The CIDA-UMich-SAO Variability/Spectroscopic Survey of the Orion OB1 Association

The history of the star formation in molecular cloud complexes in the solar vicinity is not yet well understood. Although we have no problem identifying the youngest stars (~1 million years) wich are projected on sky regions with large amounts of gas and dust, it is much more difficult to find more slightly evolved (~ < 10 million years) stellar populations, produced in previous star formation events. The reason is that the gas and dust of the clouds in which these stars were born have already dissipated, the stars themselves have dispersed far from their native places. This dispersal can happen in a few million years. It is important to identify these "adolescent" stellar populations if we want to understand not only the early evolution of stars, but the planet formation processes, that are supposed to take place during these critical first few million years in the life of a star.

One of the various methods to look for these evolved young stars has been to carry out surveys with x-ray satellites, like the [[http://heasarc.gsfc.nasa.gov/docs/rosat/rass.html][ROSAT All-Sky Survey (RASS)]. Nevertheless, the interpretation of the nature of the x-ray sources identified with the RASS is still the subject of an important controversy, because these lists of objects are contaminated by much more older stars (20-100 million years), that are not related with the young stellar populations that we are interested in detecting (Briceño et al. 1997). Furthermore, the RASS is not sensitive to less massive young stars, which happen to be the most numerous.

In order to address these problems, a team of astronomers from CIDA, University of Michigan, and the Harvard-Smithsonian Center for Astrophysics, are using a host of wide-field instruments in observatories in Venezuela and the US to conduct a large scale survey of the Orion OB1 Association, one of closest star forming regions. The imaging survey, spanning ~150 square degrees, is done using the 8000 x 8000 pixel CCD Mosaic Camera installed on the 1m Schmidt telescope of the National Astronomical Observatory, in Mérida, Venezuela. The Camera is optimized for operating in "Drifscan-mode", producing an image of a continuous strip of the sky, 2.3 x 2.3 degrees wide. It is equipped with up to 4 filters, that allow cuasi-simultaneous photometry in four wavelength ranges a a rate of 34 square degrees per hour. The selection technique is unique in that it takes advantage of the variations in brightness characteristic of young stellar objects. By analizing repeated observations (up to 35 scans in some strips of the sky) the astronomers can determine which stars are variable. Those that also exhibit colors and brightness typical of young, low-mass stars placed at the distance of Orion, are selected as candidate young stars. However, this careful pruning is not enough, spectroscopic observations are necessary to ultimately decide which objects are truly young stars. Spectra provide determinations of the star's surface temperature and allows researchers to look for certain features that are the signpost of stellar youth.

The follow up spectroscopic observations are conducted using various telescopes. The brighter candidates (V < 16) are observed with the FAST spectrograph on the Smithsonian Astrophysical Observatory's 1.5m telescope at Mt. Hopkins, Arizona. For the much more numerous faint candidates, the instrument of choice is the 300 fiber Hectospec spectrograph on the 6.5m MMT telescope, also at Mt. Hopkins. This unique wide-field instruments provides spectra of thousands of stars spread out over many square degress in only a few nights. The [http://cfa-www.harvard.edu/cfa/oir/MMT/MMTI/hectochelle.html][Hectochelle multifiber spectrograph]], also installed on the MMT, delivers high resolution spectra to measure radial velocities of the newly identified low-mass, young stars. Such data allow studies of the kinematics of these stars as a group.

Finally, infrared (IR) observations provide information about the warm dust surrounding young stars. Such data are crucial to detect the presence of circumstellar disks, estimate how frequent they are and on what timescales do they evolve, possibly into young planetary systems. The team uses the 2 Micron All sky Survey (2MASS) measurements at 1.25 to 2.2 microns, and longer IR observations obtained with the Spitzer Space Telescope, to study the properties of dusty disks around the newly identified young stars.

This study will produce the first exhaustive census of the young stellar population across all Orion, providing for the first time strong constraints on the history of star formation in this region, as well as the duration of the formation process and initial evolution of protoplanetary systems.