Astronomical Data Center

ADCADC/CDS Standard Document for Catalog:
/external/photom/E2004/

The ADC has expanded its resources in order to better serve our users, we have developed a new category called "external" to complement our catalogs and journal table directories. You can access the data set described here through the URL:
http://www.astro.wesleyan.edu/~bill/research/ttauri.html.

The following is an abbreviated "ReadMe" document that describes this external data set.
E/II/4          Photometry of T Tauri Stars  (Herbst+ 1994)
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Catalogue of UBVRI photometry of T Tauri stars and analysis of the causes
of their variability
   Herbst W., Herbst D.K., Grossman E.J.
   <Astronomical Journal, 108, 1906-1923 (1994)>
   =1994AJ....108.1906H
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ADC_Keywords: Photometry, UBVRI; Stars, pre-main sequence; External catalog

Location: http://www.astro.wesleyan.edu/~bill/research/ttauri.html

Description:
   A computer-based catalogue of UBVRI photoelectric photometry of T Tauri 
   stars and their earlier type analogs has been compiled. It presently 
   includes over 10000 entries on 80 stars and will be updated on a regular 
   basis; it is available on Internet. The catalogue is used to analyze the 
   sometimes bizarre light variations of pre-main-sequence stars on time 
   scales of days to months in an attempt to illuminate the nature and causes
   of the phenomenon. It is useful in discussing their light variations to 
   divide the stars into three groups according to their spectra. These are: 
   weak T Tauri stars (WTTS; spectral class later than K0 and W(sub H-alpha 
   less than 10 A), classical T Tauri stars (CTTS; spectral class later than 
   K0 and W(sub H-alpha) greater than 10 A), and early type T Tauri stars 
   (ETTS; spectral class of K0 or earlier). Three distinct types of variability
   are displayed by stars in the catalogue. Type I variations are periodic in 
   VRI and undoubtedly caused by rotational modulation of a star with an 
   asymmetric distribution of cool spots on its surface. Irregular flare 
   activity is sometimes seen on such stars in U and B. Type I variations are 
   easiest to see on WTTS but are clearly present on CTTS and ETTS as well. 
   Type II variations are caused by hot 'spots' or zones and, it is argued, 
   result from changes in the excess or 'veiling' continuum commonly 
   attributed to an accretion boundary layer or impact zone of a magnetically 
   channeled accretion flow. This type of variation is seen predominantly or 
   solely in CTTS. A subcategory, designated Type IIp, consists of stars which 
   display periodic variations caused by hot spots. Whereas cool spots may 
   last for hundreds or thousands of rotations, hot spots appear to come and 
   go on a much shorter time scale. This suggests that both unsteady accretion
   and rotation of the star contribute to Type II variations. It is shown that
   a third type of variation exists among ETTS, including stars as early as A 
   type. UX Ori is a typical example and we call these Type III variables or 
   UXors. Their distinguishing characteristic is that they can display very 
   large amplitudes (exceeding 2.8 mag in V) while showing little or no 
   evidence for a veiling continuum or any substantial change in their 
   photospheric spectra. If Type III variations are caused by changes in 
   accretion luminosity, then boundary layers or impact zones in ETTS must be 
   much different from CTTS which, of course, is possible since mass accretion
   rates are probably much higher. However, the leading hypothesis for 
   explaining Type III variations is variable obscuration by circumstellar 
   dust. It is argued that the putative dust clumps causing such variations 
   cannot be confined to a disk; otherwise UXors would be rare. Perhaps 
   magnetic effects are involved in levitating accreting dust out of the plane,
   as has been suggested for CTTS, or perhaps we are witnessing continuing 
   infall of clumps from placental clouds. A third possibility is that dust 
   may be condensing in an outflow. 

   Recently, most of the data has come from the ROTOR program directed by 
   Valery Shevchenko of the Maidanak Observatory in Uzbekistan. A program of 
   joint research at Maidanak and Van Vleck Observatory is proceeding with the
   support of the Civilian Research and Development Fund for States of the 
   Former Soviet Union. Contributions to the data base or corrections to it 
   are most welcome.  Please send them to William Herbst at 
   bill@astro.wesleyan.edu.

   
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(End)                         Gail L. Schneider [SSDOO/ADC]      31-Aug-1998
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