J. Stauffer

A Spitzer View of Protoplanetary Disks in the γ Velorum Cluster

We present new Spitzer Space Telescope observations of stars in the young (~5 Myr)γ Velorum stellar cluster. Combining optical and 2MASS photometry, we have selected 579 stars as candidate members of the cluster. With the addition of the Spitzer mid-infrared data, we have identified five debris disks around A-type stars and five to six debris disks around solar-type stars, indicating that the strong radiation field in the cluster does not completely suppress the production of planetesimals in the disks of cluster members. However, we find some evidence that the frequency of circumstellar primordial disks is lower, and the infrared flux excesses are smaller than for disks around stellar populations with similar ages. This could be evidence for a relatively fast dissipation of circumstellar dust by the strong radiation field from the highest mass star(s) in the cluster. Another possibility is that γ Velorum stellar cluster is slightly older than reported ages and the low frequency of primordial disks reflects the fast disk dissipation observed at ~5 Myr.

Spitzer observations of the λ orionis cluster. I. the frequency of young debris disks at 5 Myr

We present IRAC/MIPS Spitzer observations of intermediate-mass stars in the 5 Myr old λ Orionis cluster. In a representative sample of stars earlier than F5 (29 stars), we find a population of nine stars with varying degree of moderate 24 μm excess comparable to those produced by debris disks in older stellar groups. As expected in debris disks systems, those stars do not exhibit emission lines in their optical spectra. We also include in our study the star HD 245185, a known Herbig Ae object which displays excesses in all Spitzer bands and shows emission lines in its spectrum. We compare the disk population in the λ Orionis cluster with the disk census in other stellar groups studied using similar methods to detect and characterize their disks and spanning a range of ages from 3 Myr to 10 Myr. We find that for stellar groups of 5 Myr or older the observed disk frequency in intermediate-mass stars (with spectral types from late B to early F) is higher than in low-mass stars (with spectral types K and M). This is in contradiction with the observed trend for primordial disk evolution, in which stars with higher stellar masses dissipate their primordial disks faster. At 3 Myr, the observed disk frequency in intermediate-mass stars is still lower than for low-mass stars indicating that second generation dusty disks start to dominate the disk population at 5 Myr for intermediate-mass stars. This result agrees with recent models of evolution of solids in the region of the disk where icy objects form (>30 AU), which suggest that at 5-10 Myr collisions start to produce large amount of dust during the transition from runaway to oligarchic growth (reaching sizes of ~500 km) and then dust production peaks at 10-30 Myr, when objects reach their maximum size (≥1000 km).

SPITZER OBSERVATIONS OF THE λ ORIONIS CLUSTER. II. DISKS AROUND SOLAR-TYPE AND LOW-MASS STARS

We present IRAC/MIPS Spitzer Space Telescope observations of the solar-type and the low-mass stellar population of the young (~5Myr) λ Orionis cluster. Combining optical and Two Micron All Sky Survey photometry, we identify 436 stars as probable members of the cluster. Given the distance (450 pc) and the age of the cluster, our sample ranges in mass from 2 M_⊙ to objects below the substellar limit. With the addition of the Spitzer mid-infrared data, we have identified 49 stars bearing disks in the stellar cluster. Using spectral energy distribution slopes, we place objects in several classes: non-excess stars (diskless), stars with optically thick disks, stars with “evolved disks” (with smaller excesses than optically thick disk systems), and “transitional disk” candidates (in which the inner disk is partially or fully cleared). The disk fraction depends on the stellar mass, ranging from ~6% for K-type stars (R_C − J 4). We confirm the dependence of disk fraction on stellar mass in this age range found in other studies. Regarding clustering levels, the overall fraction of disks in the λ Orionis cluster is similar to those reported in other stellar groups with ages normally quoted as ~5Myr.

ERRATUM: “AN X-RAYS SURVEY OF THE YOUNG STELLAR POPULATION OF THE LYNDS 1641 AND IOTA ORIONIS REGIONS” (2013, ApJ, 768, 99)

I. Pillitteri1, S. J. Wolk1, S. T. Megeath2, L. Allen3, J. Bally4, Marc Gagne5, R. A. Gutermuth6, L. Hartman7, G. Micela8, P. Myers1, J. M. Oliveira9, S. Sciortino8, F. Walter1, L. Rebull10, and J. Stauffer10 1 SAO–Harvard Center for Astrophysics, 60 Garden St, Cambridge MA 02138, USA; ipillitteri@cfa.harvard.edu 2 Department of Physics & Astronomy, University of Toledo, OH, USA 3 National Optical Astronomy Observatory, USA 4 University of Colorado, Boulder, CO, USA 5 Department of Geology & Astronomy, West Chester University, West Chester, PA, USA 6 Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA 7 University of Michigan, Ann Arbor, MI, USA 8 INAF Osservatorio Astronomico di Palermo, Italy 9 School of Physical & Geographical Sciences, Lennard-Jones Laboratories, Keele University, Staffordshire ST5 5BG, UK 10 CALTECH, Pasadena, CA, 91125, USA Received 2013 June 27; published 2013 July 29

Mapping the Star Formation in Orion A/L1641

First phases of the process of star formation are characterized by excess in infrared and high X-ray emission. With Spitzer and XMM-Newton we have surveyed the Orion A part relative to the filamentary cloud Lynds 1641 (L1641). Furthermore, an extended spectroscopic survey has been realized to better constraint the cluster membership of stars without IR excess. We find that:

A multi-wavelength view of magnetic flaring from PMS stars

Context. Flaring is an ubiquitous manifestation of magnetic activity in low mass stars including, of course, the Sun. Although flares, both from the Sun and from other stars, are most prominently observed in the soft X-ray band, most of the radiated energy is released at optical/UV wavelengths. In spite of decades of investigation, the physics of flares, even solar ones, is not fully understood. Even less is known about magnetic flaring in pre-main sequence (PMS) stars, at least in part because of the lack of suitable multi-wavelength data. This is unfortunate since the energetic radiation from stellar flares, which is routinely observed to be orders of magnitude greater than in solar flares, might have a significant impact on the evolution of circumstellar, planet-forming disks. Aims. We aim at improving our understanding of flares from PMS stars. Our immediate objectives are constraining the relation between flare emission at X-ray, optical, and mid-infrared (mIR) bands, inferring properties of the optically emitting region, and looking for signatures of the interaction between flares and the circumstellar environment, i.e. disks and envelopes. This information might then serve as input for detailed models of the interaction between stellar atmospheres, circumstellar disks and proto-planets. Methods. Observations of a large sample of PMS stars in the NGC 2264 star forming region were obtained in December 2011, simultaneously with three space-borne telescopes, Chandra (X-rays), CoRoT (optical), and Spitzer (mIR), as part of the “Coordinated Synoptic Investigation of NGC 2264” (CSI-NGC 2264). Shorter Chandra and CoRoT observations were also obtained in March 2008. We analyzed the lightcurves obtained during the Chandra observations (∼300 ks and ∼60 ks in 2011 and 2008, respectively), to detect X-ray flares with an optical and/or mIR counterpart. From the three datasets we then estimated basic flare properties, such as emitted energies and peak luminosities. These were then compared to constrain the spectral energy distribution of the flaring emission and the physical conditions of the emitting regions. The properties of flares from stars with and without circumstellar disks were also compared to establish any difference that might be attributed to the presence of disks. Results. Seventy-eight X-ray flares (from 65 stars) with an optical and/or mIR counterpart were detected. The optical emission of flares (both emitted energy and peak flux) is found to correlate well with, and to be significantly larger than, the X-ray emission. The slopes of the correlations suggest that the difference becomes smaller for the most powerful flares. The mIR flare emission seems to be strongly affected by the presence of a circumstellar disk: flares from stars with disks have a stronger mIR emission with respect to stars without disks. This might be attributed to either a cooler temperature of the region emitting both the optical and mIR flux or, perhaps more likely, to the reprocessing of the optical (and X-ray) flare emission by the inner circumstellar disk, providing evidence for flare-induced disk heating.

Spitzer/IRAC Young Stellar Objects Candidates in 30 Doradus

We have analyzed several sets of images of the 30 Doradus (a massive Star Forming Region located in the Large Magellanic Cloud) obtained with the IRAC and MIPS cameras of the Spitzer Space Telescope. Our goal is to identify the youngest stars and even protostars in this region, either by their very red colors or by their photometric variability. In order to do this, we have cross-correlated the Spitzer photometry with data obtained at other wavelengths. We identify a set of ∼ 30 new candidate protostars, provide new data for some of the previous candidates, and attempt to place the new candidates in context. We have used these data to empirically place constraints on the physical processes that cause the photometric variability of high mass protostars.

X‐ray emission in the Outer Galaxy: the Star Forming Region NGC 1893

A key issue of the star formation process is its independence from the environmental conditions. In particular, it is not clear whether star formation in the outer Galaxy occurs in the same way as in the inner Galaxy. We present preliminary results of the analysis of a ∼440 Ks ACIS‐Chandra observation of NGC1893, a young cluster (∼3–4 Myrs), far away from the Galactic Center with the aim to study star formation in the outer region of the Galaxy and investigate the coronal properties of the cluster stars. We detect more than 1000 X‐ray sources, most of which are likely cluster members. We present also a preliminary analysis of X‐ray variability of the cluster stars.