J. Alves

Planck 2013 results

In this volume, we proudly present a special feature on science results from the data that ESA Planck mission gathered over its first 15 months and which ESA and the Planck Collaboration released in March 2013. This collection of 31 articles presents the initial scientific results extracted from this first Planck dataset, which measures the cosmic microwave background (CMB) with the highest accuracy to date. It provides major new advances in different domains of cosmology and astrophysics. We thank Jan Tauber and the Planck Science Team for coordinating this special feature.

ON THE STAR FORMATION RATES IN MOLECULAR CLOUDS

In this paper, we investigate the level of star formation activity within nearby molecular clouds. We employ a uniform set of infrared extinction maps to provide accurate assessments of cloud mass and structure and compare these with inventories of young stellar objects within the clouds. We present evidence indicating that both the yield and rate of star formation can vary considerably in local clouds, independent of their mass and size. We find that the surface density structure of such clouds appears to be important in controlling both these factors. In particular, we find that the star formation rate (SFR) in molecular clouds is linearly proportional to the cloud mass (M 0.8) above an extinction threshold of A K ≈ 0.8 mag, corresponding to a gas surface density threshold of Σgas ≈ 116 M ☉ pc2. We argue that this surface density threshold corresponds to a gas volume density threshold which we estimate to be n(H2) ≈ 104 cm–3. Specifically, we find SFR (M ☉ yr–1) = 4.6 ± 2.6 × 10–8 M 0.8 (M ☉) for the clouds in our sample. This relation between the rate of star formation and the amount of dense gas in molecular clouds appears to be in excellent agreement with previous observations of both galactic and extragalactic star-forming activity. It is likely the underlying physical relationship or empirical law that most directly connects star formation activity with interstellar gas over many spatial scales within and between individual galaxies. These results suggest that the key to obtaining a predictive understanding of the SFRs in molecular clouds and galaxies is to understand those physical factors which give rise to the dense components of these clouds.

The mass function of dense molecular cores and the origin of the IMF

Context. Stars form in the cold dense cores of interstellar molecular clouds and the detailed knowledge of the spectrum of masses of such cores is clearly a key for the understanding of the origin of the IMF. To date, observations have presented somewhat contradictory evidence relating to this issue. Aims. In this paper we propose to derive the mass function of a complete sample of dense molecular cores in a single cloud employing a robust method that uses uses extinction of background starlight to measure core masses and enables the reliable extension of such measurements to lower masses than previously possible. Methods. We use a map of near-infrared extinction in the nearby Pipe dark cloud to identify the population of dense cores in the cloud and measure their masses. Results. We identify 159 dense cores and construct the mass function for this population. We present the first robust evidence for a departure from a single power-law form in the mass function of a population of cores and find that this mass function is surprisingly similar in shape to the stellar IMF but scaled to a higher mass by a factor of about 3. This suggests that the distribution of stellar birth masses (IMF) is the direct product of the dense core mass function and a uniform star formation efficiency of 30%±10%, and that the stellar IMF may already be fixed during or before the earliest stages of core evolution. These results are consistent with previous dust continuum studies which suggested that the IMF directly originates from the core mass function. The typical density of ∼10 4 cm −3 measured for the dense cores in this cloud suggests that the mass scale that characterizes the dense core mass function may be the result of a simple process of thermal (Jeans) fragmentation.

The Luminosity and Mass Function of the Trapezium Cluster: From B Stars to the Deuterium-burning Limit

We use the results of a new multiepoch, multiwavelength, near-infrared census of the Trapezium cluster in Orion to construct and analyze the structure of its infrared (K-band) luminosity function. Specifically, we employ an improved set of model luminosity functions to derive this clusters underlying initial mass function (IMF) across the entire range of mass from OB stars to substellar 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 substellar regime. Independent of the details, we find that substellar objects account for no more than ~22% of the total number of likely cluster members. Further, the substellar Trapezium IMF breaks from a steady power-law decline and forms a significant secondary peak at the lowest masses (10-20 times the mass of Jupiter). This secondary peak may contain as many as ~30% of the substellar objects in the cluster. Below this substellar IMF peak, our K-band luminosity function (KLF) modeling requires a subsequent sharp decline toward the planetary mass regime. Lastly, we investigate the robustness of pre-main-sequence luminosity evolution as predicted by current evolutionary models, and we discuss possible origins for the IMF of brown dwarfs.

N2h+ and c18o depletion in a cold dark cloud

We present sensitive, high angular resolution molecular line observations of C18O and N2H+ toward the dark globule B68. We directly compare these data with the near-infrared extinction measurements of Alves, Lada, & Lada to derive the first evidence for the depletion of N2H+, and by inference N2, in a prestellar dark cloud. We also find widespread C18O depletion throughout the centrally condensed core of the B68 cloud. Specifically, we find the N2H+ emission to peak in a shell partially surrounding the peak of dust extinction. Moreover, N2H+ peaks inside the much larger C18O depletion hole and has a smaller depletion zone, confirming theoretical predictions. These data are analyzed through a direct coupling of time-dependent chemical models to a radiation transfer code. This analysis highlights the importance of photodissociation at cloud edges and suggests that the CO abundance declines by 2 orders of magnitude from edge to center. In contrast, N2H+ declines in abundance, at minimum, by at least a factor of 2. Indeed, it is entirely possible that both N2H+ and N2 are completely absent from the central regions of the B68 core. The depletion of N2H+, and its parent molecule N2, opens up the possibility that the centers of dense cores, prior to the formation of a star, may evade detection by conventional methods of probing cores using molecular emission. Under these conditions, H2D+ may be the sole viable molecular probe of the innermost regions of star-forming cores.

Mapping the interstellar dust with near-infrared observations: An optimized multi-band technique

We generalize Lada et al. (1994) Nice technique to map dust column density through a molecular cloud to an optimized multi-band technique that can be applied to any multi-band survey of molecular clouds. We present a first application to a ~ 625 deg2 subset of the Two Micron All Sky Survey (2MASS) data and show that when compared to Nice, the optimized Nicer (i) achieves the same extinction peak values, (ii) improves the noise variance of the map by a factor of 2 and (iii) is able to reach 3σ dust extinction measurements as low as A V ≃ 0.5 magnitudes, better than or equivalent to classical optical star count techniques and below the threshold column density for the formation of CO.

Ubvri light curves of 44 type ia supernovae

We present UBVRI photometry of 44 Type Ia supernovae (SNe 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 SNe Ia to date, nearly doubling the number of well-observed, nearby SNe Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SNe 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 the B band.

THE NATURE OF THE DENSE CORE POPULATION IN THE PIPE NEBULA : THERMAL CORES UNDER PRESSURE

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(H2) = 7 ? 103. 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 (0.2-20 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.

Infrared L-Band Observations of the Trapezium Cluster: A Census of Circumstellar Disks and Candidate Protostars

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 find 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%–85%, confirming earlier published suggestions of a high disk fraction for this young cluster. Moreover, we find that the probability of finding 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 differs 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., ~50%) 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.

CALIFA, the Calar Alto Legacy Integral Field Area survey

We present the Calar Alto Legacy Integral Field Area survey (CALIFA). CALIFAs main aim is to obtain spatially resolved spectroscopic information for ~600 galaxies of all Hubble types in the Local Universe (0.005< z <0.03). The survey has been designed to allow three key measurements to be made: (a) Two-dimensional maps of stellar populations (star formation histories, chemical elements); (b) The distribution of the excitation mechanism and element abundances of the ionized gas; and (c) Kinematic properties (velocity ?elds, velocity dispersion), both from emission and from absorption lines. To cover the full optical extension of the target galaxies (i.e. out to a 3sigma depth of ~23 mag/arcsec2), CALIFA uses the exceptionally large ?eld of view of the PPAK/PMAS IFU at the 3.5m telescope of the Calar Alto observatory. We use two grating setups, one covering the wavelength range between 3700 and 5000 AA at a spectral resolution R~1650, and the other covering 4300 to 7000 AA at R~850. The survey was allocated 210 dark nights, distributed in 6 semesters and starting in July 2010 and is carried out by the CALIFA collaboration, comprising ~70 astronomers from 8 di?erent countries. As a legacy survey, the fully reduced data will be made publically available, once their quality has been veri?ed. We showcase here early results obtained from the data taken so far (21 galaxies).