Bruno Merín

Infrared Array Camera (IRAC) Colors of Young Stellar Objects

We compare the infrared colors predicted by theoretical models of protostellar envelopes and protoplanetary disks with initial observations of young stellar objects made with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. Disk and envelope models characterized by infall and/or accretion rates found in previous studies can quantitatively account for the range of IRAC colors found in four young embedded clusters: S140, S171, NGC 7129, and Cep C. The IRAC color-color diagram ([3.6]� [4.5] vs. [5.8]� [8.0]) can be used to help distinguish between young stars with only disk emission and protostars with circumstellar envelopes. Subject heading gs: infrared: stars — stars: formation — stars: pre–main-sequence

Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope

We present the results of an infrared imaging survey of two clusters in the Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPS instruments on board the Spitzer Space Telescope. Our observations cover the wavelengthrangefrom3.6to24 � m,allowingustodetectdiskemissionoveratypicalrangeofradii � 0.1to � 20AU from the central star. In Tr 37, with an age of about 4 Myr, about 48% of the low-mass stars exhibit detectable disk emission in the IRAC bands. Roughly 10% of the stars with disks may be ‘‘transition’’ objects, with essentially photospheric fluxes at wavelengths � 4.5 � m but with excesses at longer wavelengths, indicating an optically thin inner disk. The median optically thick disk emission in Tr 37 is lower than the corresponding median for stars in the youngerTaurusregion;thedecreaseininfraredexcessislargerat6–8 � mthanat24 � m,suggestingthatgraingrowth and/or dust settling has proceeded faster at smaller disk radii, as expected on general theoretical grounds. Only about 4% of the low-mass stars in the 10 Myr old cluster NGC 7160 show detectable infrared disk emission. We also find evidence for 24 � m excesses around a few intermediate-mass stars, which may represent so-called ‘‘debris disk’’ systems. Our observations provide new constraints on disk evolution through an important age range. Subject headingg accretion, accretion disks — planetary systems: protoplanetary disks — stars: pre–main-sequence

The spatial distribution of star formation in the solar neighbourhood: do all stars form in dense clusters?

We present a global study of low mass, young stellar object (YSO) surface densities (�) in nearby (< 500 pc) star forming regions based on a comprehensive collection of Spitzer Space Telescope surveys. We show that the distribution of YSO surface densities in the solar neighbourhood is a smooth distribution, being adequately described by a lognormal function from a few to 10 3 YSOs per pc 2 , with a peak at � 22 stars pc

THE SPITZER c2d SURVEY OF LARGE, NEARBY, INTERSTELLAR CLOUDS. IX. THE SERPENS YSO POPULATION AS OBSERVED WITH IRAC AND MIPS

We discuss the combined IRAC/MIPS c2d Spitzer Legacy observations of the Serpens star-forming region. We describe criteria for isolating bona fide YSOs from the extensive background of extragalactic objects. We then discuss the properties of the resulting high-confidence set of 235 YSOs. An additional 51 lower confidence YSOs outside this area are identified from the MIPS data and 2MASS photometry. We present color-color diagrams to compare our observed source properties with those of theoretical models for star/disk/envelope systems and our own modeling of the objects that are well represented by a stellar photosphere plus circumstellar disk. These objects exhibit a wide range of disk properties, from many with actively accreting disks to some with both passive disks and even possibly debris disks. The YSO luminosity function extends down to at least a few times 10^(-3) L_☉ or lower. The lower limit may be set more by our inability to distinguish YSOs from extragalactic sources than by the lack of YSOs at very low luminosities. We find no evidence for variability in the shorter IRAC bands between the two epochs of our data set, Δt ~ 6 hr. A spatial clustering analysis shows that the nominally less evolved YSOs are more highly clustered than the later stages. The background extragalactic population can be fitted by the same two-point correlation function as seen in other extragalactic studies. We present a table of matches between several previous infrared and X-ray studies of the Serpens YSO population and our Spitzer data set. The clusters in Serpens have a very high surface density of YSOs, primarily with SEDs suggesting extreme youth. The total number of YSOs, mostly Class II, is greater outside the clusters.

EXPORT: Spectral classification and projected rotational velocities of Vega-type and pre-main sequence stars

In this paper we present the rst comprehensive results extracted from the spectroscopic campaigns car- ried out by the EXPORT (EXoPlanetary Observational Research Team) consortium. During 1998{1999, EXPORT carried out an intensive observational eort in the framework of the origin and evolution of protoplanetary sys- tems in order to obtain clues on the evolutionary path from the early stages of the pre-main sequence to stars with planets already formed. The spectral types of 70 stars, and the projected rotational velocities, v sini ,o f 45 stars, mainly Vega-type and pre-main sequence, have been determined from intermediate- and high-resolution spectroscopy, respectively. The rst part of the work is of fundamental importance in order to accurately place the stars in the HR diagram and determine the evolutionary sequences; the second part provides information on the kinematics and dynamics of the stars and the evolution of their angular momentum. The advantage of using the same observational conguration and methodology for all the stars is the homogeneity of the set of pa- rameters obtained. Results from previous work are revised, leading in some cases to completely new determinations of spectral types and projected rotational velocities; for some stars no previous studies were available.

The Spitzer c2d Survey of Large, Nearby, Insterstellar Clouds. II. Serpens Observed with IRAC

We present maps of 0.89?deg2 of the Serpens dark cloud at 3.6, 4.5, 5.8, and 8.0??m observed with the Spitzer Space Telescope Infrared Array Camera (IRAC). We discuss in detail the data processing carried out by the c2d team on IRAC data. More than 100,000 compact sources have been extracted, but we confine most of our discussion to the most reliable subset of these sources. This includes those that are detected above 7?? in all four IRAC bands or those detected in the two shorter IRAC bands together with 2MASS. We estimate completeness limits for our survey from Monte Carlo tests with artificial sources inserted into the Spitzer maps. We compare source counts, colors, and magnitudes in the Serpens cloud to two reference data sets, a 0.10 deg2 set of low-extinction regions near the dark cloud and a 1 deg2 subset of the SWIRE Elais N1 data that was processed through our pipeline. We find that it is possible to identify more than 200 young stellar object (YSO) candidates from color-magnitude and color-color diagrams, most of which were previously unknown. In addition to the dense area of new star formation known before in the core region (cluster A), we also find a moderately rich area to the south (cluster B). Our mapped area also includes the Herbig Ae star VV Ser, whose Spitzer images have been carefully modeled in a separate study. The extreme sensitivity of Spitzer IRAC allows us to search to very low luminosity limits for young substellar objects. The comparison of the Serpens region with the reference areas suggests that a population of infrared excess sources exists in Serpens at least down to luminosities of L ~ 10-3 L? and possibly lower.

A Spitzer c2d legacy survey to identify and characterize disks with inner dust holes

Understanding how disks dissipate is essential to studies of planet formation. However, identifying exactly how dust and gas dissipate is complicated due to the difficulty of finding objects that are clearly in the transition phase of losing their surrounding material. We use Spitzer Infrared Spectrograph (IRS) spectra to examine 35 photometrically selected candidate cold disks (disks with large inner dust holes). The infrared spectra are supplemented with optical spectra to determine stellar and accretion properties and 1.3 mm photometry to measure disk masses. Based on detailed spectral energy distribution modeling, we identify 15 new cold disks. The remaining 20 objects have IRS spectra that are consistent with disks without holes, disks that are observed close to edge-on, or stars with background emission. Based on these results, we determine reliable criteria to identify disks with inner holes from Spitzer photometry, and examine criteria already in the literature. Applying these criteria to the c2d surveyed star-forming regions gives a frequency of such objects of at least 4% and most likely of order 12% of the young stellar object population identified by Spitzer. We also examine the properties of these new cold disks in combination with cold disks from the literature. Hole sizes in this sample are generally smaller than in previously discovered disks and reflect a distribution in better agreement with exoplanet orbit radii. We find correlations between hole size and both disk and stellar masses. Silicate features, including crystalline features, are present in the overwhelming majority of the sample, although the 10 μm feature strength above the continuum declines for holes with radii larger than ~7 AU. In contrast, polycyclic aromatic hydrocarbons are only detected in 2 out of 15 sources. Only a quarter of the cold disk sample shows no signs of accretion, making it unlikely that photoevaporation is the dominant hole-forming process in most cases.

C2D Spitzer-IRS spectra of disks around T Tauri stars - IV. Crystalline silicates

Aims. Dust grains in the planet-forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation, and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks for a statistically significant number of TTauri stars (96). Methods. As part of the cores to disks (c2d) legacy program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars, over a spectral range of 5-35 μm where many silicate amorphous and crystalline solid-state features are present. At these wavelengths, observations probe the upper layers of accretion disks up to distances of a dozen AU from the central object. Results. More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. The Fe-rich crystalline silicates are largely absent in the c2d IRS spectra. The strength and detection frequency of the crystalline features seen at λ > 20 μm correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 μm feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (≤1 AU) emitting at ~ 10 μm and a much colder region emitting at λ > 20 μm (≤10 AU). We identify a crystallinity paradox, as the long-wavelength (λ > 20 m) crystalline silicate features are detected 3.5 times more frequently (~55% vs. ~15%) than the crystalline features arising from much warmer disk regions (λ ~ 10 μm). This suggests that the disk has an inhomogeneous dust composition within ~10 AU. The analysis of the shape and strength of both the amorphous 10 μm feature and the crystalline feature around 23 μm provides evidence for the prevalence of μm-sized (amorphous and crystalline) grains in upper layers of disks. Conclusions. The abundant crystalline silicates found far from their presumed formation regions suggest efficient outward radial transport mechanisms in the disks around TTauri stars. The presence of μm-sized grains in disk atmospheres, despite the short timescales for settling to the midplane, suggests efficient (turbulent) vertical diffusion, probably accompanied by grain-grain fragmentation to balance the expected efficient growth. In this scenario, the depletion of submicron-sized grains in the upper layers of the disks points toward removal mechanisms such as stellar winds or radiation pressure.

C2D Spitzer-IRS spectra of disks around T Tauri stars. II. PAH emission features

Aims. We search for Polycyclic Aromatic Hydrocarbon (PAH) features towards young low-mass (T Tauri) stars and compare them with surveys of intermediate mass (Herbig Ae/Be) stars. The presence and strength of the PAH features are interpreted with disk radiative transfer models exploring the PAH feature dependence on the incident UV radiation, PAH abundance and disk parameters. Methods. Spitzer Space Telescope 5-35 mu m spectra of 54 pre-main sequence stars with disks were obtained, consisting of 38 T Tauri, 7 Herbig Ae/Be and 9 stars with unknown spectral type. Results. Compact PAH emission is detected towards at least 8 sources of which 5 are Herbig Ae/Be stars. The 11.2 mu m PAH feature is detected in all of these sources, as is the 6.2 mu m PAH feature for the 4 sources for which short wavelength data are available. However, the 7.7 and 8.6 mu m features appear strongly in only 1 of these 4 sources. Based on the 11.2 mu m feature, PAH emission is observed towards at least 3 T Tauri stars, with 14 tentative detections, resulting in a lower limit to the PAH detection rate of 8%. The lowest mass source with PAH emission in our sample is T Cha with a spectral type G8. All 4 sources in our sample with evidence for dust holes in their inner disk show PAH emission, increasing the feature/continuum ratio. Typical 11.2 mu m line intensities are an order of magnitude lower than those observed for the more massive Herbig Ae/Be stars. Measured line fluxes indicate PAH abundances that are factors of 10-100 lower than standard interstellar values. Conversely, PAH features from disks exposed to stars with T(eff)

The Herschel ? first look at protostars in the Aquila Rift ??

As part of the science demonstration phase of the Herschel mission of the Gould Belt Key Program, the Aquila Rift molecular complex has been observed. The complete ~ 3.3deg x 3.3deg imaging with SPIRE 250/350/500 micron and PACS 70/160 micron allows a deep investigation of embedded protostellar phases, probing of the dust emission from warm inner regions at 70 and 160 micron to the bulk of the cold envelopes between 250 and 500 micron. We used a systematic detection technique operating simultaneously on all Herschel bands to build a sample of protostars. Spectral energy distributions are derived to measure luminosities and envelope masses, and to place the protostars in an M_env - L_bol evolutionary diagram. The spatial distribution of protostars indicates three star-forming sites in Aquila, with W40/Sh2-64 HII region by far the richest. Most of the detected protostars are newly discovered. For a reduced area around the Serpens South cluster, we could compare the Herschel census of protostars with Spitzer results. The Herschel protostars are younger than in Spitzer with 7 Class 0 YSOs newly revealed by Herschel. For the entire Aquila field, we find a total of ~ 45-60 Class 0 YSOs discovered by Herschel. This confirms the global statistics of several hundred Class~0 YSOs that should be found in the whole Gould Belt survey.