We propose to study the global process of star formation in the Orion OB1b association. Only the high mass population of Orion OB1 has been fully studied, but the majority of stars are low mass stars. Spectroscopic and photometric data on low mass stars surrounding #tex2html_wrap_inline92# Orionis (Walter et al. 1998) show that the low mass stars are concentrated around #tex2html_wrap_inline94# Orionis. This suggests that there is substructure within the OB1b subgroup. We seek to extend our previous work to include other OB stars. The spatial distribution of the low mass stars will show if low mass stars form uniformly through out the cloud or clustered around the massive stars. Knowledge of the spatial distribution of recently formed stars and ages determined from the color magnitude diagram will allow us to study the history of star formation in the Orion OB1b subgroup.

We propose to obtain UBVRI photometry of 5 #tex2html_wrap_inline96# 1 square degree fields centered on #tex2html_wrap_inline98#, #tex2html_wrap_inline100#, and #tex2html_wrap_inline102# Orionis and two control fields in the belt. We also propose to obtain JHK photometry of the inner portions of these fields to measure the extinction and study the stars which retain their circumstellar disks.

Since most stars form in OB associations (Lada 1991), that is where we should search for the details of star formation. The Orion OB association is the largest nearby OB association (Blaaw 1991; Brown et al. 1994). This makes it our best venue for learning about the details of star formation in OB associations. The high mass population has been extensively studied (Warren Hesser 1977a, 1977b, 1978). However, most of the stars which form in OB associations are low mass stars. Studying the low mass stars of the Orion OB association is crucial to fully understanding how and under what conditions most of the stars in the galaxy formed.

There are many outstanding questions about star formation in OB associations. What is the distribution of recently formed low mass stars? Do they occur uniformly throughout the cloud or are they clustered around the massive stars? Hillenbrand (1997) studied the youngest region of the Orion OB1 association, the Orion Nebular cluster (ONC; Orion OB1d), and found that the massive stars are at the center of a larger cluster of low mass stars. Similar clustering of embedded stars has been seen at IR wavelengths in the Orion B cloud (Lada et al. 1991). Our photometric and spectroscopic work on the region around #tex2html_wrap_inline104#Orionis (Walter et al. 1998), found 110 pre-main sequence (PMS) stars clustered within 0.5^o of #tex2html_wrap_inline108#~Orionis (Figure~1). This suggests that there is substructure in the OB1b subgroup.

We propose to study the clustering properties and the star-formation history of the low mass pre-main sequence population of the belt of Orion (Ori~OB1b). This entails a photometric survey of 5 fields in Ori~OB1b, extending and complementing our work on #tex2html_wrap_inline110#~Ori. Three fields are centered on the belt stars #tex2html_wrap_inline112#, #tex2html_wrap_inline114#, and #tex2html_wrap_inline116# Orionis; two lie elsewhere in the belt. Each field is about 1 square degree in size. We have X-ray observations of each field, and have proposed for WIYN spectra of the brighter stars in each field. The photometry will be used to study the structure in the spatial distribution of the low mass stars.

Using the UBVRI photometry, we will construct a color magnitude diagram (CMD) of the low mass population of the Ori~OB1b association (Figure~2). From the CMD we can pick out those stars likely to be PMS, and determine the ages of the stars. The optical and JHK photometry will be used to determine extinctions, and to study those stars which may still have residual circumstellar dust. Ages determined from the CMD will be more precise those determined from the B stars because the position of low mass stars in the CMD is more sensitive to age than is the position of B stars on the ZAMS (Brown et al. 1998).

The result of this research is likely to be a better understanding of the global processes of star formation in OB associations. What are the clustering properties of the stars? Do low mass stars form in conjunction with whatever massive cores produce the massive stars, or do they form in a more distributed backdrop. We will learn the history of the low mass star formation. Was low mass star formation triggered (or stopped) by the high mass stars, or, if not coeval, did it proceed more-or-less continuously for a long time (cf. Walter et al 1994)? What is the mass function? The photometry, supplanted by the spectroscopy, will reveal the complete population of these fields. This research complements work being done elsewhere, in the Orion Nebula Cluster and near #tex2html_wrap_inline118#~Ori. The belt of Orion is a fossil star formation region: by digging up the fossil record we can learn how stars formed in a representative OB association.

**Figure:** The radial distribution of stars brighter than V=15 near #tex2html_wrap_inline120#Ori. PMS stars (solid histogram) show a clustering towards #tex2html_wrap_inline122#Ori, while the other stars (dashed distribution) do not. The probability that the two samples are drawn from the same parent distribution is 0.0003. The lines are linear least-squares fits to the distribution. The lack of stars within 3 arcmin of #tex2html_wrap_inline124#Ori is an artifact of the glare from the V=3.8 star.
#figure66#

**Figure:** The observed color-magnitude diagram for stars with 12;SPMlt;V;SPMlt;19 in 0.15~sq.~deg within 15~arcmin of #tex2html_wrap_inline132#~Orionis. Spectroscopically-identified low mass pre-main sequence stars, marked with solid circles, define the PMS locus. Photometric PMS candidates are indicated by the open circles. These data are not corrected for extinction; the reddening vector parallels the PMS locus. We have extended the CMD to V=23, and find that the PMS locus extends to the H-burning limit. We have identified over a dozen brown dwarf candidates in this field.
#figure76#

We select 5 fields in the belt of Orion for this investigation. Three are centered on the bright OB supergiants. We will use the observations of these fields to search for evidence of clustering of the low mass stars about the massive stars. We want to know whether the clusters around #tex2html_wrap_inline134#C and #tex2html_wrap_inline136#~Ori are unique, or whether subclustering is a common aspect of star formation in OB associations. The other two fields are in the belt, but removed from the bright stars. They do have copious numbers of X-ray sources, and so are not devoid of low mass stars. These fields will be used to determine the extent of the clustering, and the DC level of any uniform background distribution of low mass stars.

<#86#>CFIM<#86#>
We plan to obtain UBVRI photometry in 5 fields within the belt of Orion. Target stars range from V#tex2html_wrap_inline138#10 to V#tex2html_wrap_inline140#23, or about the brown dwarf limit. We require 16 exposures to cover the inner 50~arcmin of each field. At each pointing position we will integrate for 90 seconds in the BVRI filters, and 120 seconds at U. Where a significant number of stars are overexposed, we will also take shorter 10 second images. At most positions, we will also take 5 to 10~minute exposures in V and I, which will push our limiting magnitude down to close to 23. A full series of observations takes about 35 minutes; we can complete all 5 fields in 8 full nights. Since we have already observed the inner portions of 3 of the fields, we request 7 nights.

<#87#>CIRIM<#87#>
We will use a 6-pointing dither pattern for the CIRIM observations. About 10 minutes per target suffices to get 2 or better JHK photometry for targets with K;SPMlt;14, in our experience. Including allowance for standards, we expect to be able to observe about 5 stars per hour, or 30-40 stars per night. There are about 60 known PMS stars (from extant low dispersion spectra) and a few hundred stars near the PMS locus identified from last year's UBVRI images. The requested CIRIM time will permit us to sample the near-IR colors of about 25 stars in each field. This will provide a good map of the mean extinction (and its variations), as well as a detailed look into stars with near-IR excesses (inferred from strong H#tex2html_wrap_inline144#~emission).

We request CIRIM F14, rather than F7, because many of the PMS stars are close visual pairs, and because the F14 secondary permits better photometric sampling of the PSF under good seeing conditions. Despite the fairly high density of PMS stars in Orion, the larger field of view with the F7 secondary does not significantly increase observational efficiency.

<#88#>Other Telescopes<#88#>
We have proposed for time with WIYN/HYDRA to obtain echelle spectra of about 200 stars in each field. These will include all the X-ray sources, objects near the photometric PMS locus (where we have photometry), and randomly-selected stars of appropriate magnitudes and colors in the USNO A1.0 catalog. These spectra will be used for PMS identifications and to measure radial velocities. We do have some low dispersion spectra of X-ray sources in these regions. We use the locations of the spectroscopically-confirmed PMS stars to define the PMS locus in the photometric CMD.

Although Orion can be observed from KPNO, it is a southern constellation. None of our targets are north of the equator. We choose to do this work from CTIO for two reasons:

Abstract: