Visual Extinction AV Research Articles

Gas phase Elemental abundances in Molecular cloudS (GEMS)

Context.Gas phase Elemental abundances in molecular CloudS (GEMS) is an IRAM 30-m Large Program aimed at determining the elemental abundances of carbon (C), oxygen (O), nitrogen (N), and sulfur (S) in a selected set of prototypical star-forming filaments. In particular, the elemental abundance of S remains uncertain by several orders of magnitude, and its determination is one of the most challenging goals of this program.Aims.This paper aims to constrain the sulfur elemental abundance in Taurus, Perseus, and Orion A based on the GEMS molecular database. The selected regions are prototypes of low-mass, intermediate-mass, and high-mass star-forming regions, respectively, providing useful templates for the study of interstellar chemistry.Methods.We have carried out an extensive chemical modeling of the fractional abundances of CO, HCO+, HCN, HNC, CS, SO, H2S, OCS, and HCS+to determine the sulfur depletion toward the 244 positions in the GEMS database. These positions sample visual extinctions fromAV~ 3 mag to >50 mag, molecular hydrogen densities ranging from a few × 103cm−3to 3 × 106cm−3, andTk~ 10–35 K. We investigate the possible relationship between sulfur depletion and the grain charge distribution in different environments.Results.Most of the positions in Taurus and Perseus are best fitted assuming early-time chemistry,t= 0.1 Myr, ζH2~ (0.5−1) × 10−16s−1, and [S/H] ~ 1.5 × 10−6. On the contrary, most of the positions in Orion are fitted witht= 1 Myr and ζH2~ 10−17s−1. Moreover, ~40% of the positions in Orion are best fitted assuming the undepleted sulfur abundance, [S/H] ~ 1.5 × 10−5. We find a tentative trend of sulfur depletion increasing with density.Conclusions.Our results suggest that sulfur depletion depends on the environment. While the abundances of sulfur-bearing species are consistent with undepleted sulfur in Orion, a depletion factor of ~20 is required to explain those observed in Taurus and Perseus. We propose that differences in the grain charge distribution might explain these variations. Grains become negatively charged at a visual extinction ofAV~ 3.5 mag in Taurus and Perseus. At this low visual extinction, the S+abundance is high,X(S+) > 10−6, and the electrostatic attraction between S+and negatively charged grains could contribute to enhance sulfur depletion. In Orion, the net charge of grains remains approximately zero until higher visual extinctions (AV~ 5.5 mag), where the abundance of S+is already low because of the higher densities, thus reducing sulfur accretion. The shocks associated with past and ongoing star formation could also contribute to enhance [S/H].

Study of dust properties nearby pulsar J0302+2252 using Iris, Akari and wise surveys

A search of dust structures nearby the pulsar J0302+2252 in IRIS, AKARI, and WISE surveys were performed using Skyview Observatory (http://skyview.gsfc.nasa.gov). The physical properties have been studied at 60µm & 100µm in the IRIS survey, 90µm & 140µm in the AKARI survey, and 12µm & 22µm in the WISE survey at right ascension (R.A.) (J2000)=03h 02m 31.99s and Declination (Decl.) (J2000)=+ 220 52m 12.10s . In this work, we have studied the physical properties such as Dust color temperature, Planck’s function, Visual extinction, and Mass profile of the dust using the data reduction software ALADIN v11. The dust color temperature is found to be 22.271 ± 0.261K in IRIS, 22.343 ± 0.939 K in AKARI, and 278.74 ± 0.067 K in WISE. Also, the dust mass is found to be 2.286 × 1027 kg (1.149 × 10-3 M⊙ ) in IRIS, 8.800 ×1026 kg (0.416× 10-3 M⊙ ) in AKARI and 5.220 ×1025 kg (0.026 ×10-3 M⊙ ) in WISE. The average visual extinction is found to be 1.372 ×10-13 mag. In IRIS, 4.970 ×10-14 mag. In AKARI and 1.673 ×10-15 mag. In WISE. The dust color temperature and dust mass distribution show a very good agreement with Gaussian. The Contour plots of dust color temperature and dust mass show inverse relation between them in all three bands IRIS, AKARI, and WISE.

Oversized Gas Clumps in an Extremely Metal-poor Molecular Cloud Revealed by ALMA’s Parsec-scale Maps

Abstract Metals are thought to have profound effects on the internal structures of the molecular clouds in which stars are born. The absence of metals is expected to prevent gas from efficient cooling and fragmentation in theory. However, this effect has not yet been observed in low-metallicity environments, such as in the early universe and local dwarf galaxies, because of the lack of high-spatial-resolution maps of gas. We carried out Atacama Large Millimeter/submillimeter Array observations of the carbon monoxide (CO) J = 2–1 emission line at 1.4 parsec resolutions of a molecular cloud in DDO 70 at 7% solar metallicity, the most metal-poor galaxy currently known with a CO detection. In total, five clumps have been identified and they are found to more or less follow Larson’s law. Since the CO emission exists in regions with visual extinction A V around 1.0, we converted this A V to the gas-mass surface density using a gas-to-dust ratio of 4594 ± 2848 for DDO 70. We found that the CO clumps in DDO 70 exhibit significantly larger (on average four times) sizes than those at the same gas-mass surface densities in the massive star formation regions of the Milky Way. The existence of such large clumps appears to be consistent with theoretical expectations that gas fragmentation in low-metallicity clouds is suppressed. While our observation is only for one cloud in the galaxy, if it is representative, the above result implies suppressed gas fragmentation during the cloud collapse and star formation in the early universe.

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS)

Aims. We present the first detection of diffuse dust in the intra-cluster medium of the Virgo cluster out to ∼0.4 virial radii, and study the radial variation of its properties on a radial scale of the virial radius. Methods. Analysing near-UV – i colours for a sample of ∼12 000 background galaxies with redshifts 0.02 < z < 0.8, we find significant colour reddening and relate it to variation in E(B − V) values. Results. The E(B − V) mean profile shows a dust component characterised by an average reddening E(B − V) ∼ 0.042 ± 0.004 mag within 1.5 degrees (∼0.3 rvir) from the cluster centre. Assuming a Large Magellanic Cloud extinction law, we derive an average visual extinction AV = 0.14 ± 0.01 for a total dust mass, Md = 2.5 ± 0.2 × 109 M⊙, hence a dust-to-gas mass ratio Md/Mg = 3.0 ± 0.3 × 10−4. Based on the upper limits on the flux density I250μm = 0.1 MJy sr−1 derived from Herschel data, we estimate an upper limit for the dust temperature of Td ∼ 10 K. However, similar densities can be obtained with dust at higher temperatures with lower emissivities. Conclusions. The Virgo cluster has diffuse dust in its intra-cluster medium characterised by different physical properties as those characterising the Milky Way dust. The diffuse dust in Virgo is transported into the cluster space through similar phenomena (stripping) as those building up the optical intra-cluster light, and it constitutes an additional cooling agent of the cluster gas.

Nobeyama 45 m mapping observations toward Orion A. II. Classification of cloud structures and variation of the 13CO/C18O abundance ratio due to far-UV radiation

Abstract We present results of the classification of cloud structures toward the Orion A Giant Molecular Cloud based on wide-field 12CO (J = 1–0), 13CO (J = 1–0), and C18O (J = 1–0) observations using the Nobeyama 45 m radio telescope. We identified 78 clouds toward Orion A by applying Spectral Clustering for Interstellar Molecular Emission Segmentation (SCIMES) to the data cube of the column density of 13CO. Well-known subregions such as OMC-1, OMC-2/3, OMC-4, OMC-5, NGC 1977, L1641-N, and the dark lane south filament (DLSF) are naturally identified as distinct structures in Orion A. These clouds can also be classified into three groups: the integral-shaped filament, the southern regions of Orion A, and the other filamentary structures in the outer parts of Orion A and the DLSF. These groups show differences in scaling relations between the physical properties of the clouds. We derived the abundance ratio between 13CO and C18O, $X_{^{13}\mathrm{CO}}/X_{\mathrm{C}^{18}\mathrm{O}}$, which ranges from 5.6 to 17.4 on median over the individual clouds. The significant variation of $X_{^{13}\mathrm{CO}}/X_{\mathrm{C}^{18}\mathrm{O}}$ is also seen within a cloud in both the spatial and velocity directions and the ratio tends to be high at the edge of the cloud. The values of $X_{^{13}\mathrm{CO}}/X_{\mathrm{C}^{18}\mathrm{O}}$ decrease from 17 to 10 with the median of the column densities of the clouds at the column density of $N_{\mathrm{C^{18}O}} \gtrsim 1 \times 10^{15}\:$cm−2 or visual extinction of AV ≳ 3 mag under the strong far-ultraviolet (FUV) environment of G0 > 103, whereas it is almost independent of the column density in the weak FUV radiation field. These results are explained if the selective photodissociation of C18O is enhanced under a strong FUV environment and it is suppressed in the dense part of the clouds.

Simulating the atomic and molecular content of molecular clouds using probability distributions of physical parameters

Modern observations of the interstellar medium (ISM) in galaxies detect a variety of atomic and molecular species. The goal is to connect these observations to the astrochemical properties of the ISM. 3D hydro-chemical simulations attempt this but due to extreme computational cost, they have to rely on simplified chemical networks and are bound to individual case studies. We present an alternative approach which models the ISM at larger scales by an ensemble of pre-calculated 1D thermo-chemical photodissociation region (PDR) calculations that determine the abundance and excitation of atomic and molecular species. We adopt lognormal distributions of column density (AV-PDFs) for which each column density is linked to a volume density as derived by hydrodynamical simulations. We consider two lognormal AV-PDFs: a diffuse, low-density medium with average visual extinction of |$\overline{{\rm A}_\mathrm{ V}}=0.75\, {\rm mag}$| and dispersion of σ = 0.5 and a denser giant molecular cloud with |$\overline{{\rm A}_\mathrm{ V}}=4\, {\rm mag}$| and σ = 0.8. We treat the UV radiation field, cosmic ray ionization rate, and metallicity as free parameters. We find that the low-density medium remains fully H i- and C ii-dominated under all explored conditions. The denser cloud remains almost always molecular (i.e. H2-dominated) while its carbon phase (CO, C i, and C ii) is sensitive to the above free parameters, implying that existing methods of tracing H2-rich gas may require adjustments depending on environment. Our numerical framework can be used to estimate the PDR properties of large ISM regions and quantify trends with different environmental parameters as it is fast, covers wide parameter space, and is flexible for extensions.

Molecular-cloud-scale Chemical Composition. III. Constraints of Average Physical Properties through Chemical Models

Abstract It is important to understand the origin of molecular line intensities and chemical composition at the molecular-cloud scale in Galactic sources because it serves as a benchmark to compare with the chemical compositions of extragalactic sources. Recent observations of 3-mm spectra averaged over a 10 pc scale show similar spectral patterns among sources for molecular lines HCN, HCO+, CCH, HNC, HNCO, c-C3H2, CS, SO, N2H+, and CN. To constrain the average physical properties emitting such spectral patterns, we model molecular spectra using a time-dependent gas-grain chemical model followed by a radiative transfer calculation. We use a grid of physical parameters such as the density n = 3 × 102–3 × 104 cm−3, temperature, T = 10–30 K, visual extinction A V = 2, 4, 10 mag, cosmic-ray ionization rate ζ = 10−17–10−16 s−1, and sulfur elemental abundance S/H = 8 ×10−8–8 × 10−7. A comparison with observations indicates that spectra are well reproduced with a relatively low density of n = (1–3) × 103 cm−3, T = 10 K, ζ = 10−17 s−1, and a short chemistry timescale of 105 yr. This short chemistry timescale may indicate that molecular clouds are constantly affected by turbulence, and exposed to low-density, low-A V regions that “refresh” the chemical clock by UV radiation. The relatively low density obtained is orders of magnitude lower than the commonly quoted critical density in the optically thin case. This range of density is consistent with results from recent observational analysis of molecular-cloud-scale mapping.

NOEMA Observations of a Molecular Cloud in the Low-metallicity Galaxy Kiso 5639

Abstract A giant star-forming region in a metal-poor dwarf galaxy has been observed in optical lines with the 10 m Gran Telescopio Canarias (GTC) and in the emission line of CO(1–0) with the Northern Extended Millimeter Array (NOEMA) mm-wave interferometer. The metallicity was determined to be , from which we estimate a conversion factor of α CO ∼ 100 M ⊙ pc−2(K km s−1)−1 and a molecular cloud mass of ∼2.9 × 107 M ⊙. This is an enormous concentration of molecular mass at one end of a small galaxy, suggesting a recent accretion. The molecular cloud properties seem normal: the surface density, 120 M ⊙ pc−2, is comparable to that of a standard giant molecular cloud; the cloud’s virial ratio of ∼1.8 is in the star formation range; and the gas consumption time, 0.5 Gyr, at the present star formation rate is typical for molecular regions. The low metallicity implies that the cloud has an average visual extinction of only 0.8 mag, which is close to the threshold for molecule formation. With such an extinction threshold, molecular clouds in metal-poor regions should have high surface densities and high internal pressures. If high pressure is associated with the formation of massive clusters, then metal-poor galaxies such as dwarfs in the early universe could have been the hosts of metal-poor globular clusters.

Dust reddening and extinction curves toward gamma-ray bursts at z > 4

Context. Dust is known to be produced in the envelopes of asymptotic giant branch (AGB) stars, the expanded shells of supernova (SN) remnants, and in situ grain growth within the interstellar medium (ISM), although the corresponding efficiency of each of these dust formation mechanisms at different redshifts remains a topic of debate. During the first Gyr after the Big Bang, it is widely believed that there was not enough time to form AGB stars in high numbers, hence the dust at this epoch is expected to be purely from SNe or subsequent grain growth in the ISM. The time period corresponding to z ~ 5−6 is thus expected to display the transition from SN-only dust to a mixture of both formation channels as is generally recognized at present. Aims. Here we aim to use afterglow observations of gamma-ray bursts (GRBs) at redshifts larger than z > 4 to derive host galaxy dust column densities along their line of sight and to test if a SN-type dust extinction curve is required for some of the bursts. Methods. We performed GRB afterglow observations with the seven-channel Gamma-Ray Optical and Near-infrared Detector (GROND) at the 2.2 m MPI telescope in La Silla, Chile (ESO), and we combined these observations with quasi-simultaneous data gathered with the XRT telescope on board the Swift satellite. Results. We increase the number of measured AV values for GRBs at z > 4 by a factor of ~2–3 and find that, in contrast to samples at mostly lower redshift, all of the GRB afterglows have a visual extinction of AV < 0.5 mag. Analysis of the GROND detection thresholds and results from a Monte Carlo simulation show that although we partly suffer from an observational bias against highly extinguished sight-lines, GRB host galaxies at 4 < z < 6 seem to contain on average less dust than at z ~ 2. Additionally, we find that all of the GRBs can be modeled with locally measured extinction curves and that the SN-like dust extinction curve, as previously found toward GRB 071025, provides a better fit for only two of the afterglow SEDs. However, because of the lack of highly extinguished sight lines and the limited wavelength coverage we cannot distinguish between the different scenarios. For the first time we also report a photometric redshift of zphot = 7.88-0.94+0.75 for GRB 100905A, making it one of the most distant GRBs known to date.

The 15 273 Å diffuse interstellar band in the dark cloud Barnard 68

High obscuration of background stars behind dark clouds precludes the detection of optical diffuse interstellar bands (DIBs) and hence our knowledge of DIB carriers in these environments. Taking advantage of the reduced obscuration of starlight in the near-infrared (NIR) we used one of the strongest NIR DIBs at 15 273 Å to probe the presence and properties of its carrier throughout the nearby interstellar dark cloud Barnard 68. We measured equivalent widths (EW) for different ranges of visual extinction AV, using VLT/KMOS H-band (1.46–1.85 μm) moderate-resolution (R ~ 4000) spectra of 43 stars situated behind the cloud. To do so, we fitted the data with synthetic stellar spectra from the APOGEE project and TAPAS synthetic telluric transmissions appropriate for the observing site and time period. The results show an increase of DIB EW with increasing AV. However, the rate of increase is much flatter than expected from the EW-AV quasi-proportionality established for this DIB in the Galactic diffuse interstellar medium. Based on a simplified inversion assuming sphericity, it is found that the volume density of the DIB carrier is 2.7 and 7.9 times lower than this expected average value in the external and central regions of the cloud, which have nH≃ 0.4 and 3.5 × 105 cm-3, respectively. Further measurements with multiplex NIR spectrographs should allow detailed modeling of such an edge effect of this DIB and other bands and help clarify its actual origin.

New insights into the study of magnetic field in the clumpy torus of AGN using near-infrared polarimetry

We present J, H and Kn imaging polarimetry of IC5063. NIR polarimetry observations may advance our understanding of the mechanisms of polarisation and hence magnetic field strength in the torus of AGN. In these proceedings we summarize the polarisation results of the central 1.2″ aperture (~263 pc) of IC5063. We present a simple polarising model to account for various mechanisms of polarisation in the central regions of IC5063. The model is consistent with dichroic absorption from diffuse stellar emission through dust in the nuclear bulge and electron scattering as dominant mechanisms of polarisation at J and H. Dichroic absorption from aligned dust grains within the torus of IC5063 is dominant at Kn with a visual extinction of Av = 48±2 mag by the torus. Through the use of various components to the central engine of IC5063, we estimated the intrinsic polarisation from dichroic absorption to be . The dust grain alignment mechanism is assumed to be produced by paramagnetic relaxation. In this case, the intrinsic polarisation, , and the visual extinction by the torus, Av, are related with the magnetic field strength. The magnetic field strength is estimated to be in the range of 12–128 mG, for the physical conditions and environments of the gas and dust in the NIR emitting regions of the torus of IC5063.

THESPITZERSURVEY OF INTERSTELLAR CLOUDS IN THE GOULD BELT. V. OPHIUCHUS NORTH OBSERVED WITH IRAC AND MIPS

We present Spitzer IRAC (2.1 deg2) and MIPS (6.5 deg2) observations of star formation in the Ophiuchus North (Oph N) molecular clouds. This fragmentary cloud complex lies on the edge of the Sco–Cen OB association, several degrees to the north of the well-known ρ Oph star-forming region, at an approximate distance of 130 pc. The Oph N clouds were mapped as part of the Spitzer Gould Belt project under the working name "Scorpius." In the regions mapped, selected to encompass all the cloud with visual extinction AV > 3, 11 young stellar object (YSO) candidates are identified, 8 from IRAC/MIPS color-based selection and 3 from Two Micron All Sky Survey (2MASS) KS/MIPS colors. Adding to one source previously identified in L43, this increases the number of YSOcs identified in Oph N to 12. During the selection process, four color-based YSOcs were rejected as probable asymptotic giant branch stars and one as a known galaxy. The sources span the full range of YSOc classifications from Class 0/1 to Class III, and starless cores are also present. Twelve high extinction (AV > 10) cores are identified with a total mass of ∼100 M☉. These results confirm that there is little ongoing star formation in this region (instantaneous star formation efficiency <0.34%) and that the bottleneck lies in the formation of dense cores. The influence of the nearby Upper Sco OB association, including the 09V star ζ Oph, is seen in dynamical interactions and raised dust temperatures but has not enhanced levels of star formation in Oph N.

Probing the anomalous extinction of four young star clusters: the use of colour-excess, main-sequence fitting and fractal analysis

Four young star clusters were studied in order to characterize their anomalous extinction or variable reddening that could be due to a possible contamination by dense clouds or circumstellar effects. The extinction law (Rv) was evaluated by adopting two methods: (i) the use of theoretical expressions based on the colour-excess of stars with known spectral type, and (ii) the analysis of two-colour diagrams, where the slope of observed colours distribution is compared to the normal distribution. An algorithm to reproduce the zero age main sequence (ZAMS) reddened colours was developed in order to derive the average visual extinction (Av) that provides the best fitting of the observational data. The structure of the clouds was evaluated by means of statistical fractal analysis, aiming to compare their geometric structure with the spatial distribution of the cluster members. The cluster NGC 6530 is the only object of our sample showing anomalous extinction. In average, the other clusters are suffering normal extinction, but several of their members, mainly in NGC 2264, seem to have high Rv, probably due to circumstellar effects. The ZAMS fitting provides Av values that are in good agreement with those found in the literature. The fractal analysis shows that NGC 6530 has a centrally concentrated distribution of stars that is different of the sub-structures found in the density distribution of the cloud projected in the Av map, suggesting that the original cloud has been changed with the cluster formation. On the other hand, the fractal dimension and the statistical parameters of Berkeley 86, NGC 2244, and NGC 2264 indicate a good cloud-cluster correlation, when compared to other works based on artificial distribution of points.

Chemical differentiation toward the Pipe nebula starless cores

We used the new IRAM 30-m FTS backend to perform an unbiased ~15 GHz wide survey at 3 mm toward the Pipe Nebula young diffuse starless cores. We found an unexpectedly rich chemistry. We propose a new observational classification based on the 3 mm molecular line emission normalized by the core visual extinction (Av). Based on this classification, we report a clear differentiation in terms of chemical composition and of line emission properties, which served to define three molecular core groups. The "diffuse" cores, Av<~15, show poor chemistry with mainly simple species (e.g. CS and CCH). The "oxo-sulfurated" cores, Av~15--22, appear to be abundant in species like SO and SO2, but also in HCO, which seem to disappear at higher densities. Finally, the "deuterated" cores, Av>~22, show typical evolved chemistry prior to the onset of the star formation process, with nitrogenated and deuterated species, as well as carbon chain molecules. Based on these categories, one of the "diffuse" cores (Core 47) has the spectral line properties of the "oxo-sulfurated" ones, which suggests that it is a possible failed core.

The deep 3.4 μm interstellar absorption feature toward the IRAS 18511+0146 cluster

Context. An absorption feature at 3.4 μm, widely observed in the diffuse interstellar medium (ISM), is produced by aliphatic hydrocarbon dust. In the local ISM there is in general a linear correlation between the 3.4 μm optical depth and the visual extinction AV . However, the 3.4 μm optical depth toward the young stellar object IRAS 18511+0146, the highest found in the local diffuse ISM apart from the Galactic centre lines of sight, is three times higher than predicted from the most likely value for the distance of the object, the extinction-distance relation for the Galactic diffuse ISM, and the value of AV /τ3.4 in the Galactic disk. Aims. IRAS 18511+0146 is associated with a small cluster of stars. We have obtained spectra of it as well as two other cluster members in order to verify the unusually high 3.4 μm optical depth toward IRAS 18511+0146 and test for patchiness in the foreground diffuse cloud material. Methods. Spectra covering 2.9–4.1 μm were recorded using the near-infrared imager/spectrograph (NIRI) at the Gemini North telescope. Results. After subtraction of the superimposed 3.47 μm band, produced in dense interstellar gas associated with the cluster, we have determined optical depths of 0.06–0.08 for the 3.4 μm absorption on the three lines of sight. We discuss the distance and extinction to the IRAS 18511+0146 cluster and possible interpretations of the high optical depth of the 3.4 μm absorption. Conclusions. The most probable distance to the IRAS 18511+0146 cluster is 3.9 kpc. Two different interpretations of the high observed τ3.4 are then possible. (i) The AV to distance ratio toward the cluster is approximately the one observed on average in the Galactic diffuse ISM, in which case AV ∼ 7a ndτ3.4 in this direction is three times higher than predicted by the usual AV /τ3.4 observed in local diffuse ISM. (ii) The visual extinction in the direction of the IRAS 18511+0146 cluster is higher (i.e. 15–20 mag), probably due to a diffuse cloud associated with the cluster, resulting in a high value of τ3.4. In either case, the high τ3.4 makes this cluster a unique and valuable line of sight for studying the Galactic hydrocarbon dust.

The dust extinction curves of gamma-ray burst host galaxies

The composition and amount of interstellar dust within gamma-ray burst (GRB) host galaxies is of key importance when addressing selection effects in the GRB redshift distribution, and when studying the properties of their host galaxies. As well as the implications for GRB research, probing the dust within the high-z hosts of GRBs also contributes to our understanding of the conditions of the interstellar medium and star-formation in the distant Universe. Nevertheless, the physical properties of dust within GRB host galaxies continues to be a highly contended issue. In this paper we explore the mean extinction properties of dust within the host galaxies of a sample of 17 GRBs with total host galaxy visual extinction Av<1 (<Av>=0.4), covering a redshift range z=0.7-3.1. We find the average host extinction curve to have an ultraviolet slope comparable to that of the LMC, but with little evidence of a 2175Angs dust extinction feature as observed along Milky Way and LMC sightlines. We cannot at present rule out the presence of a 2175Angs feature, and both the standard SMC and LMC extinction curves also provide good fits to our data. However, we can reject an extinction curve that has a UV slope as flat as the mean Milky Way extinction curve, whilst also having a 2175Angs feature as prominent as seen in the mean Milky Way extinction curve. This is in contrast to the clear detection of a 2175Angs bump and the flatter extinction curves of some more heavily extinguished GRBs (Av>1), which may be indicative of there being a dependence between dust abundance and the wavelength dependence of dust extinction, as has been previously speculated.

GRB 100614A and GRB 100615A: two extremely dark gamma-ray bursts

Dark gamma-ray bursts (GRBs) are sources with a low optical-to-X-ray flux ratio. Proposed explanations for this darkness are: i) the GRB is at high redshift ii) dust in the GRB host galaxy absorbs the optical/NIR flux iii) GRBs have an intrinsically faint afterglow emission. Within this framework, GRB 100614A and GRB 100615A are extreme. In fact, they are bright in the X-rays, but no optical/NIR afterglow has been detected for either source, despite several follow-up campaigns began early after the triggers. We build optical-to-X-ray spectral energy distributions (SEDs) at the times at which the reddest upper limits are available, and we model our SEDs with the extinction curves of the Milky Way (MW), Small Magellanic Cloud (SMC), and the attenuation curve obtained for a sample of starburst galaxies. We find that to explain the deepest NIR upper limits assuming either a MW or SMC extinction law, a visual extinction of AV > 50 is required, which is extremely unlikely. Since both GRBs are bright in X-rays, explanation iii) also cannot explain their dark classification, unless optical radiation and X-rays are not part of the same synchrotron spectrum. An alternative, or complementary explanation of the previous possibility, involves greyer extinction laws. A starburst attenuation curve gives AV>10, which is less extreme, despite still very high. Assuming high redshift in addition to extinction, implies an AV>10 at z=2 and AV>4-5 at z=5, regardless of the adopted extinction recipe. A different, exotic possibility would be an extremely high redshift origin (z>17 given the missing K detections). Population III stars are expected to emerge at z ~ 20 and can produce GRBs with energies well above those inferred for our GRBs at these redshifts. Mid- and far-IR observations of these extreme class of GRBs can help us to differentiate between the proposed scenarios.

The Relation Between Dust and Gas in the Taurus Molecular Cloud

We report a study of the relation between dust and gas over a 100 deg2 area in the Taurus molecular cloud. We compare the H2 column density derived from dust extinction with the CO column density derived from the 12CO and 13CO J = 1 → 0 lines. We derive the visual extinction from reddening determined from 2MASS data. The comparison is done at an angular size of 200′′, corresponding to 0.14 pc at a distance of 140 pc. We find that the relation between visual extinction AV and N(CO) is linear between AV ≃ 3 and 10 mag in the region associated with the B213–L1495 filament. In other regions the linear relation flattens for AV ≳ 4 mag. Accounting for the observed relation between the column density of CO and CO2 ices and AV, we find a linear relationship between the column of carbon monoxide and dust for observed visual extinctions up to the maximum value in our data ≃23 mag.

Statistical universal branching ratios for cosmic ray dissociation, photodissociation, and dissociative recombination of the C, CH and C3H2neutral and cationic species

Fragmentation branching ratios of electronically excited molecular species are of first importance for the modeling of gas phase interstellar chemistry. Despite experimental and theoretical efforts that have been done during the last two decades there is still a strong lack of detailed information on those quantities for many molecules such as Cn, CnH or C3H2. Our aim is to provide astrochemical databases with more realistic branching ratios for Cn (n=2 to 10), CnH (n=2 to 4), and C3H2 molecules that are electronically excited either by dissociative recombination, photodissociation, or cosmic ray processes, when no detailed calculations or measurements exist in literature. High velocity collision in an inverse kinematics scheme was used to measure the complete fragmentation pattern of electronically excited Cn (n=2 to 10), CnH (n=2 to 4), and C3H2 molecules. Branching ratios of dissociation where deduced from those experiments. The full set of branching ratios was used as a new input in chemical models and branching ratio modification effects observed in astrochemical networks that describe the dense cold Taurus Molecular Cloud-1 and the photon dominated Horse Head region. The comparison between the branching ratios obtained in this work and other types of experiments showed a good agreement. It was interpreted as the signature of a statistical behavior of the fragmentation. The branching ratios we obtained lead to an increase of the C3 production together with a larger dispersion of the daughter fragments. The introduction of these new values in the photon dominated region model of the Horse Head nebula increases the abundance of C3 and C3H, but reduces the abundances of the larger Cn and hydrocarbons at a visual extinction Av smaller than 4.

THE RELATION BETWEEN GAS AND DUST IN THE TAURUS MOLECULAR CLOUD

(abridged) We report a study of the relation between dust and gas over a 100deg^2 area in the Taurus molecular cloud. We compare the H2 column density derived from dust extinction with the CO column density derived from the 12CO and 13CO J= 1-0 lines. We derive the visual extinction from reddening determined from 2MASS data. The comparison is done at an angular size of 200", corresponding to 0.14pc at a distance of 140pc. We find that the relation between visual extinction Av and N(CO) is linear between Av~3 and 10 mag in the region associated with the B213--L1495 filament. In other regions the linear relation is flattened for Av > 4 mag. We find that the presence of temperature gradients in the molecular gas affects the determination of N(CO) by ~30--70% with the largest difference occurring at large column densities. Adding a correction for this effect and accounting for the observed relation between the column density of CO and CO2 ices and Av, we find a linear relationship between the column of carbon monoxide and dust for observed visual extinctions up to the maximum value in our data 23mag. We have used these data to study a sample of dense cores in Taurus. Fitting an analytical column density profile to these cores we derive an average volume density of about 1.4e4 cm^-3 and a CO depletion age of about 4.2e5 years. We estimate the H2 mass of Taurus to be about 1.5e4 M_sun, independently derived from the Av and N(CO) maps. We derive a CO integrated intensity to H2 conversion factor of about 2.1e20 cm^-2 (K km/s)^-1, which applies even in the region where the [CO]/[H_2] ratio is reduced by up to two orders of magnitude. The distribution of column densities in our Taurus maps resembles a log--normal function but shows tails at large and low column densities.