A candidate protostellar object in the L1457 / MBM12 cloud

The association of young T Tauri stars, MBM12A, indicates that L1457 was forming stars not too long ago. With our study we want to find out whether or not there are still signs for ongoing star formation in that cloud. Using the Max-Planck-Millimeter-Bolometer MAMBO at the IRAM 30m telescope we obtained a map of about 8' by 8' centered on L1457 in the dust continuum emission at 230 GHz. Towards the most intense regions in our bolometer map we obtained spectra at high angular resolution in the CS (2-1) and the N2H+(1-0) lines using the IRAM 30m telescope. We find that the cold dust in L1457 is concentrated in several small cores with high H2 column densities and solar masses. The density profiles of the cores are inconsistent with a sphere with constant density. These cores are closer to virial equilibrium than the cloud as a whole. Data from the VLA and Spitzer archives reveal two point sources in the direction of one dust core. One of the sources is probably a distant quasar, whereas the other source is projected right on a local maximum of our dust map and shows characteristics of a protostellar object.Comment: 4 pages, 4 figures, accepted by Astronomy & Astrophysic

Cold dust and its relation to molecular gas in the dwarf irregular galaxy NGC 4449

We present observations of the dwarf irregular galaxy NGC 4449 at 850 $\mu$m and 450 $\mu$m obtained with SCUBA at the JCMT. The distribution of the cold dust agrees well with that of the CO and H$\alpha$ emission. To explain the integrated mm- through far-infrared continuum spectrum three dust components are required, with temperatures of 16 K, 39 K and 168 K, respectively. The dust mass is dominated by the cold dust component; we derive a total dust mass of \~3.8 x10^6 Msun, and with the local gas-to-dust ratio of ~130 a total gas mass of M(HI+H_2}~4.9 x 10^8 Msun. Comparison with the HI mass leads to a total molecular gas mass of ~3.4 x 10^8 Msun. We derive a conversion factor of the CO line intensity to molecular hydrogen column density XCO = N(H_2)/I(CO) which is at least 11 times larger than the Galactic value. These values are in accord with the lower metallicity of NGC 4449.Comment: 6 pages, 2 figures, accepted for publication in A&

Physical State of Molecular Gas in High Galactic Latitude Translucent Clouds

The rotational transitions of carbon monoxide (CO) are the primary means of investigating the density and velocity structure of the molecular interstellar medium. Here we study the lowest four rotational transitions of CO towards high-latitude translucent molecular clouds (HLCs). We report new observations of the J = (4-3), (2-1), and (1-0) transitions of CO towards eight high-latitude clouds. The new observations are combined with data from the literature to show that the emission from all observed CO transitions is linearly correlated. This implies that the excitation conditions which lead to emission in these transitions are uniform throughout the clouds. Observed 13CO/12CO (1-0) integrated intensity ratios are generally much greater than the expected abundance ratio of the two species, indicating that the regions which emit 12CO (1-0) radiation are optically thick. We develop a statistical method to compare the observed line ratios with models of CO excitation and radiative transfer. This enables us to determine the most likely portion of the physical parameter space which is compatible with the observations. The model enables us to rule out CO gas temperatures greater than 30K since the most likely high-temperature configurations are 1 pc-sized structures aligned along the line of sight. The most probable solution is a high density and low temperature (HDLT) solution. The CO cell size is approximately 0.01 pc (2000 AU). These cells are thus tiny fragments within the 100 times larger CO-emitting extent of a typical high-latitude cloud. We discuss the physical implications of HDLT cells, and we suggest ways to test for their existence.Comment: 19 pages, 13 figures, 2 tables, emulateapj To be published in The Astrophysical Journa

ORFEUS II echelle spectra: Absorption by H_2 in the LMC

We present the first detection of molecular hydrogen (H_2) UV absorption profiles on the line of sight to the LMC. The star LH 10:3120 in the LMC was measured with the ORFEUS telescope and the Tuebingen echelle spectrograph during the space shuttle mission of Nov./Dec. 1996. 16 absorption lines from the Lyman band are used to derive the column densities of H_2 for the lowest 5 rotational states in the LMC gas. For these states we find a total column density of N(H_2)=6.6 x 10^18$ cm^-2 on this individual line of sight. We obtain equivalent excitation temperatures of T < 50 K for the rotational ground state and T = 470 K for 0 < J < 6 by fitting the population densities of the rotational states to theoretical Boltzmann distributions. We conclude that UV pumping dominates the population of the higher rotational levels, as known from the H_2 gas in the Milky Way. (Research supported in part by the DARA)Comment: Astronomy & Astrophysics, Letter, in pres

Chemical and physical small-scale structure in a pre-stellar core

We present a comparative study of several molecular lines and of the dust contiunuum at 1.2mm in a pre-stellar core that is embedded in the Galactic cirrus cloud MCLD123.5+24.9. Previous studies found that the core is gravitationally stable and shows signs of inward motion. Using the Owens Valley (OVRO) and Plateau de Bure (PdB) interferometers we obtained high-angular resolution maps of the core in the carbon monosulfide CS 2-1 and the cyanoacetylene HC3N 10-9 transitions. Together with CS 5-4, C34S 3-2, and bolometer data obtained with the IRAM 30m telescope, we analyse the excitation conditions and the structural properties of the cloud. On the one hand, the new CS 2-1 observations reveal significant substructure on a scale of about 7", i.e., the beam size, corresponding to about 1050 AU at an adopted distance of 150pc. On the other hand, the interferometric observations in the HC3N 10-9 transition shows just one single well resolved clump in the inner part of the core. This core is well described by an intensity profile following from a centrally peaked volume density distribution. We find no evidence for depletion of CS onto dust grains. The inward motion seen in the CS 2-1 occurs one-sided from the middle of the filamentary cloud towards the HC3N core.Comment: 8 pages, 8 figures, accepted by A&

Infrared Excess and Molecular Gas in the Galactic Worm GW46.4+5.5

We have carried out high-resolution (~3') HI and CO line observations along one-dimensional cuts through the Galactic worm GW46.4+5.5. By comparing the HI data with IRAS data, we have derived the distributions of I_100 excess and tau_100 excess, which are respectively the 100 mum intensity and 100 mum optical depth in excess of what would be expected from HI emission. In two observed regions, we were able to make a detailed comparison of the infrared excess and the CO emission. We have found that tau_100 excess has a very good correlation with the integrated intensity of CO emission, W_CO, but I_100 excess does not. There are two reasons for the poor correlation between I_100 excess and W_CO: firstly, there are regions with enhanced infrared emissivity without CO, and secondly, dust grains associated with molecular gas have a low infrared emissivity. In one region, these two factors completely hide the presence of molecular gas in the infrared. In the second region, we could identify the area with molecular gas, but I_100 excess significantly underestimates the column density of molecular hydrogen because of the second factor mentioned above. We therefore conclude that tau_100 excess, rather than I_100 excess, is an accurate indicator of molecular content along the line of sight. We derive tau_100/N(H)=(1.00+-0.02)*10^-5~(10^20 cm^-2)^-1, and X=N(H_2)/W_CO=~0.7*10^20 cm^-2 (K km s^-1)^-1. Our results suggest that I_100 excess could still be used to estimate the molecular content if the result is multiplied by a correction factor xi_c=_HI/_H_2 (~2 in the second region), which accounts for the different infrared emissivities of atomic and molecular gas. We also discuss some limitations of this work.Comment: 10 pages, 9 postscript figures, uses aas2pp4.sty to be published in Astrophyslcal Journa

The Discovery of a Molecular Complex in the Tidal Arms near NGC 3077

We present the discovery of a giant molecular complex (r=350 pc, M_ vir=10^7 M_sun) in the tidal arms south-east of NGC 3077, a member of the M 81 triplet. The complex is clearly detected in the 12CO (J=1-0) transition at five independent positions. The position relative to NGC 3077, the systemic velocity (v_hel=14km/s) and the cloud averaged line width (15km/s) indicate that the object is not related to Galactic cirrus but is extragalactic. The tidal HI arm where the molecular complex is located has an total HI mass of M_HI=3x10^8 M_sun. This tidal material was presumably stripped off the outer parts of NGC 3077 during the closest encounter with M 81, about 3x10^8 years ago. After the complex detected along a torn-out spiral arm of M 81 by Brouillet et al., it is the second of its kind reported so far. Based on published optical observations, we have no evidence for on--going star formation in the newly detected molecular complex. Since the system has all the ingredients to form stars in the future, we speculate that it might eventually resemble the young dwarf galaxies in the M 81 group.Comment: 12 pages (including 3 figures), accepted for publication in the ApJ Letter

Extended Star Formation and Molecular Gas in the Tidal Arms near NGC3077

We report the detection of ongoing star formation in the prominent tidal arms near NGC 3077 (member of the M 81 triplet). In total, 36 faint compact HII regions were identified, covering an area of ~4x6 kpc^2. Most of the HII regions are found at HI column densities above 1x10^21 cm^-2 (on scales of 200 pc), well within the range of threshold columns measured in normal galaxies. The HII luminosity function resembles the ones derived for other low-mass dwarf galaxies in the same group; we derive a total star formation rate of 2.6x10^-3 M_sun/yr in the tidal feature. We also present new high-resolution imaging of the molecular gas distribution in the tidal arm using CO observations obtained with the OVRO interferometer. We recover about one sixth of the CO flux (or M_H2~2x10^6 M_sun, assuming a Galactic conversion factor) originally detected in the IRAM 30m single dish observations, indicating the presence of a diffuse molecular gas component in the tidal arm. The brightest CO peak in the interferometer map (comprising half of the detected CO flux) is coincident with one of the brightest HII regions in the feature. Assuming a constant star formation rate since the creation of the tidal feature (presumably ~3x10^8 years ago), a total mass of ~7x10^5 M_sun has been transformed from gas into stars. Over this period, the star formation in the tidal arm has resulted in an additional enrichment of Delta(Z)>0.002. The reservoir of atomic and molecular gas in the tidal arm is ~3x10^8 M_sun, allowing star formation to continue at its present rate for a Hubble time. Such wide-spread, low-level star formation would be difficult to image around more distant galaxies but may be detectable through intervening absorption in quasar spectra.Comment: Accepted for publication in the Astronomical Journa

Can Reflection from Grains Diagnose the Albedo?

By radiation transfer models with a realistic power spectra of the projected density distributions, we show that the optical properties of grains are poorly constrained by observations of reflection nebulae. The ISM is known to be hierarchically clumped from a variety of observations (molecules, H I, far-infrared). Our models assume the albedo and phase parameter of the dust, the radial optical depth of the sphere averaged over all directions, and random distributions of the dust within the sphere. The outputs are the stellar extinction, optical depth, and flux of scattered light as seen from various viewing angles. Observations provide the extinction and scattered flux from a particular direction. Hierarchical geometry has a large effect on the flux of scattered light emerging from a nebula for a particular extinction of the exciting star. There is a very large spread in both scattered fluxes and extinctions for any distribution of dust. Consequently, an observed stellar extinction and scattered flux can be fitted by a wide range of albedos. With hierarchical geometry it is not completely safe to determine even relative optical constants from multiwavelength observations of the same reflection nebula. The geometry effectively changes with wavelength as the opacity of the clumps varies. Limits on the implications of observing the same object in various wavelengths are discussed briefly. Henry (2002) uses a recipe to determine the scattered flux from a star with a given extinction. It is claimed to be independent of the geometry. It provides considerably more scattering than our models, probably leading to an underestimate of the grain albedos from the UV Diffuse Galactic Light.Comment: 27 pages, including 7 figures. Accepted by Ap