EDP Sciences Journals List
Advanced Search
Subscriber Authentication Point
Home All issues Volume 411/3 Article
Free access article

Issue A&A
Volume 411, Number 3, December I 2003
Page(s) 405 - 416
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20031350



A&A 411, 405-416 (2003)
DOI: 10.1051/0004-6361:20031350

Bright stars and an optically thick inactive disk in Sgr A * and other dormant galaxy centers

J. Cuadra1, S. Nayakshin1 and R. Sunyaev1, 2

1  Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
2  Space Research Institute, Moscow, Russia

(Received 2 May 2003 / Accepted 19 August 2003 )

Abstract
Cold inactive disks are believed to exist in Low Luminosity AGN (LLAGN). They may also exist in the nuclei of inactive galaxies and in the center of our own Galaxy. These disks would then be embedded in the observed dense nuclear stellar clusters. Making the simplest assumption of an optically thick disk, we explore several ways to detect the disk presence through its interaction with the cluster. The first of these is the eclipse of close bright stars by the disk. The second is the increase in the infrared flux of the disk due to illumination of its surface by such stars during close passages. Finally the surface brightness of the star cluster should show an anisotropy that depends on the inclination angle of the disk. We apply the first two of the methods to Sgr A *, the super-massive black hole in our Galactic Center. Using the orbital parameters of the close star S2, we strongly rule out a disk optically thick in the near infrared unless it has a relatively large inner hole. For disks with no inner holes, we estimate that the data permit a disk with infrared optical depth no larger than about 0.01. Such a disk could also be responsible for the detected 3.8 $\mu$m excess in the spectrum of S2. The constraints on the disk that we obtain here can be reconciled with the disk parameters needed to explain the observed X-ray flares if dust particles in the disk have sizes greater than ${\sim} 30~\mu$m. The destruction of small dust particles by strong UV heating and shocks from star passages through the disk, and grain growth during "quiescent" times, are mentioned as possible mechanisms of creating the unusual grain size distribution. We estimate the emissivity of the thin layer photo-ionized by the star in Hydrogen Br $\gamma$ line and in the continuum recombination in the 2.2 $\mu$m band, and find that it may be detectable in the future if the disk exists.


Key words: accretion, accretion disks -- ISM: dust, extinction -- eclipses -- Galaxy: center -- stars: individual: S2

Offprint request: J. Cuadra, jcuadra@MPA-Garching.MPG.DE

SIMBAD Objects



© ESO 2003


What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.
  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.