The Rise and Fall of Debris Disks: MIPS Observations of h and chi Persei and the Evolution of Mid-IR Emission from Planet Formation

We describe Spitzer/MIPS observations of the double cluster, h and $\chi$ Persei, covering a $\sim$ 0.6 square-degree area surrounding the cores of both clusters. The data are combined with IRAC and 2MASS data to investigate $\sim$ 616 sources from 1.25-24 $\mu m$. We use the long-baseline $K_{s}$-[24] color to identify two populations with IR excess indicative of circumstellar material: Be stars with 24 $\mu m$ excess from optically-thin free free emission and 17 fainter sources (J$\sim$ 14-15) with [24] excess consistent with a circumstellar disk. The frequency of IR excess for the fainter sources increases from 4.5 $\mu m$ through 24 $\mu m$. The IR excess is likely due to debris from the planet formation process. The wavelength-dependent behavior is consistent with an inside-out clearing of circumstellar disks. A comparison of the 24 $\mu m$ excess population in h and $\chi$ Per sources with results for other clusters shows that 24 $\mu m$ emission from debris disks 'rises' from 5 to 10 Myr, peaks at $\sim$ 10-15 Myr, and then 'falls' from $\sim$ 15/20 Myr to 1 Gyr.Comment: 48 pages, 15 figures, accepted for publication in The Astrophysical Journa

Comparison of the Hα circumstellar disks in Be/X-ray binaries and Be stars

We present a comparative study of the circumstellar disks in Be/X-ray binaries and isolated Be stars based upon the Hα emission line. From this comparison it follows that the overall structure of the disks in the Be/X-ray binaries is similar to the disks of other Be stars, i.e. they are axisymmetric and rotationally supported. The factors for the line broadening (rotation and temperature) in the disks of the Be stars and the Be/X-ray binaries seem to be identical. However, we do detect some intriguing differences between the envelopes. On average, the circumstellar disks of the Be/X-ray binaries are twice as dense as the disks of the isolated Be stars. The different distribution of the Be/X-ray binaries and the Be stars seen in the full with half maximum versus peak separation diagram indicates that the disks in Be/X-ray binaries have on average a smaller size, probably truncated by the compact object.Reig Torres, Pablo, [email protected] ; Fabregat Llueca, Juan, [email protected]

Observational Constraints on Interstellar Grain Alignment

We present new multicolor photo-polarimetry of stars behind the Southern Coalsack. Analyzed together with multiband polarization data from the literature, probing the Chamaeleon I, Musca, rho Opiuchus, R CrA and Taurus clouds, we show that the wavelength of maximum polarization (lambda_max) is linearly correlated with the radiation environment of the grains. Using Far-Infrared emission data, we show that the large scatter seen in previous studies of lambda_max as a function of A_V is primarily due to line of sight effects causing some A_V measurements to not be a good tracer of the extinction (radiation field strength) seen by the grains being probed. The derived slopes in lambda_max vs. A_V, for the individual clouds, are consistent with a common value, while the zero intercepts scale with the average values of the ratios of total-to-selective extinction (R_V) for the individual clouds. Within each cloud we do not find direct correlations between lambda_max and R_V. The positive slope in consistent with recent developments in theory and indicating alignment driven by the radiation field. The present data cannot conclusively differentiate between direct radiative torques and alignment driven by H_2 formation. However, the small values of lambda_max(A_V=0), seen in several clouds, suggest a role for the latter, at least at the cloud surfaces. The scatter in the lambda_max vs. A_V relation is found to be associated with the characteristics of the embedded Young Stellar Objects (YSO) in the clouds. We propose that this is partially due to locally increased plasma damping of the grain rotation caused by X-rays from the YSOs.Comment: Accepted for publication in the Astrophysical Journa

A GLIMPSE into the Nature of Galactic Mid-IR Excesses

We investigate the nature of the mid-IR excess for 31 intermediate-mass stars that exhibit an 8 micron excess in either the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire or the Mid-Course Space Experiment using high resolution optical spectra to identify stars surrounded by warm circumstellar dust. From these data we determine projected stellar rotational velocities and estimate stellar effective temperatures for the sample. We estimate stellar ages from these temperatures, parallactic distances, and evolutionary models. Using MIPS [24] measurements and stellar parameters we determine the nature of the infrared excess for 19 GLIMPSE stars. We find that 15 stars exhibit Halpha emission and four exhibit Halpha absorption. Assuming that the mid-IR excesses arise in circumstellar disks, we use the Halpha fluxes to model and estimate the relative contributions of dust and free-free emission. Six stars exhibit Halpha fluxes that imply free-free emission can plausibly explain the infrared excess at [24]. These stars are candidate classical Be stars. Nine stars exhibit Halpha emission, but their Halpha fluxes are insufficient to explain the infrared excesses at [24], suggesting the presence of a circumstellar dust component. After the removal of the free-free component in these sources, we determine probable disk dust temperatures of Tdisk~300-800 K and fractional infrared luminosities of L(IR)/L(*)~10^-3. These nine stars may be pre-main-sequence stars with transitional disks undergoing disk clearing. Three of the four sources showing Halpha absorption exhibit circumstellar disk temperatures ~300-400 K, L(IR)/L(*)~10^-3, IR colors K-[24]< 3.3, and are warm debris disk candidates. One of the four Halpha absorption sources has K-[24]> 3.3 implying an optically thick outer disk and is a transition disk candidate.Comment: 17 figures. Accepted for publication in Ap

Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions

Open clusters offer us the means to study stellar properties in samples with well-defined ages and initial chemical composition. Here we present a survey of projected rotational velocities for a large sample of mainly B-type stars in young clusters to study the time evolution of the rotational properties of massive stars. The survey is based upon moderate resolution spectra made with the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs, and the target stars are members of 19 young open clusters with an age range of approximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387, 4471 and Mg II 4481 using model theoretical profiles to find projected rotational velocities for a total of 496 OB stars. We find that there are fewer slow rotators among the cluster B-type stars relative to nearby B stars in the field. We present evidence consistent with the idea that the more massive B stars (M > 9 solar masses) spin down during their main sequence phase. However, we also find that the rotational velocity distribution appears to show an increase in the numbers of rapid rotators among clusters with ages of 10 Myr and higher. These rapid rotators appear to be distributed between the zero age and terminal age main sequence locations in the Hertzsprung-Russell diagram, and thus only a minority of them can be explained as the result of a spin up at the terminal age main sequence due to core contraction. We suggest instead that some of these rapid rotators may have been spun up through mass transfer in close binary systems.Comment: 33 pages, 11 figures, accepted by Ap

Identification of particles with Lorentz factor up to 10410^{4} with Transition Radiation Detectors based on micro-strip silicon detectors

This work is dedicated to the study of a technique for hadron identification in the TeV momentum range, based on the simultaneous measurement of the energies and of the emission angles of the Transition Radiation (TR) X-rays with respect to the radiating particles. A detector setup has been built and tested with particles in a wide range of Lorentz factors (from about $10^3$ to about $4 \times 10^4$ crossing different types of radiators. The measured double-differential (in energy and angle) spectra of the TR photons are in a reasonably good agreement with TR simulation predictions.Comment: 31 pages, 12 figures, paper published on Nuclear Instruments & Methods

Quantitative PCR Is Faster, More Objective, and More Reliable Than Immunohistochemistry for the Diagnosis of Cytomegalovirus Gastrointestinal Disease in Allogeneic Stem Cell Transplantation

Diagnosis of gastrointestinal (GI) cytomegalovirus (CMV) disease relies on the presence of GI symptoms and detection of CMV, mainly by immunohistochemistry (IHC), in GI biopsy specimens. Thus, in a symptomatic patient, a positive CMV-IHC result is accepted as a diagnosis of CMV disease. However, a positive CMV-PCR in GI tissue is considered "possible" CMV disease. Therefore, it would be very useful if, in practice, both techniques showed equal sensitivity and reliability. This is because PCR has many practical advantages over IHC for detecting CMV. The aim of this study was to compare quantitative PCR with IHC for the diagnosis of GI CMV disease. A total of 186 endoscopic GI biopsy specimens from 123 patients with GI symptoms after an allogeneic stem cell transplantation (allo-SCT; 2004-2017) were analyzed by IHC and PCR on 113 paraffin-embedded and 73 fresh samples. The results were then compared. Of the patients with macroscopic lesions in the mucosa and CMV-IHC-positive biopsy specimens (eg, "proven" CMV disease, n = 28), all but 1 were CMV-PCR positive. Of the patients without macroscopic lesions in the mucosa and CMV-IHC-positive biopsy specimens (eg, probable CMV disease, n = 4), only 1 was CMV-PCR positive. Eight patients had CMV-IHC-negative/CMV-PCR-positive gut biopsy specimens. These cases fall within the current definition of possible CMV disease. In 6 of these 8 cases (75%), the viral load in GI tissue was very high (>10,000 copies/µg). Taken together, the results from the proven and probable cases revealed that CMV-PCR shows the same sensitivity (100%), specificity (98%), and positive (93%) and negative predictive value (100%) as CMV-IHC. Detection of CMV in fresh GI mucosa by quantitative PCR is as useful as IHC for the diagnosis of GI CMV disease. The results show that quantitative PCR has the same sensitivity, specificity, and positive/negative predictive value as IHC

Photometry and membership for low mass stars in the young open cluster NGC 2516

We present the results of a 0.86 square degree CCD photometric survey of the open cluster NGC 2516, which has an age of about 150 Myr and may have a much lower metallicity than the similarly-aged Pleiades. We select a preliminary catalogue of 1254 low mass (between 0.2 and 2.0M_{sun}) cluster candidates, of which about 70--80 percent are expected to be genuine. The mass function is metallicity dependent, but consistent with a Salpeter-like law (dN/dlog M ~ M^{-alpha}, alpha=+1.47+/-0.11 or alpha=+1.67+/-0.11 for solar and half-solar metallicities) between 0.7 and 3.0M_{sun}. At lower masses (between 0.3 and 0.7M_{sun}) there is a sharp fall in the mass function, with alpha=-0.75+/-0.20 (solar metallicity) or alpha=-0.49+/-0.13 (half-solar metallicity), which seems inconsistent with the much flatter mass functions seen in the Pleiades and field populations. We explain this by demonstrating that mass segregation has been at work in NGC 2516 -- more than half the cluster low mass stars are expected to lie outside out survey. The mass of NGC 2516 stars with mass greater than 0.3M_{sun} inside our survey is 950-1200M_{sun}, depending on metallicity and what corrections are applied for unresolved binarity. Correcting for mass segregation increases this to ~1240-1560M_{sun}, about twice the total mass of the Pleiades.Comment: 27 pages, accepted for Astronomy & Astrophysic