arXiv subjects

T. M. Dame

Publications and source records attributed to T. M. Dame.

49 records · Page 3Linked to original sources

arXiv2006

The X-ray Halo of GX5-1

Using Chandra observations we have measured the energy-resolved dust-scattered X-ray halo around the low-mass X-ray binary GX5-1, which shows signs of both singly- and multiply-scattered X-rays. We compared the observed X-ray halo at various energies to predictions from a range of dust models. These fits used both smoothly-distributed dust as well as dust in clumped clouds, with CO and 21 cm observations helping to determine the position of the clouds along the line of sight. We found that the BARE-GR-B model of Zubko, Dwek & Arendt (2004 generally led to the best results, although inadequacies in both the overall model and the data limit our conclusions. We also found that the composite dust models of Zubko, Dwek & Arendt (2004), especially the ``no carbon'' models, gave uniformly poor results. Although models using cloud positions and densities derived naively from CO and 21 cm data gave generally poor results, plausible adjustments to the distance of the largest cloud and the mass of a cloud in the expanding 3 kpc Arm lead to significantly improved fits. We suggest that combining X-ray halo, CO, and 21 cm observations will be a fruitful method to improve our understanding of both the gas and dust phases of the interstellar medium.

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arXiv2005

CH 3 GHz Observations of Molecular Clouds Along the Galactic Plane

Spectra in the CH $^2Π_{1/2}$, J=1/2, F=1-1 transition at 3335 MHz were obtained in three 5-point crosses centered on the Galactic plane at $\ell =$ 50$\arcdeg$, 100$\arcdeg$, and 110$\arcdeg$. The lines of sight traverse both Giant Molecular Clouds (GMCs) and local, smaller entities. This transition is a good tracer of low-density molecular gas and the line profiles are very similar to CO(1-0) data at nearly the same resolution. In addition, the CH 3335 MHz line can be used to calibrate the CO-H$_2$ conversion factor (X$_{\rm CO}$) in low-density molecular gas. Although this technique underestimates X$_{\rm CO}$ in GMCs, our results are within a factor of two of X$_{\rm CO}$ values calibrated for GMCs by other techniques. The similarity of CH and CO line profiles, and that of X$_{\rm CO}$ values derived from CH and more traditional techniques, implies that most of the molecular gas along the observed lines of sight is at relatively low densities ($n \le$ 10$^3$ cm$^{-3}$).

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arXiv2005

CH 3 GHz Observations of the Galactic Center

A 3 $\times$ 3 map of the Galactic Center was made at 9$\arcmin$ resolution and 10$\arcmin$ spacing in the CH $^2Π_{1/2}$, J=1/2, F=1-1 transition at 3335 MHz. The CH emission shows a velocity extent that is nearly that of the CO(1-0) line, but the CH line profiles differ markedly from the CO. The 3335 MHz CH transition primarily traces low-density molecular gas and our observations indicate that the mass of this component within $\sim$ 30 pc of the Galactic Center is $\sim$ 9 $\times$ 10$^6$ M$_\odot$. The CO-H$_2$ conversion factor obtained for the low-density gas in the mapped region is greater than that thought to apply to the dense molecular gas at the Galactic Center. In addition to tracing the low-density molecular gas at the Galactic Center, the CH spectra show evidence of emission from molecular clouds along the line of sight both in the foreground and background. The scale height of these clouds ranges from 27 - 109 pc, consistent with previous work based on observations of molecular clouds in the inner Galaxy.

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arXiv2005

High-Latitude Molecular Clouds as gamma-ray Sources for GLAST

For about two decades, a population of relative small and nearby molecular clouds has been known to exist at high Galactic latitudes. Lying more than 10$^\circ$ from the Galactic plane, these clouds have typical distances of $\sim$150 pc, angular sizes of $\sim1^\circ$, and masses of order tens of solar masses. These objects are passive sources of high-energy $γ$-rays through cosmic ray-gas interactions. Using a new wide-angle CO survey of the northern sky, we show that typical high-latitude clouds are not bright enough in $γ$-rays to have been detected by EGRET, but that of order 100 of them will be detectable by the Large Area Telescope (LAT) on GLAST. Thus, we predict a new steady population of $γ$-ray sources at high Galactic latitudes, perhaps the most numerous after active galactic nuclei.

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arXiv2004

A Uniform CO Survey of the Molecular Clouds in Orion and Monoceros

We report the results of a new large scale survey of the Orion-Monoceros complex of molecular clouds made in the J = 1->0 line of CO with the Harvard-Smithsonian 1.2m millimetre-wave telescope. The survey consists of 52,288 uniformly spaced spectra that cover an area of 432 square degrees on the sky and is the most sensitive large-scale survey of the region to date. Distances to the constituent molecular clouds of the complex, estimated from an analysis of foreground and background stars, have provided information on the three dimensional structure of the entire complex.

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arXiv2004

Chandra detection of ejecta in the small-diameter supernova remnant G349.7+0.2

We present high-resolution X-ray observations of the small-diameter supernova remnant (SNR) G349.7+0.2 with the Chandra X-ray Observatory. The overall SNR spectrum can be described by two spectral components. The soft component is in ionization equilibrium and has a temperature of ~0.8 keV; the hard spectral component has a temperature of ~1.4 keV, an ionization timescale of ~5x10^{11} cm{-3}s and enhanced abundances of Si. The spatially resolved spectral modeling shows that S may also be enhanced, at least in some regions. The enhanced abundances clearly point to the presence of an ejecta component in this remnant. The X-ray morphology of G349.7+0.2 is strikingly similar to that at radio wavelengths - an irregular shell with a brighter eastern side - which is consistent with expansion in a medium with a large scale density gradient. The remnant is known to be interacting with a molecular cloud (from the presence of OH(1720 MHz) masers), but this interaction is probably limited to the central portion of the SNR, as seen in SNR IC 443. We found that HI clouds are present in the SNR region, which supports the notion that G349.7+0.2 belongs to a class of remnants evolving in the intercloud medium (such are IC 443 and W44, Chevalier 1999), which is also responsible for the remnant's morphology. G349.7+0.2 does not have the mixed-morphology found for other maser-emitting SNRs studied to date in X-rays, but its morphology can be explained by a projection model for mixed-morphology SNRs. We have identified a point source close to the center of the SNR with a luminosity of Lx(0.5-10.0 keV) ~ (0.2-2.3)x10^{34} d_{22} erg/s, which is consistent with that of the compact central objects found in a few other Galactic SNRs.

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arXiv2003

A Large Extension of the CfA Galactic CO Survey

The Galactic CO survey of Dame, Hartmann, & Thaddeus (2001; hereafter DHT) is composed of both large-scale unbiased surveys, mainly concentrated within 10-deg of the Galactic plane, and targeted observations of clouds at higher latitudes. Analysis of all-sky IRAS and 21 cm maps suggests that the DHT survey is nearly complete for clouds larger than ~1 deg, even though roughly half of the total area at |b| < 30 deg was not observed. In October 2001 we began a new survey of all of this unobserved area that is accessible to the northern 1.2 meter telescope, approximately 6,600 deg^2 between l = 0 deg and 230 deg, mainly in the latitude range |b| = 10-30 deg. At least 12 hours per day is being dedicated to this large project, which is sampled every 1/4-deg (every other beamwidth) to an rms sensitivity of 0.19 K at a velocity resolution of 0.65 km/s. As of May 2003, we have obtained 90,000 of the 106,000 spectra required to complete the survey. While, as expected, the new observations do not substantially change the DHT map, ~68 relatively small and isolated molecular clouds at intermediate latitudes have so far been discovered. The survey has also been extended to b < -30 deg in two regions, one in the vicinity of the large MBM clouds 53-55 at l ~ 90 deg and the other south of the Taurus clouds at l ~170 deg. Substantial amounts of molecular gas were detected in both of these high-latitude regions.

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arXiv2001

The Impact of Massive Stars on the Formation of Young Stellar Clusters

Massive OB stars play an important role in the evolution of molecular clouds and star forming regions. The OB stars both photo--ionize molecular gas as well as sweep up and compress interstellar gas through winds, ionization fronts, and supernovae. In this contribution, we examine the effect massive stars have on the formation of young stellar clusters. We first discuss the processes by which OB stars destroy cluster--forming molecular cores, and hence terminate star formation. We overview observational evidence that OB stars forming in young stellar clusters destroy their parental cores on a timescale of 0.1 Myr, and we discuss some of the implications of this result. We then summarize extensive observations of the NGC 281 molecular cloud complex, and present evidence that two types of triggered star formation are occurring in this complex. Our goal is to underscore the impact massive stars have on cluster formation over distances ranging from 0.1 pc to 300 pc.

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arXiv2000

The Milky Way in Molecular Clouds: A New Complete CO Survey

New large-scale CO surveys of the first and second Galactic quadrants and the nearby molecular cloud complexes in Orion and Taurus, obtained with the CfA 1.2 m telescope, have been combined with 31 other surveys obtained over the past two decades with that instrument and a similar telescope on Cerro Tololo in Chile, to produce a new composite CO survey of the entire Milky Way. The survey consists of 488,000 spectra that Nyquist or beamwidth (1/8-deg) sample the entire Galactic plane over a strip 4-10 deg wide in latitude, and beamwidth or 1/4-deg sample nearly all large local clouds at higher latitudes. A map of molecular column density predicted from complete and unbiased far-infrared and 21 cm surveys of the Galaxy was used both to determine the completeness of the present survey and to extrapolate it to the entire sky at |b| < 32 deg. The close agreement of the observed and predicted maps implies that only ~2% of the total CO emission at |b| < 32 deg lies outside our current sampling. Taking into account this small amount of unobserved emission, the mean molecular column density decreases from ~3 E20 cm-2 at |b| = 5 deg to ~0.1 E20 cm-2 at |b| = 30 deg. The ratio of the predicted molecular column density map to the observed CO intensity map provides a calibration of the CO-to-H2 mass conversion factor X = N(H2)/Wco. Out of the Galactic plane (|b| > 5 deg), X shows little systematic variation with latitude from a mean value of 1.8 +/- 0.3 E20 cm-2 K-1 km-1 s. Given the large sky area and large quantity of CO data analyzed, we conclude that this is the most reliable measurement to date of the mean X value in the solar neighborhood.

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arXiv2000

Comparing Giant Molecular Clouds in M31, M33 & the Milky Way

We present new observations of a 2' field in the north-eastern spiral arm of M31. In the 0.8 x 3.6 kpc mosaicked region, we have detected six distinct, large complexes of molecular gas, most of which lie along the spiral arm dust lane or in the vicinity of HII regions. The mean properties of these complexes are as follows: diameter ~ 57+/-13 pc, fwhm velocity ~ 6.5+/-1.2 km/s, M(CO) ~ 3.0+/-1.6 x 10^5 solar masses, peak brightness temperatures ~ 1.6--4.2 K. We investigate the effects of spatial filtering on the quantitative comparison of Local Group and Milky Way giant molecular clouds properties and distributions. We also discuss different cloud identification techniques and their impact on derived cloud properties. When we employ the same cloud identification method and account for differences in data acquisition for M31, Milky Way, and M33, we find that the molecular cloud complexes in all three galaxies are similar. While the global distribution of molecular gas may vary from galaxy to galaxy, cloud complexes are similar, suggesting that cloud formation and destruction is determined by local physics. This work is supported by grants AST-9613716 & AST-9981289 from the National Science Foundation.

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arXiv1999

The Distance to the Soft Gamma Repeater SGR 1627-41

We report millimeter observations of the line of sight to the recently discovered Soft Gamma Repeater, SGR 1627-41, which has been tentatively associated with the supernova remnant SNR G337.0-0.1 Among the eight molecular clouds along the line of sight to SGR 1627-41, we show that SNR G337.0-0.1 is probably interacting with one of the most massive giant molecular clouds (GMC) in the Galaxy, at a distance of 11 kpc from the sun. Based on the high extinction to the persistent X-ray counterpart of SGR 1627-41, we present evidence for an association of this new SGR with the SNR G337.0-0.1; they both appear to be located on the near side of the GMC. This is the second SGR located near an extraordinarily massive GMC. We suggest that SGR 1627-41 is a neutron star with a high transverse velocity (~ 1,000 \kms) escaping the young (~ 5,000 years) supernova remnant G337.0-0.1

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arXiv1998

Evidence for the Galactic X-ray Bulge II

A mosaic of 5 \ros~PSPC pointed observations in the Galactic plane ($l\sim25^{\circ}$) reveals X-ray shadows in the $0.5-2.0$ keV band cast by distant molecular clouds. The observed on-cloud and off-cloud X-ray fluxes indicate that $\sim15$% and $\sim37$% of the diffuse X-ray background in this direction in the \tq~keV and 1.5 keV bands, respectively, originates behind the molecular gas which is located at $\sim$3 kpc from the Sun. The implication of the derived background X-ray flux beyond the absorbing molecular cloud is consistent with, and lends further support to recent observations of a Galactic X-ray bulge.

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arXiv1996

The Distance of the Soft Gamma Repeater SGR 1806-20

We present CO(J=1-0) observations in the direction of the Soft Gamma Repeater SGR 1806-20 with the SEST telescope. We detected several molecular clouds, and we discuss in this paper the implications of these observations for the distance to the X-ray counterpart AX 1805.7-2025, the supernova remnant G10.0-0.3 and the very luminous O9-B2 star detected in the line of sight. The distance of SGR 1806-20 is estimated to be 14.5 +/- 1.4 kpc and this Soft Gamma Repeater is very likely associated with one of the brightest HII regions in the Galaxy, W31. The large size of G10.0-0.3 (25 x 38 pc) for a young supernova remnant possibly powered by a central pulsar (AX 1805.7-2025) indicates that G10.0-0.3 could be expanding in the very low density region produced by the wind of the blue star.

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