We present a new census of the stellar and substellar members of the young cluster IC 348. We have obtained images at I and Z for a 42'x28' field encompassing the cluster and have combined these measurements with previous optical and near-infrared photometry. From spectroscopy of candidate cluster members appearing in these data, we have identified 122 new members, 15 of which have spectral types of M6.5-M9, corresponding to masses of 0.08-0.015 M_sun by recent evolutionary models. The latest census for IC 348 now contains a total of 288 members, 23 of which are later than M6 and thus are likely to be brown dwarfs. From an extinction-limited sample of members (A_V<=4) for a 16'x14' field centered on the cluster, we construct an IMF that is unbiased in mass and nearly complete for M/M_sun>=0.03 (<=M8). In logarithmic units where the Salpeter slope is 1.35, the mass function for IC 348 rises from high masses down to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M_sun, and then declines into the substellar regime. In comparison, the similarly-derived IMF for Taurus from Briceno et al. and Luhman et al. rises quickly to a peak near 0.8 M_sun and steadily declines to lower masses. The distinctive shapes of the IMFs in IC 348 and Taurus are reflected in the distributions of spectral types, which peak at M5 and K7, respectively. These data provide compelling, model-independent evidence for a significant variation of the IMF with star-forming conditions.Comment: 47 pages, 14 figures, 3rd para of 4.5.3 has been added, this is final version in press at ApJ, also found at http://cfa-www.harvard.edu/sfgroup/preprints.htm

We present near-and mid-infrared photometry obtained with the Spitzer Space Telescope of $300 known members of the IC 348 cluster. We merge this photometry with existing ground-based optical and near-infrared photometry in order to construct optical-infrared spectral energy distributions (SEDs) for all the cluster members and present a complete atlas of these SEDs. We employ these observations to investigate both the frequency and nature of the circumstellar disk population in the cluster. The Spitzer observations span a wavelength range between 3.6 and 24 m, corresponding to disk radii of $0.1-5 AU from the central star. The observations are sufficiently sensitive to enable the first detailed measurement of the disk frequency for very low mass stars at the peak of the stellar initial mass function. Using measurements of infrared excess between 3.6 and 8.0 m, we find the total frequency of diskbearing stars in the cluster to be 50% AE 6%. However, only 30% AE 4% of the member stars are surrounded by optically thick, primordial disks, while the remaining disk-bearing stars are surrounded by what appear to be optically thin, anemic disks. Both these values are below previous estimates for this cluster. The disk fraction appears to be a function of spectral type and stellar mass. The fraction of stars with optically thick disks ranges from 11% AE 8% for stars earlier than K6 to 47% AE 12% for K6-M2 stars to 28% AE 5% for M2-M6 stars. The disk longevity and thus conditions for planet formation appear to be most favorable for the K6-M2 stars, which are objects of comparable mass to the Sun for the age of this cluster. The optically thick disks around later type (>M4) stars appear to be less flared than the disks around earlier type stars. This may indicate a greater degree of dust settling and a more advanced evolutionary state for the late M disk population. Finally, we find that the presence of an optically thick dust disk is correlated with gaseous accretion, as measured by the strength of H emission. A large fraction of stars classified as classical T Tauri stars possess robust, optically thick disks, and very few such stars are found to be diskless. The majority (64%) of stars classified as weak-lined T Tauri stars are found to be diskless. However, a significant fraction (12%) of these stars are found to be surrounded by thick, primordial disks. These results suggest that it is more likely for dust disks to persist in the absence of active gaseous accretion than for active accretion to persist in the absence of dusty disks.

We use the results of a new, multi-epoch, multi-wavelength, near-infrared census of the Trapezium Cluster in Orion to construct and to analyze the structure of its infrared (K band) luminosity function. Specifically, we employ an improved set of model luminosity functions to derive this cluster's underlying Initial Mass Function (IMF) across the entire range of mass from OB stars to sub-stellar objects down to near the deuterium burning limit. We derive an IMF for the Trapezium Cluster that rises with decreasing mass, having a Salpeter-like IMF slope until near ∼ 0.6 M ⊙ where the IMF flattens and forms a broad peak extending to the hydrogen burning limit, below which the IMF declines into the sub-stellar regime. Independent of the details, we find that sub-stellar objects account for no more than ∼ 22% of the total number of likely cluster members. Further, the sub-stellar Trapezium IMF breaks from a steady 1 Visiting Astronomer, European Southern Observatory

We present a description of the data reduction methods and the derived catalog of more than 1600 X-ray point sources from the exceptionally deep January 2003 Chandra X-ray Observatory (Chandra) observation of the Orion Nebula Cluster and embedded populations around OMC-1. The observation was obtained with Chandra's Advanced CCD Imaging Spectrometer (ACIS) and has been nicknamed the Chandra Orion Ultradeep Project (COUP). With an 838 ks exposure made over a continuous period of 13.2 days, the COUP observation provides the most uniform and comprehensive dataset on the X-ray emission of normal stars ever obtained in the history of X-ray astronomy.

In this paper we present the results of a systematic investigation of an entire population of predominately starless dust cores within a single molecular cloud, the Pipe Nebula. Analysis of extinction data shows the cores to be dense objects characterized by a narrow range of density with a median value of n(H 2 ) ¼ 7 ; 10 3 . The nonthermal velocity dispersions measured in molecular emission lines are found to be subsonic for the large majority of the cores and show no correlation with core mass (or size). Thermal pressure is found to be the dominate source of internal gas pressure and support for most of the core population. The total internal pressures of the cores are found to be roughly independent of core mass over the entire (0.2Y20 M ) range of the core mass function (CMF) indicating that the cores are in pressure equilibrium with an external source of pressure. This external pressure is most likely provided by the weight of the surrounding molecular cloud. Most of the cores appear to be pressure confined, gravitationally unbound entities whose fundamental physical properties are determined by only a few factors, which include self-gravity, gas temperature, and the simple requirement of pressure equilibrium with the surrounding environment. The entire core population is found to be characterized by a single critical Bonnor-Ebert mass of approximately 2 M . This mass coincides with the characteristic mass of the Pipe CMF suggesting that the CMF (and ultimately the stellar IMF) has its origin in the physical process of thermal fragmentation in a pressurized medium.

Spitzer mid-infrared surveys enable accurate census of young stellar objects by sampling large spatial scales, revealing very embedded protostars and detecting low luminosity objects. Taking advantage of these capabilities, we present a Spitzer based census of the IC 348 nebula and embedded star cluster, covering a 2.5 pc region and comparable in extent to the Orion nebula. Our Spitzer census supplemented with ground based spectra has added 42 class II T-Tauri sources to the cluster membership and identified ∼ 20 class 0/I protostars. The population of IC 348 likely exceeds 400 sources after accounting statistically for unidentified diskless members. Our Spitzer census of IC 348 reveals a population of class I protostars that is anti-correlated spatially with the class II/III T-Tauri members, which comprise the centrally condensed cluster around a B star. The protostars are instead found mostly at the cluster periphery about ∼ 1 pc from the B star and spread out along a filamentary ridge. We further find that the star formation rate in this protostellar ridge is consistent with that rate which built the older exposed cluster while the presence of fifteen cold, starless, millimeter cores intermingled with this protostellar population indicates that the IC 348 nebula has yet to finish forming stars. Moreover, we show that the IC 348 cluster is of order 3-5 crossing times old, and, as evidenced by its smooth radial profile and confirmed mass segregation, is likely relaxed. While it seems apparent that the current cluster configuration is the result of dynamical evolution and its primordial structure has been erased, our finding of a filamentary ridge of class I protostars supports a model where embedded clusters are built up from numerous smaller sub-clusters. Finally, the results of our Spitzer census indicate that the supposition that star formation must progress rapidly in a dark cloud should not preclude these observations that show it can be relatively long lived.Subject headings: infrared: stars -circumstellar matter -open clusters and associations: individual (IC 348) 1 Smithsonian Astrophysical Observatory.

We report the results of a sensitive near-infrared JHKL imaging survey of the Trapezium cluster in Orion. We use the JHKL colors to obtain a census of infrared excess stars in the cluster. Of (391) stars brighter than 12th magnitude in the K and L bands, 80%^7% are found to exhibit detectable infrared excess on the J[H, K[L color-color diagram. Examination of a subsample of 285 of these stars with published spectral types yields a slightly higher infrared excess fraction of 85%. We Ðnd that 97% of the optical proplyds in the cluster exhibit excess in the JHKL color-color diagram indicating that the most likely origin of the observed infrared excesses is from circumstellar disks. We interpret these results to indicate that the fraction of stars in the cluster with circumstellar disks is between 80%È85%, conÐrming earlier published suggestions of a high disk fraction for this young cluster. Moreover, we Ðnd that the probability of Ðnding an infrared excess around a star is independent of stellar mass over essentially the entire range of the stellar mass function down to the hydrogen burning limit. Consequently, the vast majority of stars in the Trapezium cluster appear to have been born with circumstellar disks and the potential to subsequently form planetary systems, despite formation within the environment of a rich and dense stellar cluster. We identify 78 stars in our sample characterized by K[L colors suggestive of deeply embedded objects. The spatial distribution of these objects di †ers from that of the rest of the cluster members and is similar to that of the dense molecular cloud ridge behind the cluster. About half of these objects are detected in the short wavelength (J and H) bands, and these are found to be characterized by extreme infrared excess. This suggests that many of these sources could be protostellar in nature. If even a modest fraction (i.e., D50%) of these objects are protostars, then star formation could be continuing in the molecular ridge at a rate comparable to that which produced the foreground Trapezium cluster.

We present UBVRI photometry of 44 type-Ia supernovae (SN Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SN Ia to date, nearly doubling the number of well-observed, nearby SN Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SN Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U −B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ∼40% intrinsic scatter compared to B-band.Subject headings: supernovae: general -techniques: photometric Data and Reduction DiscoveryOur program of supernova photometry consists solely of follow-up; we search only our email, not the sky, to find new supernovae. A number of observers, both amateur and professional, are engaged in searching for supernovae. We rely on these searches, as well as prompt notification of candidates, coordinated by Dan Green and Brian Marsden of the IAU's Central Bureau for Astronomical Telegrams (CBAT), with confirmed SN reported in the IAU Circulars. In some cases the SN discoverers provide spectroscopic classification of the new objects, but generally spectroscopy is obtained by others, and reported separately in the IAU Circulars. With our spectroscopic SN follow-up program at the F. L. Whipple Observatory 1.5m telescope and FAST spectrograph (Fabricant et al. 1998), we have classified a large fraction of the new, nearby supernovae reported over the last several years and compiled a large spectroscopic database (Matheson et al. 2005, in preparation).Given a newly discovered and classified supernova, several factors help determine whether or not we include it in our monitoring program. Because of their importance, SN Ia are often given higher priority over other types, but factors such as ease of observability (southern targets and those discovered far to the west are less appealing), supernova phase (objects whose spectra indicate they are after maximum light are given lower priority), redshift (more nearby objects are favored), as well as the number of objects we are already monitoring are significant. Our final sample of well-observed SN Ia is not obtained from a single well-defined set of criteria, and selection effects in both the searches and follow-up may make this sample unsuitable for some applications (such as determining the intrinsic luminosity function of SN Ia, for example). A thorough discussion of the selection biases in the Calán/Tololo supernova search and follow-up campaign can be found in Hamuy & Pinto (1999).The discovery data for the sample of SN Ia presented here are given in Table 1. All of the ...