Variability of Be Stars:A Key to the Structure of their Circumstellar Environments

Anatoly Miroshnichenko

University of Toledo

Variable Star Meeting 2004. BGSU, April 3

In Collaboration with

Karen Bjorkman

Jon Bjorkman

Alex Carciofi

University of Toledo

Outline

• Brief history of studies of Be stars

• Basic properties of Be stars

• Variations of the spectrum and brightness

• How can we use the variability to get the physics?

• Current state of the research

Classical Be Stars

First discovered group of emission-line stars

Emission lines in the spectrum of Cassiopeae were found in 1867 by visual spectroscopy

~200 Be stars are currently known among 1660 B-type stars brighter than V=6.5 mag

Main properties of classical Be stars

• Non-luminous rapidly-rotating objects displaying emission-line spectra

• Emission line profile shapes are usually double- or single-peaked at a low or moderate spectral resolution

• Infrared (IR) radiation excesses• Polarization of the continuum radiation• Active emission-line phases may last for decades and are

followed by no-emission or shell-line phases• Metallic line profiles (e.g., Fe II) suggest that the

circumstellar gas is involved in a Keplerian motion around the star with small radial velocities (a few km s-1)

Basic Stellar Parameters

Some H profiles

IR Excess

Aqr

1998 1983

Polarization

Tau

Circumstellar Disks

Origin of the Observed Features

Line emission – ionized circumstellar gas

IR excess – free-free emission

Polarization – Thomson scattering

The polarization spectrum and spectral line profiles imply a flattened, disk-like, envelope

Theories of the Be phenomenon

Elliptical disk model (Struve 1931, ApJ, 73, 94) - Keplerian rotation of particles in a circumstellar disk.

No explanation for the disk long-term stability.

Rotation-pulsation model - changing inflow and outflow superposed onto the rotational motion in the disk. Variable stellar wind as triggering mechanism for the V/R variations (Doazan et al. 1987, A&A, 182, L25).

No explanation for the disk formation.

Theories of the Be phenomenon

A disk can be produced by mass transfer in binary systems (Kriz & Harmanec 1975, BAICz, 26, 65), where the mass gainer spins-up to critical rotation.Can explain formation of 20-40 % of all Be stars (Pols et al. 1991, A&A, 241, 419) or even less (~5%, Van Bever & Vanbeveren (1997, A&A, 322, 116).

Wind-compressed disk model (Bjorkman & Cassinelli 1993, ApJ, 409, 429) - a rotating wind produces a disk.

Assumption: the outward acceleration is smaller than the rotation the material will orbit down to the equator before it is accelerated outwards.

But: small non-radial forces act against disk formation.

Theories of the Be phenomenon

Non-Radial Pulsations may be a triggering mechanism for the mass loss from at least early-type Be stars (Rivinius et al. 2003, A&A, 411, 229)

Modeling the Be Star Disks

Input parameters:

1. Stellar (Teff , log g , L)

2. Circumstellar: 0 ,Te(r) , geometry, density distribution (=0 r-n)

Observational data:

1. Spectral energy distribution

2. Spectral line profiles

3. Polarization spectrum

Typical Modeling Results

Disk opening angle - a few degrees

Statistical studies suggest opening angles from 5 to 25-40 degrees

Density at the disk base - ~10-11- 10-12 g cm-3

Density distribution slope - 2.5-3.5

Theoretical Disk Structure

from Carciofi & Bjorkman (2004, Polariz. Conf.)

Modeling Results for Tauri

from Carciofi & Bjorkman (2004, Polariz. Conf.)

Problems with Snapshot Modeling

Theoretical (simplified assumptions):

Smooth density distribution

Uncertain disk size

Parameter space degeneracy (0 – disk scale height)

Observational:

Non-contemporaneous data

Limited spectral range

Line Strength Variations

Complex Profiles

Oph

Variations

Aquarii: H Variations

What can We Get from Variability?

Reveal the true basic stellar parameters and content of the system

Determine the circumstellar contribution to the continuum

Learn about the mass loss history and mass distribution in the disk

Be star spectroscopy at the Ritter Observatory

• 9 non-overlapping orders, 70 Å each, range 5285-6600 Å. Includes spectral lines of FeII 5317 & 6383, HeI 5876, NaI 5889 & 5895, SiII 6347 & 6371, and H

• Spectral resolving power R (/) ~ 26000

• 1-meter telescope with a fiberfed echelle spectrograph and a 1150x1150-pixel CCD in the Coude focus

• Spectra of stars brighter than 7.5 mag can be obtained in 1 hour with a signal-to-noise ratio of ~100

•~1700 spectra of ~ 45 Be stars obtained in 1991-2004

Ritter data statistics (as of 2004/03/26)

Name 93-95 96 97 98 99 00 01 02 03 04 Tot

Cas 3 3 23 14 28 40 50 38 22 5 221

Lyr 81 36 44 17 177

Dra 1 8 16 29 19 5 34 42 26 1 180

Tau 6 2 13 20 31 15 19 10 25 4 143

Aqr 8 17 8 30 14 23 7 19 126

Sco 1 20 40 45 41 2 149

And 30 10 14 13 1 1 70

CrB 1 3 8 26 1 1 8 8 10 66

Per 5 11 2 8 9 11 5 8 58

CMi

8 23 22 20 3 76

Total 89 36 201 176 210 161 324 273 223 16 1709

Some results from Ritter program• Discovery of 2 new Be stars, HD 4881 (V=6.2, B9 V) and HD 5839

(V=6.7, B9 V) (Miroshnichenko et al. 1999, MNRAS, 302, 612).

• Periodic RV variations (84.3 d) of the emission peak and absorption wings of the H profile in Aquarii are discovered during the normal B-star phase (Bjorkman et al. 2002, ApJ, 573, 812).

• New orbital solution for the Scorpii binary and monitoring of the initial disk formation stages (Miroshnichenko et al. 2001, A&A, 377, 485; 2003, A&A, 408, 305).

• Periodic RV variations (205 d) in the H line of Cassiopeae (Miroshnichenko, Bjorkman, Krugov 2002, PASP, 114, 1226).

Cassiopeae

Aquarii

Aquarii

Scorpii

Scorpii

Scorpii

Conclusions

• Such observations give us information about:

- the disk structure

- the system content and fundamental parameters

- the mass loss origin and evolutionary state

• Long-term multi-wavelength and multi-technique observational studies are needed

• Most of the needed observations can be obtained with relatively small telescopes