SPITZER/IRAC-MIPS Survey of NGC2451A and B: Debris Disks at 50-80 million years

We present a Spitzer IRAC and MIPS survey of NGC 2451 A and B, two open clusters in the 50-80 Myr age range. We complement these data with extensive ground-based photometry and spectroscopy to identify the cluster members in the Spitzer survey field. We find only two members with 8 micron excesses. The incidence of excesses at 24 microns is much higher, 11 of 31 solar-like stars and 1 of 7 early-type (A) stars. This work nearly completes the debris disk surveys with Spitzer of clusters in the 30-130 Myr range. This range is of inte rest because it is when large planetesimal collisions may have still been relatively common (as indicated by the one that led to the formation of the Moon during this period of the evolution of the Solar System). We review the full set of surveys and find that there are only three possible cases out of about 250 roughly solar-mass stars where very large excesses suggest that such collisions have occurred recently.Comment: Accepted for publication in ApJ. 25 pages 17 figure

On the CO Near-Infrared Band and the Line-splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae

We report on multiepoch optical and near-infrared spectroscopy around the first-overtone rovibrational band of CO in the pulsating yellow hypergiant ρ Cas, one of the most massive stars in the Galaxy and a candidate SN II progenitor. We argue that the double cores of the CO absorption lines, which have previously been attributed to separate circumstellar shells expelled during its recurrent outbursts, result in fact from a superposition of a wide absorption line and a narrow central emission line. The CO line doubling returns over subsequent pulsation cycles, where the superposed line emission assumes its largest intensity near phases of maximum light. We find that the morphology and behavior of the CO band closely resemble the remarkable "line-splitting phenomenon" also observed in optical low-excitation atomic lines. Based on radiative transport calculations, we present a simplified model of the near-infrared CO emission emerging from cooler atmospheric layers in the immediate vicinity of the photosphere. We speculate that the kinetic temperature minimum in our model results from a periodic pulsation-driven shock wave. We further discuss a number of alternative explanations for the origin of the ubiquitous emission-line spectrum, possibly due to a quasi-chromosphere or a steady shock wave at the interface of a fast expanding wind and the interstellar medium. We present a number of interesting spectroscopic similarities between ρ Cas and other types of cool variable supergiants, such as the RV Tau and R CrB stars. We further propose a possibly common mechanism for the enigmatic outburst behavior of these luminous pulsating cool stars

Spitzer 24 micron Survey of Debris Disks in the Pleiades

We performed a 24 micron 2 Deg X 1 Deg survey of the Pleiades cluster, using the MIPS instrument on Spitzer. Fifty four members ranging in spectral type from B8 to K6 show 24 micron fluxes consistent with bare photospheres. All Be stars show excesses attributed to free-free emission in their gaseous envelopes. Five early-type stars and four solar-type stars show excesses indicative of debris disks. We find a debris disk fraction of 25 % for B-A members and 10 % for F-K3 ones. These fractions appear intermediate between those for younger clusters and for the older field stars. They indicate a decay with age of the frequency of the dust-production events inside the planetary zone, with similar time scales for solar-mass stars as have been found previously for A-stars.Comment: accepted to Ap

Spitzer 24 μm Survey of Debris Disks in the Pleiades

We performed a 24 μm 2° × 1° survey of the Pleiades cluster, using the MIPS instrument on Spitzer. Fifty-four members ranging in spectral type from B8 to K6 show 24 μm fluxes consistent with bare photospheres. All Be stars show excesses attributed to free-free emission in their gaseous envelopes. Five early-type stars and four solar-type stars show excesses indicative of debris disks. We find a debris disk fraction of 25% for B-A members and 10% for F-K3 ones. These fractions appear intermediate between those for younger clusters and for the older field stars. They indicate a decay with age of the frequency of the dust production events inside the planetary zone, with similar timescales for solar-mass stars as have been found previously for A stars

Near and Mid-IR Photometry of the Pleiades, and a New List of Substellar Candidate Members

We make use of new near and mid-IR photometry of the Pleiades cluster in order to help identify proposed cluster members. We also use the new photometry with previously published photometry to define the single-star main sequence locus at the age of the Pleiades in a variety of color-magnitude planes. The new near and mid-IR photometry extend effectively two magnitudes deeper than the 2MASS All-Sky Point Source catalog, and hence allow us to select a new set of candidate very low mass and sub-stellar mass members of the Pleiades in the central square degree of the cluster. We identify 42 new candidate members fainter than Ks =14 (corresponding to 0.1 Mo). These candidate members should eventually allow a better estimate of the cluster mass function to be made down to of order 0.04 solar masses. We also use new IRAC data, in particular the images obtained at 8 um, in order to comment briefly on interstellar dust in and near the Pleiades. We confirm, as expected, that -- with one exception -- a sample of low mass stars recently identified as having 24 um excesses due to debris disks do not have significant excesses at IRAC wavelengths. However, evidence is also presented that several of the Pleiades high mass stars are found to be impacting with local condensations of the molecular cloud that is passing through the Pleiades at the current epoch.Comment: Accepted to ApJS; data tables and embedded-figure version available at http://spider.ipac.caltech.edu/staff/stauffer/pleiades07

New debris disk candidates: 24 micron stellar excesses at 100 Myr

Sixty three members of the 100 Myr old open cluster M47 (NGC 2422) have been detected at 24 micron with Spitzer. The Be star V 378 Pup shows an excess both in the near-infrared and at 24 micron (K-[24] = 2.4 mag), probably due to free-free emission from the gaseous envelope. Seven other early-type stars show smaller excesses, K-[24] = 0.6-0.9. Among late-type stars, two show large excesses: P922 - a K1V star with K-[24] = 1.08 pm 0.11 and P1121 - an F9V star with K-[24] = 3.72 pm 0.02. P1121 is the first known main-sequence star showing an excess comparable to that of beta Pic, which may indicate the presence of an exceptionally massive debris disk. It is possible that a major planetesimal collision has occurred in this system, consistent with the few hundred Myr time scales estimated for the clearing of the Solar System.Comment: 14 pages, 3 figures, accepted to the special Spitzer issue of the ApJ

On the CO Near-IR Band and the Line Splitting Phenomenon in the Yellow Hypergiant Rho Cassiopeiae

We report on multi-epoch optical and near-infrared spectroscopy around the first overtone ro-vibrational band of CO in the pulsating yellow hypergiant Rho Cas, one of the most massive stars in the Galaxy and a candidate SN II progenitor. We argue that the double cores of the CO absorption lines, that have previously been attributed to separate circumstellar shells expelled during its recurrent outbursts, result in fact from a superposition of a wide absorption line and a narrow central emission line. The CO line doubling returns over subsequent pulsation cycles, where the superposed line emission assumes its largest intensity near phases of maximum light. We find that the morphology and behavior of the CO band closely resemble the remarkable "line-splitting phenomenon" also observed in optical low-excitation atomic lines. Based on radiative transport calculations we present a simplified model of the near-IR CO emission emerging from cooler atmospheric layers in the immediate vicinity of the photosphere. We speculate that the kinetic temperature minimum in our model results from a periodical pulsation-driven shock wave. We further discuss a number of alternative explanations for the origin of the ubiquitous emission line spectrum, possibly due to a quasi-chromosphere or a steady shock wave at the interface of a fast expanding wind and the ISM. We present a number of interesting spectroscopic similarities between Rho Cas and other types of cool variable supergiants such as the RV Tau and R CrB stars. We further propose a possibly common mechanism for the enigmatic outburst behavior of these luminous pulsating cool stars.Comment: accepted to ApJ; 3 color fig