Xavier Paul Koenig

Clustered and Triggered Star Formation in W5: Observations with Spitzer

We present images and initial results from our extensive Spitzer Space Telescope imaging survey of the W5 H II region with the Infrared Array Camera (IRAC) and Multiband Imaging Photometer for Spitzer (MIPS). We detect dense clusters of stars, centered on the O stars HD 18326, BD +60 586, HD 17505, and HD 17520. At 24 μm, substantial extended emission is visible, presumably from heated dust grains that survive in the strongly ionizing environment of the H II region. With photometry of more than 18,000 point sources, we analyze the clustering properties of objects classified as young stars by their IR spectral energy distributions (a total of 2064 sources) across the region using a minimal-spanning-tree algorithm. We find ~40%-70% of infrared excess sources belong to clusters with ≥10 members. We find that within the evacuated cavities of the H II regions that make up W5, the ratio of Class II to Class I sources is ~7 times higher than for objects coincident with molecular gas as traced by -->12CO emission and near-IR extinction maps. We attribute this contrast to an age difference between the two locations and postulate that at least two distinct generations of star formation are visible across W5. Our preliminary analysis shows that triggering is a plausible mechanism to explain the multiple generations of star formation in W5 and merits further investigation.

X-Ray Processing of ChaMPlane Fields: Methods and Initial Results for Selected Anti-Galactic Center Fields

We describe the X-ray analysis procedure of the ongoing Chandra Multiwavelength Plane (ChaMPlane) Survey and report the initial results from the analysis of 15 selected anti-Galactic center observations (90° < l < 270°). We describe the X-ray analysis procedures for ChaMPlane using custom-developed analysis tools appropriate for Galactic sources but also of general use: optimum photometry in crowded fields using advanced techniques for overlapping sources, rigorous astrometry and 95% error circles for combining X-ray images or matching to optical/IR images, and application of quantile analysis for spectral analysis of faint sources. We apply these techniques to 15 anti-Galactic center observations (of 14 distinct fields), in which we have detected 921 X-ray point sources. We present log N-log S distributions and quantile analysis to show that in the hard band (2-8 keV) active galactic nuclei dominate the sources. Complete analysis of all ChaMPlane anti-Galactic center fields will be given in a subsequent paper, followed by papers on sources in the Galactic center and bulge regions.

Chandra Multiwavelength Plane (ChaMPlane) Survey: An Introduction

We introduce the Chandra Multiwavelength Plane (ChaMPlane) survey, designed to measure or constrain the populations of low-luminosity (LX 1031 ergs s-1) accreting white dwarfs, neutron stars, and stellar mass black holes in the Galactic plane and bulge. ChaMPlane incorporates two surveys, X-ray (Chandra) and optical (NOAO 4 m Mosaic imaging), and a follow-up spectroscopy and IR identification program. The survey has now extended through the first 6 yr of Chandra data using serendipitous sources detected in 105 distinct ACIS-I and ACIS-S fields observed in 154 pointings and covered by 65 deep Mosaic images in V, R, I, and Hα. ChaMPlane incorporates fields with Galactic latitude 12° and selected to be devoid of bright point or diffuse sources, with exposure time 20 ks and (where possible) minimum NH. We describe the scientific goals and introduce the X-ray and optical/IR processing and databases. We derive preliminary constraints on the space density or luminosity function of cataclysmic variables (CVs) from the X-ray/optical data for 14 fields in the Galactic anticenter. The lack of ChaMPlane CVs in these anticenter fields suggests that their space density is ~3 times below the value (3 × 10-5 pc-3) found for the solar neighborhood by previous X-ray surveys. Companion papers describe the X-ray and optical processing in detail, optical spectroscopy of ChaMPlane sources in selected anticenter fields, and IR imaging results for the Galactic center field. An appendix introduces the ChaMPlane Virtual Observatory (VO) for online access to the X-ray and optical images and source catalogs for ready display and further analysis.

A SPITZER VIEW OF STAR FORMATION IN THE CYGNUS X NORTH COMPLEX

Original article can be found at: http://iopscience.iop.org/0004-637X Copyright American Astronomical Society. [Full text of this article is not available in the UHRA]

Constraining the Nature of the Galactic Center X-Ray Source Population

We searched for infrared counterparts to the cluster of X-ray point sources discovered by Chandra in the Galactic center region (GCR). While the sources could be white dwarfs, neutron stars, or black holes accreting from stellar companions, their X-ray properties are consistent with magnetic cataclysmic variables (CVs), or high-mass X-ray binaries (HMXBs) at low accretion rates. A direct way to decide between these possibilities, and hence between alternative formation scenarios, is to measure or constrain the luminosity distribution of the companions. Using infrared (J, H, K, Brγ) imaging, we searched for counterparts corresponding to typical HMXB secondaries: spectral type B0 V with K < 15 at the GCR. We found no significant excess of bright stars in Chandra error circles, indicating that HMXBs are not the dominant X-ray source population and that they account for fewer than 10% of the hardest X-ray sources.

ChaMPlane Optical Survey: Mosaic Photometry

The Chandra Multiwavelength Plane (ChaMPlane) survey to identify and analyze the serendipitous X-ray sources in deep Galactic plane fields incorporates the ChaMPlane Optical Survey, which is one of NOAOs Long-term Survey Programs. We started this optical imaging survey in 2000 March and completed it in 2005 June. Using the NOAO 4 m telescopes with the Mosaic cameras at CTIO and KPNO, deep images of the ChaMPlane fields are obtained in V, R, I, and Hα bands. This paper describes the process of observation, data reduction, and analysis of fields included in the ChaMPlane Optical Survey and describes the search for Hα emission objects and Chandra optical counterparts. We illustrate these procedures using the ChaMPlane field for the black hole X-ray binary GRO J0422+32 as an example.

DISK EVOLUTION IN W5: INTERMEDIATE-MASS STARS AT 2-5 Myr

We present the results of a survey of young intermediate-mass stars (age < 5 Myr, 1.5 M ☉ <M ≤ 15 M ☉) in the W5 massive star-forming region. We use combined optical, near-infrared, and Spitzer Space Telescope photometry and optical spectroscopy to define a sample of stars of spectral types A and B and examine their infrared excess properties. We find objects with infrared excesses characteristic of optically thick disks, i.e., Herbig AeBe stars. These stars are rare: <1.5% of the entire spectroscopic sample of A and B stars, and absent among stars more massive than 2.4 M ☉. 7.5% of the A and B stars possess infrared excesses in a variety of morphologies that suggest their disks are in some transitional phase between an initial, optically thick accretion state and later evolutionary states. We identify four morphological classes based on the wavelength dependence of the observed excess emission above theoretical photospheric levels: (1) the optically thick disks; (2) disks with an optically thin excess over the wavelength range 2-24 μm, similar to that shown by Classical Be stars; (3) disks that are optically thin in their inner regions based on their infrared excess at 2-8 μm and optically thick in their outer regions based on the magnitude of the observed excess emission at 24 μm; (4) disks that exhibit empty inner regions (no excess emission at λ ≤ 8 μm) and some measurable excess emission at 24 μm. A sub-class of disks exhibit no significant excess emission at λ≤ 5.8 μm, have excess emission only in the Spitzer 8 μm band and no detection at 24 μm. We discuss these spectral energy distribution types, and suggest physical models for disks exhibiting these emission patterns and additional observations to test these theories.

Investigating ChaMPlane X-Ray Sources in the Galactic Bulge with Magellan LDSS2 Spectra

We have carried out optical and X-ray spectral analyses on a sample of 136 candidate optical counterparts of X-ray sources found in five Galactic bulge fields included in our Chandra Multiwavelength Plane Survey. We use a combination of optical spectral fitting and quantile X-ray analysis to obtain the hydrogen column density toward each object, and a three-dimensional dust model of the Galaxy to estimate the most probable distance in each case. We present the discovery of a population of stellar coronal emission sources, likely consisting of pre-main-sequence, young main-sequence, and main-sequence stars, as well as a component of active binaries of RS CVn or BY Dra type. We identify one candidate quiescent low-mass X-ray binary with a subgiant companion; we note that this object may also be an RS CVn system. We report the discovery of three new X-ray-detected cataclysmic variables (CVs) in the direction of the Galactic center (at distances 2 kpc). This number is in excess of predictions made with a simple CV model based on a local CV space density of 10 -->−5 pc -->−3, and a scale height ~200 pc. We discuss several possible reasons for this observed excess.

ChaMPlane Discovery of Candidate Symbiotic Binaries in Baade's and Stanek's Windows

We have searched the OGLE-II archive for candidate counterparts of X-ray sources detected in two low-extinction windows included in our Galactic bulge Chandra-HST survey. We find that a significant number—that is, in excess of the expected level of random associations—can be matched with probable M giants. Their X-ray properties can be understood if these sources are symbiotic binaries in which the X-rays are typically, either directly or indirectly, the result of a white dwarf accreting from the wind of a cool giant. Optical and near-infrared properties of selected sources are consistent with a symbiotic nature, although none of the spectra collected for eight out of 13 candidate counterparts show the high-ionization nebular emission lines observed for many symbiotics. The hard X-ray emission for several sources (power-law photon indices -1.5 Γ 1.5) suggests that our sample includes systems similar to the symbiotics recently detected with INTEGRAL and Swift.

Dusty Cometary Globules in W5

We report the discovery of four dusty cometary tails around low-mass stars in two young clusters belonging to the W5 star-forming region. Fits to the observed emission profiles from 24 μm observations with the Spitzer Space Telescope give tail lifetimes < 30 Myr, but more likely 5 Myr. This result suggests that the cometary phase is a short lived phenomenon, occurring after photoevaporation by a nearby O star has removed gas from the outer disk of a young low-mass star.