Visual Extinction AV Research Articles

Injection of molecules from the surfaces of grains into the gas phase chemistry of an expanding cloud

We present model results for the chemistry in an expanding cloud or clump in which molecules are injected into the gas phase from grain surfaces when the clump reaches a certain visual extinction Av during the expansion. We consider separately injection at two different values of Av, and include a representative large hydrocarbon, C6H, and sulphur-bearing molecule, H2SO4, as well as H2O and CO. We examine the abundances of certain molecules which have been observed in diffuse and translucent clouds, and compare the results obtained for these abundances with and without an injection during expansion. We also compare our results withpublished observations, and conclude that in most clouds an injection of molecules has occurred.

Spatial Study with the Very Large Telescope of a New Resolved Edge‐on Circumstellar Dust Disk Discovered at the Periphery of the ρ Ophiuchi Dark Cloud

We report the discovery in NIR with SofI at the NTT of a resolved circumstellar dust disk around a 2MASS source at the periphery of the rho Ophiuchi dark cloud. We present follow-up observations in J, H, and Ks-band obtained with ISAAC at the VLT, under 0.4"-seeing conditions, which unveil a dark dust lane oriented East-West between two characteristic northern and southern reflection nebulae. This new circumstellar dust disk has a radius of 2.15" (300 AU at 140 pc), and a width of 1.2" (170 AU at 140 pc). Thanks to its location at the periphery of the dense cores, it suffers small foreground visual extinction (Av=2.1\pm2.6 mag). Although this disk is seen close to edge-on, the two reflection nebulae display very different colors. We introduce a new NIR data visualization called ``Pixel NIR Color Mapping'', which allows to visualize directly the NIR colors of the nebula pixels. Thanks to this method we identify a ridge, 0.3" (40 AU at 140 pc) to the north of the dark lane and parallel to it, which displays a NIR color excess. This ridge corresponds to an unusual increase of brightness from J to Ks, which is also visible in the NTT observation obtained 130 days before the VLT one. We also find that the northern nebula shows ~3 mag more extinction than the southern nebula. We compute axisymmetric disk models to reproduce the VLT scattered light images and the spectral energy distribution from optical to NIR. Our best model, with a disk inclination i=86\pm1 deg, correctly reproduces the extension of the southern reflection nebula, but it is not able to reproduce either the observed NIR color excess in the northern nebula or the extinction difference between the two reflection nebulae. We discuss the possible origin of the peculiar asymmetrical NIR color properties of this object.

Theoretical Models of Polarized Dust Emission from Protostellar Cores

We model the polarized thermal dust emission from protostellar cores that are assembled by supersonic turbulent flows in molecular clouds. Self-gravitating cores are selected from a three-dimensional simulation of supersonic and super-Alfvenic magnetohydrodynamic (MHD) turbulence. The polarization is computed in two ways. In model A it is assumed that dust properties and grain alignment efficiency are uniform; in model B it is assumed that grains are not aligned at visual extinction larger than AV,0 = 3 mag, consistent with theoretical expectations for grain alignment mechanisms. Instead of using a specific set of grain properties, we adopt a maximum degree of polarization Pmax = 15%. Results are therefore sensitive mainly to the topology of the magnetic field (model A) and to the gas distribution that determines the distribution of AV (model B). Furthermore, the radiative transfer in the MHD model is solved with a non-LTE Monte Carlo method, to compute spectral maps of the J = 1-0 transition of CS. The CS spectral maps are used to estimate the turbulent velocity, as in the observations. The main results of this work are the following: (1) Values of P between 1% and 10% (up to almost Pmax) are typical, despite the super-Alfvenic nature of the turbulence. (2) A steep decrease of P with increasing values of the submillimeter dust continuum intensity I is always found in self-gravitating cores selected from the MHD simulations if grains are not aligned above a certain value of visual extinction AV,0 (model B). (3) The same behavior is hard to reproduce if grains are aligned independently of AV (model A). (4) The Chandrasekhar-Fermi formula, corrected by a factor f ≈ 0.4, provides an approximate estimate of the average magnetic field strength in the cores. Submillimeter dust continuum polarization maps of quiescent protostellar cores and Bok globules have recently been obtained. They always show a decrease in P with increasing value of I consistent with the predictions of our model B. We therefore conclude that submillimeter polarization maps of quiescent cores do not map the magnetic field inside the cores at visual extinction larger than AV,0 ≈ 3 mag. The use of such maps to constrain models of protostellar core formation and evolution is questionable. This conclusion suggests that there is no inconsistency between the results from optical and near-IR polarized absorption of background stars and the observed polarization of submillimeter dust continuum from quiescent cores. In both cases, grains at large visual extinction appear to be virtually unaligned.

A Highly Reddened Star Cluster Embedded in the Galactic Plane

We found a cluster of early-type star candidates embedded in the Galactic plane by using optical to near-infrared broadband imaging. The observed region is around the 10 highly reddened stars found as a possible new and very remote open cluster by Stephenson. The 10 stars are identified as red supergiants, and several similar stars are newly found in our images. Furthermore, 25 early-type star candidates suffering visual extinction of AV ~ 11.2 have been identified from various color-magnitude and color-color diagrams. These stars are likely to make a star cluster, and we estimate its age and distance to be ~107.7 yr and ~1.5 kpc, respectively.

Near‐Infrared Integral Field Spectroscopy and Mid‐Infrared Spectroscopy of the Starburst Galaxy M82

We present new infrared observations of the central regions of the starburst galaxy M82. The observations consist of near-infrared integral field spectroscopy in the H and K bands obtained with the MPE 3D instrument and of λ = 2.4-45 μm spectroscopy from the Short Wavelength Spectrometer (SWS) onboard the Infrared Space Observatory. These measurements are used, together with data from the literature, to (1) reexamine the controversial issue of extinction, (2) determine the physical conditions of the interstellar medium (ISM) within the star-forming regions, and (3) characterize the composition of the stellar populations. Our results provide a set of constraints for detailed starburst modeling, which we present in a companion paper. We find that purely foreground extinction cannot reproduce the global relative intensities of H recombination lines from optical to radio wavelengths. A good fit is provided by a homogeneous mixture of dust and sources, and with a visual extinction of AV = 52 mag. The SWS data provide evidence for deviations from commonly assumed extinction laws between 3 and 10 μm. The fine-structure lines of Ne, Ar, and S detected with SWS imply an electron density of ≈300 cm-3, and abundance ratios Ne/H and Ar/H nearly solar and S/H about one-fourth solar. The excitation of the ionized gas indicates an average effective temperature for the OB stars of 37,400 K, with little spatial variation across the starburst regions. We find that a random distribution of closely packed gas clouds and ionizing clusters and an ionization parameter of ≈10-2.3 represent well the star-forming regions on spatial scales ranging from a few tens to a few hundreds of parsecs. From detailed population synthesis and the mass-to-K-light ratio, we conclude that the near-infrared continuum emission across the starburst regions is dominated by red supergiants with average effective temperatures ranging from 3600 to 4500 K and roughly solar metallicity. Our data rule out significant contributions from older, metal-rich giants in the central few tens of parsecs of M82.

A near-infrared polarized bipolar cone in the Circinus galaxy

We present near-infrared broad-band polarization images of the nuclear regions of the Circinus galaxy in the J, H and K bands. For the first time the south-eastern reflection cone is detected in polarized light, which is obscured at optical wavelengths behind the galactic disk. This biconical structure is clearly observed in J and H band polarized flux whilst in the K band a more compact structure is detected. Total flux J-K and H-K colour maps reveal a complex colour gradient toward the south-east direction (where the Circinus galactic disk is nearer to us). We find enhanced extinction in an arc shaped structure, at about 200pc from the nucleus, probably part of the star-formation ring. We model the polarized flux images with the scattering and torus model of Young etal, with the same basic input parameters as used by Alexander etal in the spectropolarimetry modelling of Circinus. The best fit to the polarized flux is achieved with a torus radius of ~16pc, and a visual extinction Av, through the torus, to the near-infrared emission regions of >66 mags.

The Phoenix Survey: optical and near-infrared observations of faint radio sources

Using a deep Australia Telescope Compact Array (ATCA) radio survey covering an area of ≈3 deg2 to a 4σ sensitivity of ≥ 100 μJy at 1.4 GHz, we study the nature of faint radio galaxies. About 50 per cent of the detected radio sources are identified with an optical counterpart revealed by CCD photometry to mR=22.5 mag. Near-infrared (K-band) data are also available for a selected sample of the radio sources, while spectroscopic observations have been carried out for about 40 per cent of the optically identified sample. These provide redshifts and information on the stellar content. Emission-line ratios imply that most of the emission-line sources are star-forming galaxies, with a small contribution (≈ 10 per cent) from Sy1/Sy2 type objects. We also find a significant number of absorption-line systems, likely to be ellipticals. These dominate at high flux densities (>1 mJy) but are also found at sub-mJy levels. Using the Balmer decrement we find a visual extinction AV=1.0 for the star-forming faint radio sources. This moderate reddening is consistent with the V-R and R-K colours of the optically identified sources. For emission-line galaxies, there is a correlation between the radio power and the Hα luminosity, in agreement with the result of Benn et al. This suggests that the radio emission of starburst radio galaxies is a good indicator of star formation activity.

Near‐Infrared Integral Field Spectroscopy of Markarian 231

The ultraluminous infrared Seyfert 1 galaxy Mrk 231 has been spectrally imaged in the K band with the new three-dimensional MPE integral field spectrometer. The combined images of the H2 emission lines show, for the first time in an ultraluminous infrared galaxy, the presence of an extended circumnuclear structure of hot molecular gas. The H2 emitting region has a size of ~2.4 kpc and a hot molecular gas mass M ~2 × 104 M☉. The H2 emission-line ratios indicate that the gas is most likely thermally excited. If as in NGC 7469 star formation is associated with the H2 emission, the starburst would have a far-IR luminosity LFIR ~ 1 × 1012 L☉. This value represents an upper limit, since a fraction of the hot molecular gas may be excited by the radiation field emerging from the nucleus. The K-band three-dimensional data cube also shows for the first time the presence of extended narrow Paα emission blueshifted by ~1400 km s-1 with respect to the systemic velocity, and located ~0.6 kpc northwest of the nucleus. The detection of CO absorption bands with a spatial distribution peaking on the K-band continuum provides evidence for a central stellar concentration. The low CO spectroscopic index indicates, however, dilution by hot dust emission or by a nonthermal active galactic nucleus. The Paα/Hα ratio confirms previous extinction measurements based on Balmer line ratios, i.e., visual extinction of AV ~ 2.0-6.6 mag. The quasar-type nucleus of Mrk 231 should then be transparent at 2 μm and also in hard X-rays. A weak nuclear He I λ2.058 μm (He I/Paα = 0.032) is detected, and no detection of [Si VI] λ1.962 μm is made, placing an upper limit of 4 × 10-18 Wm-2 for the coronal gas emission. The ionizing source could either be a far-UV and X-ray quiet quasar or else a nuclear starburst with an upper mass limit ≥60 M☉.

Near Infrared Observations of IRAS 09104+4109

Near infrared imaging and grism spectroscopy of the high luminosity infrared bright galaxy IRAS 09104+4109 have been obtained with the W. M. Keck Telescope. The imaging shows 6 “knots” of emission projected against the extended stellar envelope of the cD galaxy thought to be the source of the large far infrared luminosity. The luminosities of the knots are consistent with the bulges of galaxies accreting onto the central galaxy. In addition, there are 11 companion galaxies seen at radii of 40-150 kpc from the cD nucleus. These objects have colors in the range R—K~ 3.5±0.5 mag, J-H~0.9±0.2 mag H-K ~0.7±0.2 mag, which are consistent with galaxies at a redshift of 0.4. The companion galaxies have luminosities comparable to or less than the characteristic luminosity (L^*) of field galaxies. While the central cD galaxy is identified with the luminous infrared source, it appears to be a quiescent, radio-quiet galaxy, showing no evidence from its near infrared colors for a highly reddened nucleus as seen in other infrared luminous galaxies. The grism spectroscopy shows forbidden lines of low ionization stages of sulfer, iron, and oxygen, as well as hydrogen recombination lines and a strong line of neutral helium. A visual extinction of Av—2 mag is derived to the narrow line region surrounding the galaxy nucleus, based on the line ratios [S II]1.03 µm/0.407 µm and Pδ/Hβ. The near infrared spectrum is consistent with the optical classification of this system being a Seyfert 2 nucleus.

The Growth of Solids, Destruction of Molecules, and Shielding of Radiation in the Young Stellar Disk of HD 45677

view Abstract Citations (9) References (62) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Growth of Solids, Destruction of Molecules, and Shielding of Radiation in the Young Stellar Disk of HD 45677 Brown, Thomas M. ; Buss, Richard, Jr. ; Grady, Carol ; Bjorkman, Karen Abstract Ultraviolet spectra (912-3300 Å) from the Astro-1 and lUE space missions of two Herbig Be stars, HD 45677 and HD 200775, show that the HD 200775 intrinsic continuum shape generally coincides with a T = 20,000 K Kurucz model and that the HD 45677 continuum exhibits additional line blanketing, extinction, and emission from gas and dust in its disk and bipolar wind. With log N(H I) = 21.4±0.1 cm-1, our measured upper limit on the fraction of HD 45677 disk H atoms in molecules (f < 1.5 × 10-2) is much less than that seen in general Galactic molecular clouds (f ≍ 0.5). Also, our derived HD 45677 circumstellar dust extinction for λ > 1400 Å is produced by an inhomogeneous disk of material: during periods of moderate visual extinction (Av ≍ 1.0±0.3 mag) the disk as a whole causes mid-UV extinction with an Rv ≍ 4-5 (values found in Galactic molecular clouds), yet at the thickest optical depths, there are regions with larger grains (inferred Rv > 7) than those in molecular clouds (Rv < 5.6). Alternately, we find in the FUV (1300-1000 A) a strong, steeply curved circumstellar extinction for HD 45677 that has a very similar shape as extinction produced by interstellar polycyclic aromatic hydrocarbon molecules (PAHs) in an illuminated molecular cloud (NGC 2023). The PAHs around HD 45677, like NGC 2023, are more abundant relative to the large grains (Aλ/Av) than predicted by the extinction parameter Rv, because HD 45677 has evaporated the PAHs off large grains. With a measured FUV depth corresponding to τλ ≍ 1.4-3.5, the excess PAHs around HD 45677 partly shield the outer disk from FUV radiation. Both the disk gas and dust have evolved greatly since the formation of the mid-mass star HD 45677 from molecular clouds: in some regions, the disk grains have grown to micron sizes suitable for the formation of planetesimals, the ISM H_{2 }has been photodissociated into H I in the disk, and CO (McGregor, Hyland, & Hillier 1988) has disappeared. In addition, the molecular PAHs that are condensed onto grains in molecular clouds have photoevaporated off grains into the gaseous disk of HD 45677, becoming dehydrogenated and ionized. Thus, because of strong FUV radiation from HD 45677 and moderate FUV shielding, the disk around this intermediate-mass star differs dramatically from the condensate-rich, molecular gas around low-mass stars and would seem to lead to a different chemical and physical evolution toward planets than those around low-mass stars such as our Sun. Publication: The Astrophysical Journal Pub Date: February 1995 DOI: 10.1086/175323 Bibcode: 1995ApJ...440..865B Keywords: STARS: CIRCUMSTELLAR MATTER; ISM: DUST; EXTINCTION; STARS: EMISSION-LINE; BE; ULTRAVIOLET: STARS; STARS: INDIVIDUAL HENRY DRAPER NUMBER: HD 45677 full text sources ADS | data products SIMBAD (5) MAST (2)

About the Suspected Very Young PN IRAS 17516-2525

Infrared photometry suggests that IRAS 17516-2525 is in transition from the AGB to a young PN (Van der Veen et al. 1989a, b, Manchado et al. 1989). IR spectra (van der Veen et al. 1989b, Käufl et al. 1992) revealed that the object has an ionized core. Optical observations show a weak Hα emitting unresolved object (Käufl et al. 1992) at the coordinates (α1950 : 17h51m37.8s, δ1950 : −25° 25′ 58″) suggested by van der Veen et al. (1989a). The spectrum between 2.0 and 2.4μm shows e.g. Brγ, 2.0875μm (unidentified) or the NaI-doublet but no molecular Hydrogen (present in other compact PNs). The ratio of Pfβ to Brα (hardly affected by extinction, insensitive to electron density and temperature) has been studied in IRAS 17516-2525 and Hen1044. For Hen1044 we find this ratio to be in accordance with theoretical calculations (Hummer and Storey, 1987). For IRAS 17516-2525, however, Pfβ is approximately twice as strong. Hence presumably Hen1044 is optically thin whereas IRAS 17516-2525 is optically thick at λ ≈4–5μm. Line fluxes appear to be constant over a time scale of 4 years. If the visual extinction AV in the object is ≥ 2mag than the infrared luminosity equals the total luminosity (i.e. L = 1400L⊙ ∗ (distance/kpc)2). A main-sequence object accidentally extincted by an intervening cloud can be excluded because the observed line-width of Brα (≈ 40km/s, Van der Veen et al. 1989a) is too small as compared to that of O-stars (≈1000km/s, Käufl, 1992). Assuming an expansion-velocity of ≈ 20km/s the stellar appearance constrains dynamic age and size (age ≤ 360y ∗ distance/kpc, size ≤ 1500AU ∗ distance/kpc). A pre-main sequence object can also be excluded. The max. luminosity (LPMS ≤ 5000L⊙) then requires the object to be closer than 2 kpc where it should be resolvable. In conclusion the observational evidence strongly suggests that IRAS 17516-2525 represents the searched ‘missing link’ between AGB stars and young compact Planetary Nebulae.

Giant H II regions in M81

view Abstract Citations (87) References (37) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Giant H II Regions in M81 Kaufman, Michele ; Bash, Frank N. ; Kennicutt, R. C., Jr. ; Hodge, P. W. Abstract H-alpha and VLA radio continuum observations at wavelengths of 6 and 20 cm are used to study the distribution of extinction and the distribution of giant radio H II regions along the spiral arms in M81. The radial distribution of visual extinction Av in the plane of M81 shows no trend, with an upper limit 0.1 mag/kpc for the radial extinction gradient and a mean Av of 1.1 + or - 0.4 mag. Nearly all the giant radio H II regions lie along the spiral arms or the inner H I ring. The radial distribution of the set of giant radio H II regions exhibits a strong maximum at a galactocentric distance R roughly 300 arcsec. Unless molecular hydrogen in M81 is also concentrated near R roughly 300 arcsec, the data disagree with Visser's (1980) model for star formation associated with a density wave. Some suggestions are made about how to change the ballistic particle model of Leisawitz and Bash to agree with the observed radial distribution of giant radio H II regions. Publication: The Astrophysical Journal Pub Date: August 1987 DOI: 10.1086/165433 Bibcode: 1987ApJ...319...61K Keywords: Astronomical Spectroscopy; H Alpha Line; Hydrogen Clouds; Radio Galaxies; Very Large Array (Vla); Nebulae; Radial Distribution; Radiant Flux Density; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M81; NEBULAE: H II REGIONS; RADIO SOURCES: GALAXIES full text sources ADS | data products SIMBAD (1) NED (1)