields. These phenomena include cyclic wind variability observed in UV lines, other types of wind variability in particular in H$\alpha$, chemical peculiarity, specific pulsation behaviour, and non-thermal emission in the radio and X-ray region. At the theoretical side the evidence is accumulating that stable magnetic field configurations can exist in massive stars throughout their evolution. On the observational side, however, direct magnetic detections in these stars are very rare. In the light of the above, the present status of observations of magnetic fields in massive stars will be discussed.


POSTER: Magnetic fields of B stars measured with FORS1 at the VLT

S. Hubrig, Th. Szeifert, P. North, M. Schoeller

European Southern Observatory, Casilla 19001, Santiago, Chile

Although magnetic fields in hot normal stars have so far escaped detection, they are definitely discovered in a number of helium peculiar stars between spectral types B1 and B7. We will present new results of measurements of hot stars with FORS1 in polarimetric mode at the VLT and discuss the differences between various groups of stars (e.g. He strong, He weak Si, HgMn, SPB, Be stars) in terms of magnetic fields, rotational and pulsational periods and other stellar fundamental parameters.


POSTER: Photo-Evaporation of Planetary Companions to Massive Stars

Richard Ignace and Mark L. Giroux

Department of Physics, Astronomy, and Geology, East Tennessee State University

One class of planetary formation models indicates that gas giants may form rapidly, on the order of centuries, owing to strong gravitational instabilities in disks. If this planet formation mechanism is effective for hot massive stars, it is possible that planets may have formed even around O and B stars. Given this possibility, we address the extent to which high levels of EUV radiation from early-type stars can lead to heating and signficant atmospheric losses from a planet. Observationally, we know that such heating can lead to hydrodynamic ``blow-off'' in the case of HD209485B, a planet in a small orbit around a solar-type star. Assuming a simplified energetics argument, we use a grid of stellar spectral models to calculate the heating and expected mass loss of a Jupiter-like planet.


POSTER: Spectroastrometry: Eta Carinae as an Example

K. Ishibashi

Center for Space Research, Massachusetts Institute of Technology, 77 Massachusetts Ave. NE80-6011, Cambridge, MA, 02139

As described by Porter et al. here in this conference, the use of a long-slit imaging spectrograph to {\it scan} a stellar disc and its neighboring media can bring a voluminous amount of data that cannot easily be obtained via 2-d imaging. As for a proof of concept, the Hubble Space Telescope / Space Telescope Imaging Spectrograph has been utilized to scan through the ejecta of the super-massive star Eta Carinae. The 1" x 1" central region of Eta Carinae will be used to demonstrate the technique and some neat science that may come from it.


POSTER: The Stability of Viscous Circumstellar Envelopes

C. E. Jones 1, T. A. A. Sigut 1, J. M. Porter 2

1The University of Western Ontario

2Liverpool John Moores University

We have constructed models of axisymmetric, circumstellar envelopes for Be star disks which incorporate a realistic chemical composition including, H, He, C I-II, O I-II, Mg II and Ca II in order to further investigate Be disk energetics in a quantitative manner. The temperature distribution throughout the disk is determined self-consistently. The predicted temperatures, combined with other model parameters such as the disk density and velocity distribution, are used to investigate the stability of the out-flowing viscous disk models for Be stars of Porter (1999).


REVIEW TALK: Disks and Peculiar Abundances in a Variety of Hot Stars

M. Jura

Department of Physics and Astronomy, UCLA, Los Angeles CA 90095-1562

At least three different classes of hot stars exhibit peculiar surface abundances which can be attributed to selective accretion from cirumstellar dusty rings or disks. Examples are the post-main-sequence binary, the Red Rectangle, the DA white dwarf, G29-38 and the main sequence star, ${\lambda}$ Boo. The evidence for such accretion is assessed, and the origin and evolution of this circumstellar matter is considered. We discuss the interactions between gas and dust in circumstellar disks and rings around hot stars, and the observability of each component. Selective accretion from rings and disks may be common.


REVIEW TALK: Winds from Hot Star Disks

Timothy Kallman

Laboratory for High Energy Astrophysics, NASA/Goddard Space Flight Center, Greenbelt, MD

In this talk I will briefly review the observational motivation and evidence for mass loss from disks around hot stars. Direct evidence for these outflows comes from line profiles in, eg. young stellar objects. Indirect evidence comes from the implied mass loss rates and wind speeds, along with dynamical models which can account for these properties. Mechanisms for disk wind driving include thermal, radiation pressure, and MHD. These will be reviewed and discussed, as will the relation to non-disk winds, and to disk winds in other contexts.


POSTER: The outflowing disks of B[e] supergiants and unclassified B[e] stars

M. Kraus, H. J. G. L. M. Lamers

Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands

One well-known group of hot and massive stars are the B[e] supergiants, which are post-main sequence stars of spectral type B with a huge zoo of permitted and forbidden emission lines. The evolutionary connection to other post-main sequence stars like the LBVs is still unknown. Their spectra show a hybrid character indicating a fast CAK type low density wind in polar direction as well as a slow but high density disk forming wind in equatorial direction. An other group of B[e] stars are the unclassified stars, i.e. stars with yet unknown or controversial evolutionary stage. They also show hints for the existence of a high density circumstellar disk. We investigate the disk structure and mass loss history of stars from both groups in terms of their forbidden emission lines.


REVIEW TALK: Generating Magnetic fields in Early-type Stars

K. B. MacGregor

HAO/NCAR

Although magnetic fields have been detected in many upper main sequence stars (most chemically peculiar, a few not), the physical origins of this magnetism are not well understood. One possibility is that these fields are the product of a hydromagnetic dynamo, located not at the base of an outer convective envelope as in the Sun and (presumably) late-type stars, but within a central convective core. Model results suggest that dynamo action can take place in such an environment, but that the high electrical conductivity and large spatial extent of the overlying, stably stratified, radiative envelope represent significant obstacles to the appearance of the fields so generated at the stellar surface. Advection by internal circulatory flows or the buoyant rise of tube-like concentrations of magnetic flux can, under certain conditions, contribute to the outward transport of the fields produced by a core dynamo. Alternatively, these difficulties can be alleviated in part if the generation process takes place closer to the surface of the star; waves, instabilities, and "battery"-driven currents are potential mechanisms for producing magnetic fields within the envelope itself. Yet another possibility is that the observed magnetic fields are relicts, generated by processes that operated only during an earlier evolutionary stage, and remaining inside the present-day star because of the long time scale for resistive decay. Such fossil fields could reflect the magnetization of the material from which the star formed, or they could have been produced by dynamo activity in the past, made possible by the internal structural characteristics and rapid rotation that existed at that time.


POSTER: A Magnetic Rotator Wind-Disk Model for Be Stars

M. Maheswaran

University of Wisconsin - Marathon County

We consider a Magnetic Rotator Wind-Disk (MRWD) model for the formation of Keplerian disks around Be stars. Material from low latitudes of the stellar surface flows along magnetic flux tubes and passes through a shock surface near the equatorial plane to form a \textit{pre-Keplerian} disk region. Initially, the density in this region is small and the magnetic field helps to maintain super-Keplerian rotation speeds. After a fill-up time, the density of the disk is significantly larger and the magnetic force in this region becomes negligible compared with the centrifugal force. The material then moves outwards to form a quasi-steady Keplerian disk. During the fill-up stage, the meridional component $B$ of the magnetic field at the stellar surface must be larger than a minimum value $B_{min}$. The radial extent of the quasi-steady Keplerian region is larger for larger values of $B$. It is largest when $B$ equals an optimal value $B_{opt}$. In B-type stars, the values of $B_{min}$ are of order 1 G to 10 G. The minimum surface magnetic fields required in the MRWD model are less than those required in the MTD model of Cassinelli et al (Ap.J., 578, p 951, 2002) because the MTD model requires fields that produce an effective torque on the disk material after fill-up. In the MRWD model, the magnetic torque supplies the necessary angular momentum to the wind material, which is much less dense. Because the magnetic force in the quasi-steady Keplerian region is negligible, the one-armed spiral pattern of the Global Disk Oscillation model (e.g., Okazaki, A\&A, 318, p548, 1997) for the V/R variability in disks of Be stars may be relevant for the MRWD model. In stars with weaker stellar fields, meridional circulation near the photosphere may play a role in some of the time-variation observed in disks of Be stars through its effect on the magnetic field.


POSTER: The Polarization of Achernar

David McDavid

Department of Astronomy, University of Virginia

Recent near-infrared measurements of the angular diameter of Achernar (the bright Be star alpha Eridani) with the ESO VLT interferometer have been interpreted as the detection of an extremely oblate photosphere, with a ratio of equatorial to polar radius of at least 1.56 +/- 0.05 and a minor axis orientation of 39 +/- 1 degrees (from North to East). The optical linear polarization of this star during an emission phase in 1995 September was 0.12 +/- 0.02% at position angle 37 +/- 8 degrees (in equatorial coordinates), which is the direction of the projection of the rotation axis on the plane of the sky according to the theory of polarization by electron scattering in an equatorially flattened circumstellar disk. These two independent determinations of the orientation of the rotation axis are therefore in agreement. The observational history of correlations between H-alpha emission and polarization as found in the literature is that of a typical Be star, with the exception of an interesting question raised by the contrast between Schroder's measurement of a small negative polarization in 1969-70 and Tinbergen's measurement of zero polarization in 1974.5, both at times when emission was reportedly absent.


POSTER: A Search for Be Stars in Open Clusters

M. V. McSwain, D. R. Gies

Georgia State University

Be stars are a class of B stars with circumstellar disks that cause Balmer and other line emission. This phenomenon is observed both in pre-main sequence and evolved stars, and the age at which Be stars appear may correspond to a phase of spin up caused by mass transfer in a close binary system or by an internal redistribution of angular momentum. Not all Be stars are observed in binary systems, so it is possible that the spin up phase is a common stage in the evolution of single B stars. To test this evolutionary model, I have conducted a photometric survey of 27 evolved open clusters to determine their fraction of Be stars relative to B stars as a function of cluster age.


REVIEW TALK: The Influence of Rotation for Massive Star Evolution: Principles and Uncertainties

G. Meynet

Geneva Observatory

The various physical processes induced by rotation in stars and the way they are implemented in the stellar models will be discussed. Then it will be showed how the inclusion of rotation in massive star models improves the agreement between theory and observations on at least three important points: 1) Rotational mixing allows to produce variations of the surface abundances already during the Main--Sequence phase as is observed. The changes of the surface abundances are more important when, for a given initial velocity, the initial masses are larger, and/or the metallicities are lower; 2) The observed number of red supergiants at the metallicity of the Small Magellanic Cloud (SMC) can be accounted for; 3) The observed variation of the number ratio of Wolf--Rayet to O--type stars as a function of the metallicity can be reproduced. For all these comparisons non--rotating models give unsatisfactory fits. Rotating models results also give interesting insights on questions such as the origin of Be stars, the mechanisms responsible for the huge mass loss rates undergone by the Luminous Blue Variables, the rotation rates of pulsars, the progenitors of collapsars and the sources of primary nitrogen at low metallicity.


POSTER: Magnetic fields in Be stars

C. Neiner {12}, \& A.-M. Hubert 2

1RSSD, Estec, ESA, Netherlands

2GEPI, Observatoire de Paris-Meudon, France

Be stars remain a mystery since their discovery 140 years ago. The two presently most favored explanations for the Be phenomenon are the beating of non-radial pulsations and the presence of a magnetic field. In this poster we will present the recent development in the search for magnetic fields in Be stars and the role of the magnetic field in the presence of the circumstellar disk.


POSTER: Exciting new vistas on high mass protostars and their circumstellar envelopes\,/\,disks

Dieter E.\,A.\ N\"urnberger

European Southern Observatory, Casilla 19001, Santiago 19, Chile

High mass protostars are usually deeply embedded in their natal environment which can be penetrated only at wavelengths beyond the mid IR. In addition, sites of high mass star formation are generally quite distant. In my presentation, I will report on our recent efforts to search for high mass protostars in two Galactic H\,{\sc ii} regions, namely NGC\,3603 (N\"urnberger, 2003, A\&A) and M\,17 (Chini et\,al., 2004, Nature). Taking advantage of "curtain-lifting" stellar winds and ionizing photons from the central clusters of early type main sequence stars and making use of sensitive, high angular resolution observations in the near and mid IR, we have identified promising candidates which may play a decisive role in our understanding of high mass star formation processes. In particular, as we see strong evidence for the existence of (accretion) disks around these sources, one may favour the accretion scenario against the collision (coalescence) scenario.


POSTER: Dynamics of Be-star decretion disks in Be/X-ray binaries

Atsuo Okazaki

Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan

The Be/X-ray binaries, which consist of a Be star and a neutron star in a wide and eccentric orbit, represent the dominant subclass of high-mass X-ray binaries. Most of these systems show transient X-ray activity as a result of a complicated interaction between the Be disk and the neutron star. In this paper, we report the results from three dimensional SPH simulations of the interaction between the viscous decretion disk around the Be star and the neutron star for a wide range of orbital parameters. Our simulations show that the Be disk is tidally truncated at a radius smaller than the periastron distance, except in systems with very high orbital eccentricity and/or large inclination angles. The mass-capture rate by the neutron star is higher in systems with higher eccentricity, longer periods and/or lower inclination angles between the Be disk and the orbital plane. Due to the eccentric orbit, the shape of the Be disk modulates with the orbital phase. In inclined systems, the disk is warped twice per orbit and precesses in the retrograde direction. We discuss how Balmer line profiles vary with the evolution/modulation of the Be disk.


POSTER: Chandra HETGS Multi-phase Spectroscopy of the Young Magnetic O Star $\theta^1$ Ori C

Mary Oksala 1, Marc Gagn\'e 1, David Cohen 2, Stephanie Tonnesen 3, Asif ud-Doula 4, Stanley Owocki 5, Joseph MacFarlane 6

1Department of Geology and Astronomy, West Chester University

2Department of Physics and Astronomy, Swarthmore College

3Department of Astronomy, Columbia University

4Physics Department, North Carolina State University

5Bartol Institute, University of Delaware

6Prism Computational Sciences

{\it Chandra} High-Energy Grating spectra obtained at four phases of the oblique magnetic rotator $\theta^1$~Ori~C (O6 V) are used to constrain the temperature, spatial location, and kinematics of the X-ray emitting plasma. In all four observations, the emission-measure distribution peaks at $\log T \approx 7.4$. The 25\% flux variability and the He-like lines of Mg, Si, S, and Ar suggest that the emitting region is close to the photosphere ($R \leq 1.7 R_{\star}$). The line profiles have Doppler widths of a few 100 km~s$^{-1}$; thus the plasma is moving, but much more slowly than the terminal wind speed. We analyze the X-ray diagnostics in conjunction with new MHD simulations of the magnetically channeled wind shock mechanism on $theta^1$~Ori~C. This model fits the data surprisingly well, producing relatively narrow, unshifted lines and very high-temperature shocks directly above the magnetic equator.


REVIEW TALK: Dynamical Processes that Drive the Formation and Evolution of Hot-Star Disks

S. P. Owocki

Bartol Research Institute, University of Delaware

Over the years, numerous scenarios have been proposed for forming hot-star disks, but nowadays it has become possible to study the dynamical processes underlying these scenarios through direct hydrodynamical simulation. In this talk I will use such simulations to examine the viability of various models, including Wind Compressed Disks (WCD), Radiatively Driven Orbital Mass Ejection (RDOME), Magnetically Torqued Disks (MTD), Pulsationally Driven Orbital Mass Ejection (PDOME), and Rigidly Rotating Magnetospheres (RRM). For most classical Be stars, I argue that the disks are likely to be Keplerian, and formed by orbital mass ejection (in some cases possibly linked to pulsations, as in the PDOME model) from a nearly critically rotating stellar surface. For Bp stars, which have measured, very strong magnetic fields, I argue in favor of the Rigidly Rotating Magnetosphere model. For both scenarios, I also briefly summarize dynamical mechanims for disk destruction, by line-driven ablation in the former case, and by centrifugal mass ejection in the latter.


POSTER: Very Short-Term $lpv$ in the Be Star $\pi$ Aquarii

G. J. Peters 1, D. R. Gies 2

1Space Science Center, University of Southern California

2CHARA, Georgia State University

The photospheric activity observed in a Be star during a non-emission phase provides important insight into the cause(s) for its mass loss and disk buildup. We present evidence for recent short period NRP in the Be star $\pi$ Aqr that, after about 5 decades of emission, lost its CS disk in the mid-1990s. A series of spectra containing H$\alpha$, C II 6578,82, and He I 6678 obtained with the Coud\'{e} Feed Telescope at KPNO during observing runs in 1999 November, 2000 October, and 2001 January show distinct traveling bumps in all the aforementioned features. The same period of 1.88 hr was present during all three observing runs. If the activity is due to NRP, the data are consistent with a sectorial mode with $\ell$=5 $\pm$1. The period is much shorter than NRP periods typically seen in Be stars and also the orbital period for material in an inner disk. The width of the bumps argue for an origin in the upper photosphere of the Be star. It is presently unclear just how the NRP might be related to future mass loss, but it would be interesting to determine if the modes have changed recently (most Be stars display sectorial modes of $\ell$=2) and whether the star has multiple, closely-spaced periods as $\mu$ Cen displayed during its recent non-emission phase.


POSTER: Spectroastrometry: A new method for measuring disc kinematics

John M. Porter 1, Rene D. Oudmaijer 2, Debbie Baines {23}

1Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, United Kingdom

2School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, United Kingdom

3Department of Physics, Denys Wilkinson Laboratory, Keble Road, University of Oxford, OX1 3RH, United Kingdom

The kinematics of discs around hot stars have never been directly measured as the sizes of the discs ($\sim$milli-arcseconds for the closest stars) is small compared to the typical seeing disc ($\sim$arcseconds). We describe a technique -- spectroastrometry -- which has the potential to probe disc kinematics directly on scales below the seeing disc. This technique makes complete use of long-slit spectra, (it is not interoferometry!), and hence can be used with simple spectrometers. Spectroastrometry may be used on a variety of astrophysical sources with a non-symmetric velocity field, and so it is not a technique confined to hot star research.


REVIEW TALK: Links Between Hot Stars and Their Disks

Th. Rivinius

Landessternwarte Heidelberg K\"onigstuhl, Germany

Hot stars typically do not have disks left over from star formation. The typical times to erode the disk by radiative driving are relatively short, and as soon as the interstellar matter in the close vicinity is consumed or the star has moved away from its birthplace the natal disk will be blown away. This review will thus concentrate on disks formed by stellar material itself, and which are the mechanims both to form and erode these disks as far as the central object is involved. Observational evidence for interaction of stellar mechanisms with the circumstellar disk will be discussed, as there are rotation, magnetic fields, and nonradial pulsation (nrp). For several stars firm evidence is established that material is forced into corotation by magnetism. But most, though not all, of these objects are slow rotators and rather possess localized clouds than disks. In turn, nonradial pulsation seems present in most Be stars, quite undoubtedly having disks, but assuming canonical rotation rates energy and angular momentum considerations do not allow to connect nrp directly to the ejection of matter. However, observations tell us that the nrp variability changes and is particularly strong at times when the disk seems to be replenished with fresh matter from the central star. In addition, during such times secondary periods are transiently present that seem not to originate from the stellar surface, but from the close circumstellar environment. Finally, observational evidence is reviewed that disks are eroded by radiative processes from the inner edge outwards and that the disk material is blown away in the form of a latitude dependant wind.


POSTER: The Geometry of $theta^1$\,Ori\,C's Magnetic Wind

Myron Smith 1, and Alex Fullerton 2

1CUA and STScI/CSC

2FUSE/Johns Hopkins University

Because the optical and UV line and X-ray continuum fluxes of $\theta^1$\,Orionis\,C (O6-7 V) modulate on the magnetic/rotational period, it is likely that this star is a hot analog of the Bp variables with an oblique dipolar magnetic field. In this paradigm emissions and absorptions in its UV resonance and strong optical lines are produced over the cycle as an anchored magnetosphere appears to wobble around the rotational axis. We report that we have reconciled internal contradictions in published discussions of the optical/UV behavior of this star by reexamining the detailed behavior of the strong UV lines across the line profiles. Whereas earlier discussions focused on variations of the full profile, we discovered that the blue and red wings exhibit anticorrelated relations. For example, at phase $\phi$ = 0.5, when the observer views the system along the magnetic equator, the resonance line shows strong high-velocity absorption and positive velocity emission. A second dissimilarity with the phenomenalogy of Bp stars, is the lack of evidence for a {\it static} disk among strong optical or UV resonance lines. In fact, the presence of a disk in the magnetic plane works against the observed blue shift noted by Stahl et al. in the H$\alpha$ emission profile at pole-on phase since the this profile can be understood as the product of the wind flowing out from the pole toward the observer at $\phi$ = 0. We suggest that the contrasting blue/red wing behaviors of the UV resonance and several other optical and UV lines can be explained by two components of a wind. One accelerates to a free flow at the magnetic equator and is visible as enhanced high velocity absorption in the C IV when viewed along the magnetic equator. The other, responsible for redshifted emission in a high density environment, requires another, probably more novel interpretation. Out of necessity we suggest one in which the wind, initially following closed loop lines near the star, is diverted inward along with the field lines toward the star as it approaches the magnetic equator. The resulting shocks produces the observed redshifted (+200 km/s) emission. Because this flow circulates material back to the star at the equator, we further suggest these kinematics are responsible for the absence of a static disk.


POSTER: Non-isotropic circumstellar disk in the $\phi$\,Per-type binary FY CMa

S.~\v{S}tefl 1, T.~Rivinius 2, M.~Maintz 2, O.~Stahl 2, D.~Baade 3

1Astronomical Institute, Academy of Sciences, CZ-251\,65 Ond\v{r}ejov, Czech Republic

2Landessternwarte K\"onigstuhl, D-69117 Heidelberg, Germany

3European Southern Observatory, Karl-Schwarzschild-Str.~2, D-85748 Garching bei M\"{u}nchen, Germany

Optical spectra of the Be star FY\,CMa show transient shell phases and variable RV and shape of \ion{He}{i} emission components. Therefore, FY CMa closely resembles the Be+sdO binary $\phi$ Per. Moreover these variations in the optical spectrum are consistent with the period of $P=37.26\pm0.03$\,d derived from the UV \ion{Ne}{v} lines. They are due to the Be primary's circumstellar disk, which is ionized where it faces the hot companion. Next to $\phi$\,Per and 59 Cyg, FY\,CMa is only the third candidate to the group, although such systems should be relatively common.


POSTER: 3D Magnetostatic models of centrifugally supported, rigid-body disks around rotating hot-stars with a strong, tilted dipole field

R.H.D Townsend, S.P. Owocki

Bartol Research Institute, University of Delaware

We present a semi-analytic approach for modeling circumstellar emission from rotating hot-stars with a strong dipole magnetic field of arbitrary tilt angle between the magnetic and rotation axes. By assuming the very strong, rigid-field limit in which material driven (e.g. in a wind outflow) from the star is forced to remain in strict rigid-body corotation, we are able to solve for the effective centrifugal-plus-gravitational potential along field lines, and thereby identify the locus of points defining potential minima where material is prone to accumulate. Using basic scalings for radiatively driven stellar wind surface mass flux, we calculate the circumstellar density distribution that obtains once the ejected plasma settles into magnetohydrostatic equilibrium. The envelope formed by these accumulation points resembles a warped disk, with average surface normal lying between the rotation and magnetic axes. The plasma in the warped disk corotates rigidly with the magnetic field. Using simple models for the plasma opacity and source function, we calculate formal solutions of the equation of radiative transfer, and thereby obtain time-resolved synthetic emission-line spectra for the disk. Initial comparisons show an encouraging level of correspondence with the observed rotational phase variations of Balmer line emission profiles from magnetic Bp stars like sigma Ori E.


POSTER: Centrifugal Breakout of Magnetically Torqued Line-Driven Stellar Winds

A. ud-Doula 1, R.H.D.T. Townsend 2, S. P. Owocki 2

1Department of Physics, North Carolina State Universiyt

2Bartol Research Institute, University of Delaware

We present 2D MHD simulations of radiatively driven stellar winds from a hot star with a dipole magnetic field aligned to the star's rotation axis. We focus in particular on models with a moderately large equatorial rotation speed (about half the critical value), and also a strong magnetic confinement parameter, $\eta_{\ast} \equiv B_\eq^2 R_\ast^2/{\dot M} v_\infty = 600-1000$. The strong magnetic field channels and torques the wind outflow into an equatorial, rigidly rotating disk near and above the Keplerian co-rotation radius. But the strong net outward centrifugal force on material in the outer edge of this disk stretches the magnetic loops, leading to episodic breakout of mass when the field reconnects. The associated dissipation of magnetic energy heats material to temperature of nearly $10^{8}~K$, high enough to emit hard (several keV) X-rays. Such ``centrifugal mass ejection'' represents a novel mechanism for explaining X-ray flares recently observed in the magnetic Bp star sigma Ori E.


POSTER: Do A-type Supergiants have magnetic fields?

E. Verdugo 1, H.F. Henrichs 2, A. Talavera 3, R.S. Schnerr ^2, V.C. Geers 2, A.I. G\'omez de Castro 4

1ISO Data Centre. VILSPA, Madrid. Spain.

2Astronomical Institute Anton Pannekoek. University of Amsterdam. Netherlands

3XMM-Newton Science Operations Centre. VILSPA, Madrid. Spain.

4Instituto de Astronom\'\i a y Geodesia. Universidad Complutense de Madrid. Spain

Radiation pressure is accepted as the dominant driving mechanism in the mass loss phenomenon of A-supergiants. There are now, however, several clues which point to a model of co-rotating clouds of gas and the existence of a weak magnetic field on the surface, corotating with the star. We report on spectropolarimetric observations of 12 A-type supergiants taken with the MuSiCoS echelle spectropolarimeter mounted at the 2m Bernard Lyot telescope at the Pic Du Midi, during several observing runs from 1999 to 2003. The significance of the results is discussed.


POSTER: Chandra HETGS Observations of the Rapidly Rotating O-Star, Zeta Oph

W. L. Waldron

L-3 Communications Government Services, Inc.

The late main sequence O-star, Zeta Oph, is an ideal X-ray target for investigating how the observed X-ray characteristics depend on a variety of interesting stellar phenomena, such as rapid rotation, disk structures, non-radial pulsations, and a low density fast wind. Although the HETGS spectra of Zeta Oph are similar in appearance with those of other OB stars, there are some differences in the X-ray emission line characteristics (e.g., centroid velocity shifts, HWHM, He-like ion fir-inferred radii, and line profile structures). We discuss the implication of these differences with regards to understanding the outer atmospheric structure of Zeta Oph, and provide comparisons of the observed emission line profiles with several emission line model calculations. In addition, we also discuss the short term (hours) and long term (years) X-ray variability.


POSTER: Circumstellar Disk Systems in the LMC, SMC and Milky Way Galaxy

J.P. Wisniewski 1, K.S. Bjorkman 1, A.M. Magalh\~aes 2, J.E. Bjorkman 1, A.C. Carciofi 1

1University of Toledo

2IAG, University of S\~ao Paulo

Recent studies have suggested that either age (evolutionary stage) or metallicity, or both, may play key roles (in addition to rotation) in promoting the formation of circumstellar disks around hot stars. We have set out to test these suggestions via an observational program, using optical photometry, imaging polarimetry, and infrared spectroscopy to probe the circumstellar environments of B-type stars in various cluster environments. We have detected roughly 600 B-type emission line stars in 17 star clusters and discuss our efforts to quantify whether these objects are true classical Be star-disk systems. Our preliminary results seem to contradict the suggestion that the Be phenomenon occurs in the mid- to later-half of the main-sequence. We also discuss our photometric observations of a sample of candidate extragalactic Herbig AeBe stars. We acknowledge support provided by a NASA LTSA grant (NAG5-8054) and a Research Corporation Cottrell Scholar Award to KSB, NASA GSRP (NGT5-50469) and Sigma Xi GIAR awards to JPW, and FAPESP and CNPq support to AMM.


POSTER: Differential Emission Measure Determination of Collisionally Ionized Plasma: Application to Hot Stars

Patrick S. Wojdowski and Norbert S. Schulz

Center for Space Research, Massachusetts Institute of Technology

We describe a technique to derive constraints on the differential emission measure (DEM) distribution, a measure of the temperature distribution, of collisionally ionized hot plasmas from their X-ray emission line spectra. This technique involves fitting spectra using a number of components, each of which is the entire X-ray line emission spectrum for a single ion. It is applicable to high-resolution X-ray spectra of any collisionally ionized plasma and particularly useful for spectra in which the emission lines are broadened and blended such as those of the winds of hot stars. This method does not require that any explicit assumptions about the form of the DEM distribution be made and is easily automated. We apply our technique to the {\it Chandra} HETGS spectra of nine hot stars.


POSTER: Circumstellar disks around Herbig Ae/Be and Vega-type stars and local magnetic fields

R.V. Yudin

Pulkovo Observatory, Russian Academy of Sciences

There is a reason to believe that during the evolution the compression of circumstellar (CS) envelopes around young stars takes place along the line-of force of local magnetic fields that results to the formation of CS discs. Nowadays, there is a consensus that most young stellar objects exhibit such CS dusty discs. To verify this hypothesis we compare the parameters of intrinsic polarization (that is determined by the orientation of CS envelopes) with the direction of interstellar polarization (that is determined by the local magnetic field). The results of the statistical analysis confirm well the above suggestion.


POSTER: A Short Characteristic Solution for 2.5D Transfer Equation in the Envelopes of O and B Stars

J. Zsarg\'{o} 1, J. D. Hillier 1, L. N. Georgiev 2

1University of Pittsburgh, 3941 O'Hara str., Pittsburgh, PA 15260

2Instituto de Astronomia, UNAM, Apartado Postal 70-264, Cuidad Universitaria, 04510 Mexico, D. F. Mexico

We discuss work toward developing a 2.5D non-LTE radiative transfer code. As part of this effort we present the results for a short characteristic method to solve the radiative transfer equation for bound-bound transitions in stellar envelopes. Our code is based on the approach of Busche \& Hillier (2000), with modifications to handle realistic 3D wind velocities. Although non-vectorial quantities, such as electron density and temperature, are axial symmetric, we solve for the radiation field in a spherical coordinate system that naturally accommodates the inherent discontinuity at the limb of the stellar surface and the symmetries due to the forward-peaking nature of the radiation field. The effects of the velocity field are allowed for by increasing, as needed, the number of grid points along a short characteristic. This allows us to accurately map the variation of the opacities and emissivities as a function of frequency and spatial coordinate. We designed this code for parallel computing facilities and to allow for an adjustable compromise between computing efficiency and accuracy. In the future we plan to use this transfer code with a 2D non-LTE stellar atmosphere program (Georgiev \& Hillier, 2003) to self-consistently solve for level populations, the radiation field and temperature structure for stars with winds and without spherical symmetry.