qingkang li department of astronomy beijing normal university the third workshop of song, april,...

Qingkang Li

Department of AstronomyBeijing Normal University

The Third Workshop of SONG, April, 2010

Disks of Be Stars & Their Pulsations &

Collaborators:

• J. C. Brown, Dept. of Physics and Astronomy, University of Glasgow, UK

• J. P. Cassinelli, Dept of Astronomy, University of Wisconsin-Madison, USA

• R. Ignace, Dept. of Physics and Astronomy, University of East Tennessee, USA

• And many others.

2. The mystery of the Be stars

Contents

3. Be star disk models

1. Introduction

4. Discussion

• Over the course of their lifetimes, hot, luminous, massive (OB-type) stars lose large amount of mass in nearly continuous outflow called stellar winds.

• These winds are driven by scattering of the star’s continuum radiation in a large ensemble of spectral lines (Castor, Abbott & Klein 1975).

• There is extensive evidence for variability and structure on both small and large scales.

1. Introduction

Solar Activities: Coronal Expansion

SOHO Extreme ultraviolet (171 Angstrom)

Wind-Blown Bubbles in ISM

WR wind bubble NGC 2359

Superbubble in the Large Magellanic Cloud

Eta Carinae

OB stars in the HR diagram

O, B spectral type starsTeff > 10,000 KL* > 100 Lsun

M* > 3 Msun

Hot massive pulsators

Pulsating stars in the HR diagram

Pulsating Be stars: P~similar to SPB up to 100-200 hrs

βCep variables: pulsation periods P~2-10 hrs, low-order p and g modes, B0-B2

Slowly pulsating B (SPB) stars: P 10-∽50 hrs, high-order g modes, B3-B9

2. The Mystery of the Be Stars

Be stars are non-supergiant B-type stars whose spectra have, or had at some time, one or more Balmer lines in emission. The mystery of the "Be phenomenon" is that the emission, which is well understood to originate from a flattened circumstellar disk (e.g. Struve, 1931), can come and go episodically on time scales of days to decades.

Be stars• Hot, bright, & rapidly rotating stars.• The “e” stands for emission lines in the star’s spectrum

• Detailed spectra show emission intensity is split into peaks to blue and red of line-center.

o

Inte

nsi

ty

Wavelength

• Indicates a disk of gas orbits the star.

• This is from Doppler shift of gas moving toward and away from the observer.

Hydrogenspectrum

HH

Cartoon of a Be star and its emission lines

From http://www.astrosurf.org/buil/us/bestar.html

Variability of line profile

From http://www.astrosurf.org/buil/us/bestar.html

• Strong, radiatively driven stellar winds

---Mdot ~ 10-10 to 10-6 MO/yr; v > 1000 km/s– Driven by line-scattering of the star’s radiation

Key Properties

• Magnetic Activity– Some have observed dipole field ~103-104 G – stable; from convective dynamo or fossil?

• Stellar Rotation– Be stars are generally rapid rotators

– Vrot ~ 200-400 km/s ~ Vcrit/2

– Prot ~ few days

– oblateness and gravity darkening

• Stellar Pulsation– Many Be stars show Non-Radial Pulsations

(NRP) with m < l = 1 – 4

– Give rise to the line variability.

The Key Puzzle of Be Disks

• And most Be stars are not in close binary systems.

• Be stars are too old to still have protostellar disk.

How do Be starsdo this?

• They thus lack outside mass source to fall into disk.

• So disk matter must be launched from the star.

The key theoretical concerns on the Be star disks are as follows: 1. What source of energy elevates matter to orbits well above the stellar surface?

2. How does the matter obtain the angular momentum and orbit around the star with Keplerian speed with no observed outflow?

3. How are the very high observed disk density attained?

3. Be star disk models(1) Wind compressed disk model (Bjorkman, Cassinelli, 1993, ApJ, 409, 429)

(2) Episodic mass ejection model (Ando, 1986, A&A, 163, 97; Owocki & Cranmer, 2002, ASP 259, 512, 951; Cranmer, 2009, ApJ, 701, 396)

Observations confirm that Be star line-profile variability is due to the non-radial pulsations (NRP). Some mixed modes of NRP (retrograde) are associated with the transport of angular momentum up to the surface and beyond, saying, pulsational energy may leak out of a hot star into circumstellar medium.

• low-frequency evanescent pulsations;• high-frequency resonant wave excitation at the acoustic cutoff;• shock steepening and dissipation;• wave pressure & angular momentum transfer.

(3) Magnetically torqued disk (MTD) model (Cassinelli, et al., 2002, ApJ, 578, 951)

MTD+GD (Brown, et al., 2004, MNRAS, 352, 1061)

X-ray emission from MTD+GD Li, et al., 2008, ApJ, 672,1174

X-ray emission from MTD+GD Li, et al., 2008, ApJ, 672,1174

(4) Combination of all stuff

?

Wind + B-field + Rotation + Pulsation

Painted by John C. Brown

4. Discussion• The disk formation mechanism of Be stars is one of the key issues to hot star research both observational and theoretical.

• We need new observational facilities, such as SONG.

• We require the development of simulations.

• We require the advancement of model atmospheres and radiative transfer techniques

• We need correct stellar parameters (mass loss rate, rotation, magnetic fields, and pulsations).

Thank you very much for your time!