No abstract

We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the young (∼ 3 Myr) σ Orionis cluster. The populous nature of this cluster makes it a good target for statistically-significant studies of disk emission as a function of mass. We identify 336 stars as members of the cluster using optical and near-infrared color magnitude diagrams. Using the spectral energy distribution (SED) slopes in the IRAC spectral range, we place objects in several classes: non-excess stars, stars with optically thick disks (like classical T Tauri stars), class I (protostellar) candidates, and stars with "evolved disks"; the last exhibit smaller IRAC excesses than optically thick disk systems. In general, this classification agrees with the location expected in IRAC-MIPS color-color diagrams for these objects. We find that the evolved disk systems are mostly a combination of objects with optically thick but non-flared disks, suggesting grain growth and/or settling, and transition disks, systems in which the inner disk is partially or fully cleared of small dust. In all, we identify 7 transition disk candidates and 3 possible debris disk systems. There appears to be a spatial extension of infrared excess sources to the north-east, which may be associated with the young (< 1 Myr) embedded cluster NGC 2024. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ∼10%

No abstract

We present Hubble Space Telescope ultraviolet spectra and supporting ground-based data for a sample of nine intermediate-mass T Tauri stars (IMTTSs; 1.5-4 M ). The targets belong to three star-forming regions: T Tau, SU Aur, and RY Tau in the Taurus clouds; EZ Ori, P2441, and V1044 Ori in the Ori OB1c association surrounding the Orion Nebula cluster; and CO Ori, GW Ori, and GX Ori in the ring around k Ori. The supporting groundbased observations include nearly simultaneous UBV(R I ) C photometry, 6 8 resolution spectra covering the range 3900-7000 8, optical echelle observations in the range 5800-8600 8, and K-band near-infrared spectra. We use these data to determine improved spectral types and reddening corrections and to obtain physical parameters of the targets. We find that an extinction law with a weak 2175 8 feature but high values of A UV =A V is required to explain the simultaneous optical-UV data; the reddening laws for two B-type stars located behind the Taurus clouds, HD 29647 and HD 283809, meet these properties. We argue that reddening laws with these characteristics may well be representative of cold, dense molecular clouds. Spectral energy distributions and emission-line profiles of the IMTTSs are consistent with expectations from magnetospheric accretion models. We compare our simultaneous optical-UV data with predictions from accretion shock models to get accretion luminosities and mass accretion rates (Ṁ ) for the targets. We find that the average mass accretion rate for IMTTSs is $3 ; 10 À8 M yr À1 , a factor of $5 higher than that for their low-mass counterparts. The new data extend the correlation betweenṀ and stellar mass to the intermediate-mass range. Since the IMTTSs are evolutionary descendants of the Herbig Ae/ Be stars, our results put limits to the mass accretion rates of their disks. We present luminosities of the UV lines of highly ionized metals and show that they are well above the saturation limit for magnetically active cool stars but correlate strongly with accretion luminosity, indicating that they are powered by accretion, in agreement with previous claims but using a sample in which reddening and accretion luminosities have been determined self-consistently. Finally, we find that the relation between accretion luminosity and Br luminosity found for low-mass T Tauri stars extends to the intermediate-mass regime.

We present a study of accretion in a sample of 45 young, low-mass objects in a variety of star-forming regions and young associations, about half of which are likely substellar. Based primarily on the presence of broad, asymmetric H emission, we have identified 13 objects ($30% of our sample) that are strong candidates for ongoing accretion. At least three of these are substellar. We do not detect significant continuum veiling in most of the accretors with late spectral types (M5-M7). Accretion shock models show that lack of measurable veiling allows us to place an upper limit to the mass accretion rates of d10 À10 M yr À1 . Using magnetospheric accretion models with appropriate (sub)stellar parameters, we can successfully explain the accretor H emission-line profiles and derive quantitative estimates of accretion rates in the range 10 À12 M yr À1 < _ M M < 10 À9 M yr À1 . There is a clear trend of decreasing accretion rate with stellar mass, with mean accretion rates declining by 3-4 orders of magnitude over $1-0.05 M .

We present multiband photometry of 185 type-Ia supernovae (SNe Ia), with over 11,500 observations. These were acquired between 2001 and 2008 at the F. L. Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics (CfA). This sample contains the largest number of homogeneously observed and reduced nearby SNe Ia (z 0.08) published to date. It more than doubles the nearby sample, bringing SN Ia cosmology to the point where systematic uncertainties dominate. Our natural system photometry has a precision of 0.02 mag in BV RI r i and 0.04 mag in U for points brighter than 17.5 mag. We also estimate a systematic uncertainty of 0.03 mag in our SN Ia standard system BV RI r i photometry and 0.07 mag for U. Comparisons of our standard system photometry with published SN Ia light curves and comparison stars, where available for the same SN, reveal agreement at the level of a few hundredths mag in most cases. We find that 1991bg-like SNe Ia are sufficiently distinct from other SNe Ia in their color and light-curve-shape/ luminosity relation that they should be treated separately in light-curve/distance fitter training samples. The CfA3 sample will contribute to the development of better light-curve/distance fitters, particularly in the few dozen cases where near-infrared photometry has been obtained and, together, can help disentangle host-galaxy reddening from intrinsic supernova color, reducing the systematic uncertainty in SN Ia distances due to dust.

We present observations of H emission-line profiles taken at Magellan Observatory for a sample of 39 young low-mass stars and brown dwarfs in the Taurus and Chamaeleon I star-forming regions. We have identified 11 new substellar accretors, more than tripling the number of known brown dwarfs with measurable accretion activity. These include the lowest-mass objects yet seen with accretion, with masses down to $0.015 M . Using models of H emission produced in magnetospheric accretion flows, the most widely applicable primary calibrator now available, we determine the first estimates of mass accretion rates for objects at such extremely low masses. For the six objects with masses P0.03 M , we find accretion rates of $5 ; 10 À12 M yr À1 , among the smallest yet measured. These new results continue the trend of decreasing mass accretion rate with decreasing (sub)stellar mass that we have noted previously for samples of more massive objects; the overall correlation isṀ / M 2:1 , and it now extends over a mass range of over 2 orders of magnitude. Finally, the absence of a discontinuity in the distribution of accretion rates with mass tends to suggest that stars and brown dwarfs share similar formation histories.

We present results of a large-scale, multiepoch optical survey of the Orion OB1 association, carried out with the QUEST camera at the Venezuela National Astronomical Observatory. We identify for the first time the widely spread low-mass, young population in the Ori OB1a and OB1b subassociations. Candidate members were picked up by their variability in the V band and position in color-magnitude diagrams. We obtained spectra to confirm membership. In a region spanning $68 deg 2 , we found 197 new young stars; of these, 56 are located in the Ori OB1a subassociation and 141 in Ori OB1b. The spatial distribution of the low-mass young stars is spatially coincident with that of the highmass members but suggests a much sharper edge to the association. Comparison with the spatial extent of molecular gas and extinction maps indicates that the subassociation Ori OB1b is concentrated within a ringlike structure of radius $2 ($15 pc at 440 pc), centered roughly on the star Ori in the Orion belt. The ring is apparent in 13 CO and corresponds to a region with an extinction A V ! 1. The stars exhibiting strong H emission, an indicator of active accretion, are found along this ring, whereas the center is populated with weak H-emitting stars. In contrast, Ori OB1a is located in a region devoid of gas and dust. We identify a grouping of stars within a $3 deg 2 area located in Ori OB1a, roughly clustered around the B2 star 25 Ori. The Herbig Ae/Be star V346 Ori is also associated with this grouping, which could be an older analog of Ori. Using several sets of evolutionary tracks, we find an age of 7-10 Myr for Ori OB1a and of $4-6 Myr for Ori OB1b, consistent with previous estimates from OB stars. Indicators such as the equivalent width of H and near-IR excesses show that the number of accreting low-mass stars decreases sharply between Ori OB1b and Ori OB1a. These results indicate that although a substantial fraction of accreting disks remain at ages $5 Myr, inner disks are essentially dissipated by 10 Myr.