Observations and modeling of classical Be stars

• Thesis title in Czech: Pozorování a modelování klasických Be hvězd
• Thesis title in English: Observations and modeling of classical Be stars
• Key words: Be hvězdy, okolohvězdné prostředí, rádiová astronomie, interferometrie, dvojhvězdnost
• English key words: Be stars, circumstellar envelopes, radio astronomy, interferometry, binarity
• Academic year of topic announcement: 2012/2013
• Thesis type: dissertation
• Thesis language: angličtina
• Department: Astronomical Institute of Charles University (32-AUUK)
• Supervisor: doc. Mgr. Daniela Korčáková, Ph.D.
• Author: hidden - assigned and confirmed by the Study Dept.
• Date of registration: 26.09.2012
• Date of assignment: 26.09.2012
• Confirmed by Study dept. on: 07.12.2012
• Date and time of defence: 21.06.2017 10:00
• Date of electronic submission: 16.05.2017
• Date of submission of printed version: 02.05.2017
• Date of proceeded defence: 21.06.2017
• Opponents: Carol Evelyn Jones
• Advisors: Stanislav Štefl

Guidelines

Student by se během svého studia měl seznámit s problematikou týkající se Be a B[e] hvězd a to z pozorovatelského úhlu pohledu i po stránce teoretické. Práce bude zaměřena převážně na dlouhovlnnou oblast spektra - infračervené a zvláště pak milimetrové záření. To umožní studium vnějších částí disků okolo těchto hvězd, kde je v současnosti řada nevyjasněných otázek. Cílem práce je u vybraných objektů sestavit model okolohvězdného prostředí na základě pozorovaných vlastností. K dispozici bude numerický kód HDUST (Carciofi & Bjorkman, 2008, ApJ, 684, 1374; 2006, ApJ, 639, 1081),
s pomocí kterého je možné modelovat záření vycházející z disků okolo horkých hvězd. Získané rozložení energie ve spektrech či intenzitní mapy lze porovnávat s pozorováními. Student by měl pracovat nejen s archivními daty, ale také by se měl pokusit získat data z interferometru ALMA. Jeho úkolem bude také příprava proposalu pro ALMA a to včetně vytipování vhodných objektů pro pozorování.

Práce bude probíhat pod vedením zahraničních konzultantů:
PhD Stanislav Štefl, Joint ALMA Observatory, ESO, Chile
Prof. Alex Carciofi, University of Sao Paulo, Brazil
Prof. Jon Bjorkman, University of Toledo, USA

References

Baade, D., Rivinius, T., Štefl, S. & Martayan, Ch. 2011, IAUS, 272, 1
Carciofi, A.C. & Bjorkman, J. E. 2006, ApJ, 639, 1081
Carciofi, A.C. & Bjorkman, J. E. 2008, ApJ, 684, 1374
Carciofi, A.C. et al. 2009, A&A, 504, 915
Carciofi, A.C. 2011, IAUS, 272, 325
Emerson, D., 1999, "Interpreting Astronomical Spectra", John Wiley & Sons
Fullerton, A. W. 2011, IAUS, 272, 136
Glatzel, W. 2007 ASPC, 361, 337
Gray, D.F. 2008, "The Observation and Analysis of Stellar Photospheres", 3rd Edition, Cambridge University Press
Hillier, D.J. 2006 ASPC, 355, 39
Howarth, I.D. 2007 ASPC, 361, 15
Lamers, H.J.G.L.M., Zickgraf, F.J., de Winter, D., Houziaux, L., & Zorec, J., 1998, A&A, 340, 117
Meynet, G., Ekström, S., Maeder, A. & Barblan, F. 2007, ASPC, 361, 325
Miroshnichenko, A.S. 2006, ASPC, 355, 13
Miroshnichenko, A.S. 2007, ApJ, 667, 497
Miroshnichenko, A.S., Manset, N., Kusakin, A. V., et al. 2007, ApJ, 671, 828
Moffat, A.F.J. 2008, cihw.conf, 17
Negueruela, I. 2007, ASPC, 361, 376
Owocki, S. 2006, ASPC, 355, 219
Owocki, S. P. 2007, ASPC, 361, 3
Porter, J, Rivinius, T 2003, PASP, 115, 1153
Puls, J., Sundqvist, J.O., Rivero G., & Jorge G. 2011, IAUS, 272, 554
Štefl, S et al. 2009, A&A, 504, 929
Štefl, S et al. 2010, workshop Resolving the Future of Astronomy with Long-Baseline Interferometry, Socorro, PASP Conf. Ser., in press
Waters, L.B.F. et al. 1991, A&A, 244, 120
ALMA Cycle 1 Technical Handbook: http://www.almaobservatory.org/en

Preliminary scope of work in English

The multitechnic and mutiwavelength observations of the Be star zeta Tau (Stefl et al. 2009, A&A 504, 929) and their modeling (Carciofi et al. 2009, A&A 504, 915) as well as our sub-mm APEX and CARMA observations of delta Sco and 28 CMa (Stefl et al. 2012, workshop Circumstellar Dynamics at High Resolution, in press) will be the starting points for the PhD project.

Although the zeta Tau model derived by Carciofi et al. using the code HDUST (Carciofi and Bjorkman, 2008, ApJ, 684, 1374; 2006, ApJ, 639, 1081) is considered to be the most physically consistent one ever derived for a gaseous disk of a Be star, it suffers from uncertainty in its outer parts, which are not constrained by any observational data. Such constrain can come only from sub-mm observations. The modelling of the outer disks will bring new insights on the geometrical and dynamical structure of outer disks, their dissipation and their interaction with the interstellar medium. Because circumstellar disks of classical Be stars can be observed in bright and relatively close galactic targets, they can be considered to be a laboratory for other targets containing viscous disk, which are still not reachable by the present interferometric facilities, e.g. AGNs or protostellar and protoplanetary disks. However, these targets will become not only observable with ALMA, when the longest baselines are available, but they will represent the main focus of this starting and superb observatory. The studies of the outer Be star disks may become not
only a continuation of the half century long traditional research of Be stars in the Czech Republic, but they may open a path to new fields, which have the highest priority in the present European and world astrophysics.

Because temperatures in the disks of classical Be stars are two hot to keep a dust, they can be observed only in the free-free radiation in the sub-mm region. After the pioneering work by Waters et al (1991, 244,120) bringing a few detections of Be stars in sub-mm region, early-B emission-line stars have not been observed by radio astronomers for almost 20 years. The recent APEX and CARMA observations (Stefl et al. 2012, workshop Circumstellar Dynamics at High Resolution, in press) shows that sub-mm fluxes or even their variations can be well measured for well selected Be stars with the present sub-mm telescopes and they are more than sufficient for ALMA direct interferometric imaging in the near future. Typical fluxes at 1mm are of the order of tens of mJy, about 3 orders larger than the expected ALMA sensitivity. Even routinely used sub-mm telescopes (CARMA array or APEX) can provide reliable continuum fluxes in the 0.8-3mm region for modeling of the
outer Be/B[e] star disks for studies, which may help to select the most suitable targets for ALMA imaging and to elaborate needed procedures.

On the theoretical side, the modeling of the circumstellar disks with the help of the HDUST code is an unique opportunity and provides an unlimited possibilities for extension to other astrophysical problems. The code (as it is reflected already in its name) offers a possibility to model both the gaseous and dust component of circumstellar disks and in principle can also
be applied to protoplanetary disks, although it was not applied to them yet. The program enables to compute different observables including spectral energy distribution, magnitudes, spectral line profiles, interferometric visibilities and phases, or polarization. Although its first version was limited to the hydrogen, spectrum, it is being upgraded also for other elements.

In order to prepare a strong program for the hard competition for ALMA, the two research directions will be followed. The basic data for the project and well selected targets can be found in the data archives (HR spectroscopy, near IR-interferometry) and proposed for the available radio-telescopes (CARNMA, APEX). The follow-up ALMA proposals with the aim to get an interferometric image of the studied circumstellar disks should be based on the first step of modeling.

1) Observations and modeling of the outer disks of classical Be stars. Our overview of HR AMBER/VLTI data (Stefl et al. 2010, workshop Resolving the Future of Astronomy with Long-Baseline Interferometry, Socorro, PASP Conf. Ser., in press) as well as our own VLTI programs (zeta Tau - Stefl et al 2009, 504, 929, 48 Lib - Stefl et al. 2012, in press; delta Sco -
Le Bouquin et al. 2012, in preparation) allow us to select a few closest Be stars with huge circumstellar disks, the fluxes of which are comparable with those measured in delta Sco and for which the flux measurements with a sufficient accuracy are feasible with a very high feasibility. The observing proposals to ALMA, CARMA, APEX and possibly other (sub-)mm radiotelescopes
should be submitted for the next proposal periods. The PhD student can immediately start with installation of the HDUST code and getting the first experience with it by modelling the archival data (1st year of the PhD). Inclusion of the sub-mm data in the modelling and involvement in the ALMA related projects (optimally at the ALMA observatory, Chile) would be scheduled for the second year. During the third year of the PhD project, a proposal for the direct imaging of a most suitable target should be prepared on basis of the complete SED and other data analysis. If successful, the ALMA interferometric data should complete the PhD project.

2) Extension of the circumstellar disk modeling to the outer gas and dust region of selected B[e] stars. Unlike classical Be stars, less studied B[e] stars have outer dust disk,
extending to large angular diameters and producing very high fluxes (of the order of ~1 Jy) in the mid-IR and sub-mm region. The closest of the B[e] stars are expected to be easy targets for for ALMA or CARMA. Some of the B[e] stars (V921 Sco - Kraus et al. 2012, ApJ accepted, HD 87643 - Millour et al. 2009, A&A507,317) were observed and studied with AMBER/VLTI and the data are already available in the ESO archive. Nevertheless, the data were not modeled at a level offered by the HDUST code.

Preliminary plan for the PhD project:

1st year:
Selection of suitable B[e] stars, reduction of the available archival interferometric and spectroscopic data, training with and possible modification of the HDUST code for B[e] systems (the systems contain not only an equatorial disk, but also extended polar nebulae). Proposal for complementary sub-mm/mm flux observations (ALMA, CARMA, APEX).

2nd year:
Modeling of available archival data.

Sub-mm/mm radio observations, inclusion of the radio observations in the HDUST models, involvement of the PhD student in the ALMA related projects (optimally at the ALMA observatory, Chile).

3rd year:
ALMA proposal for a direct imaging of selected B[e] stars, sub-mm/mm radio observations.

4th year:
Analysis of the ALMA observations using the HDUST model.