Abstract We present a systematic study of the 3.0 μm H2O ice and the 3.4 μm aliphatic carbon absorption features toward 48 local ultraluminous infrared galaxies (ULIRGs) using spectra obtained by the AKARI Infrared Camera to investigate the UV environment in their star-forming regions. All the ULIRGs in our sample exhibit a ratio of optical depth of H2O ice to silicate dust (τ3.0/τ9.7) that is lower than that in the Taurus dark cloud. This implies that ULIRGs cannot be described as an ensemble of low-mass star-forming regions and that a significant amount of high-mass star-forming regions contribute to star-forming clouds in local ULIRGs. The results also show that the ratios of optical depth of aliphatic carbon to silicate dust, τ3.4/τ9.7, exhibit diverse values. We investigate two effects that can affect this ratio: the geometric temperature gradient (which increases the ratio) and the intense UV environment (which decreases it). The geometric temperature gradient is typically considered as a sign of active galactic nuclei (AGN). ULIRGs with AGN signs (optical classification, near-infrared color, and a polycyclic aromatic hydrocarbon emission strength of $3.3\, \mu$m) indeed tend to exhibit a large τ3.4/τ9.7 ratio. However, we find that the presence of buried AGN is not the only cause of the geometric temperature gradient, because the enhancement of the ratio is also evident in pure starburst-like ULIRGs without these AGN signs. Regarding the intense UV environment in star-forming regions, the correlation between the aliphatic carbon ratio and the ratio of the [C ii] 158 μm line luminosity to the far-infrared luminosity ($L_{[\mathrm{C}\,\,\small {ii}]}/L_{\mathrm{FIR}}$), which represents the UV environment in photodissociation regions, implies that the intense UV environment causes the decrease of the aliphatic carbon ratio. We find that an intense UV environment (G/nH > 3) in star-forming regions is needed for the aliphatic carbon ratio to be suppressed.

This paper presents spectra in the 2 to 20 micron range of quiescent cloud material located in the IC 5146 cloud complex. The spectra were obtained with NASA's Infrared Telescope Facility (IRTF) SpeX instrument and the Spitzer Space Telescope's Infrared Spectrometer. We use these spectra to investigate dust and ice absorption features in pristine regions of the cloud that are unaltered by embedded stars. We find that the H2O-ice threshold extinction is 4.03+/-0.05 mag. Once foreground extinction is taken into account, however, the threshold drops to 3.2 mag, equivalent to that found for the Taurus dark cloud, generally assumed to be the touchstone quiescent cloud against which all other dense cloud and embedded young stellar object observations are compared. Substructure in the trough of the silicate band for two sources is attributed to CH3OH and NH3 in the ices, present at the ~2% and ~5% levels, respectively, relative to H2O-ice. The correlation of the silicate feature with the E(J-K) color excess is found to follow a much shallower slope relative to lines of sight that probe diffuse clouds, supporting the previous results by Chiar et al. (2007).

Abstract The value of different forms of energy contained in TDC region as well as the structure and physical characteristics of star formation in the Taurus Dark Clouds are analyzed.

Normal field stars located behind dense clouds are a valuable resource in interstellar astrophysics, as they provide continua in which to study phenomena such as gas-phase and solid-state absorption features, interstellar extinction, and polarization. This paper reports the results of a search for highly reddened stars behind the Taurus Dark Cloud complex. We use the Two Micron All Sky Survey (2MASS) Point Source Catalog to survey a ~50 deg2 area of the cloud to a limiting magnitude of Ks = 10.0. Photometry in the 1.2-2.2 μm passbands from 2MASS is combined with photometry at longer infrared wavelengths (3.6-12 μm) from the Spitzer Space Telescope and the Infrared Astronomical Satellite to provide effective discrimination between reddened field stars and young stellar objects (YSOs) embedded in the cloud. Our final catalog contains 248 confirmed or probable background field stars, together with estimates of their total visual extinctions, which span the range 2 < AV < 29 mag. We also identify the 2MASS source J04292083+2742074 (IRAS 04262+2735) as a previously unrecognized candidate YSO, based on the presence of infrared emission greatly in excess of that predicted for a normal reddened photosphere at wavelengths >5 μm.

We present spectropolarimetry of the solid CO feature at 4.67 μm along the line of sight to Elias 16, a field star background to the Taurus dark cloud. A clear increase in polarization is observed across the feature with the peak of polarization shifted in wavelength relative to the peak of absorption. This shows that dust grains in dense, cold environments (temperatures ∼20 K or less) can align and produce polarization by dichroic absorption. For a grain model, consisting of a core with a single mantle, the polarization feature is best modelled by a thick CO mantle, possibly including 10 per cent water-ice, with the volume ratio of mantle to bare grain of ∼5. Radiative torques could be responsible for the grain alignment provided the grain radius is at least 0.5 μm. This would require the grain cores to have a radius of at least 0.3 μm, much larger than grain sizes in the diffuse interstellar medium. Sizes of this order seem reasonable on the basis of independent evidence for grain growth by coagulation, as well as mantle formation, inside dense clouds.

Galactic extinction is tested using galaxy number counts at low Galactic latitude obtained from five-band photometry of the Sloan Digital Sky Survey. The spatial variation of galaxy number counts for low-extinction regions of E(B - V) 0.15, however, the map of Schlegel and coworkers overestimates the reddening by a factor of up to 1.4, which can likely be ascribed to the departure from proportionality of reddening to infrared emissivity of dust. This result is consistent with the analysis of Arce & Goodman for the Taurus dark cloud complex.

In this study I present new K and L$'$-band infrared photometry and 2-4$\mu$m spectra of ten Class I and flat spectrum stars forming within the Taurus dark cloud complex. Nine sources have H$_2$ {\it v}=0-1 S(1) emission, and some show multiple H$_2$ emission features in their K-band spectra. Photospheric absorptions characteristic to low mass stars are detected in five of the targets, and these stars were fit with models to determine spectral type, infrared accretion excess veiling (r$_K$ and r$_{L'}$) and dust temperatures, estimates of visual extinction and characteristics of the 3$\mu$m water-ice absorption. On average, the models found high extinction values, infrared accretion excess emission with blackbody temperatures in the 900-1050K range, and 3$\mu$m absorption profiles best fit by water frozen onto cold grains rather than thermally processed ice. Five techniques were used to estimate the extinction toward the stellar photospheres; most gave vastly different results. Analysis of emission line ratios suggests that the effect of infrared scattered light toward some protostars should not be neglected. For stars that exhibited Br$\gamma$ in emission, accretion luminosities were estimated using relations between L$_{acc}$ and Br$\gamma$ luminosity. The young stars in this sample were preferentially chosen as variables, but they do not have the accretion dominated luminosities necessary to put them in their main stage of mass-building. The characteristics of the 2-4$\mu$m spectra are placed in the context of existing multi-wavelength data, and five of the stars are more consistent with reddened Class IIs or stars in transition between Class I and II, rather than protostars embedded within massive remnant envelopes.

Ultraviolet extinction curves for three early-type stars that sample dust in the Taurus dark cloud in the vicinity of TMC-1 are analyzed to examine the contribution of internal (intraclump) dust to the total line-of-sight extinction. One of our selected targets samples only dust in the "diffuse-screen" component of the cloud, where the dust grains lack ice mantles and have optical properties similar to average "diffuse-ISM" dust. Comparison with data for more heavily reddened stars that also sample dust within TMC-1 (where grains are mantled) allows us to determine the intraclump extinction. Our principal result is that the 2175 Å "extinction bump" is extremely weak or absent inside the clump. The feature present in the observed extinction curves appears to arise almost entirely in the diffuse screen. This dramatic reduction in bump amplitude in the high-density environment is consistent with the prediction of the graphite model that the feature is suppressed by hydrogenation. Amplitude reduction is accompanied by a more subtle change in the profile of the bump, specifically, an increase in width. Comparison of Taurus and other clouds strongly suggests that the increase in width results from accumulation of surface coatings on the carrier grains rather than from particle clustering.

Abstract Seven-color photometry in the Vilnius system has been obtained for 420 stars down to V = 16 mag in the area containing the overlapping open clusters NGC 1750 and NGC 1758 in Taurus. Spectral and luminosity classes, color excesses, interstellar extinctions and distances are given for 287 stars. The classification of stars is based on their reddening-free Q-parameters. 18 stars observed photoelectrically were used as standards. The extinction vs. distance diagram exhibits the presence of one dust cloud at a distance of 175 pc which almost coincides with a distance of other dust clouds in the Taurus complex. The clusters NGC 1750 and NGC 1758 are found to be at the same distance of ~760 pc and may penetrate each other. Their interstellar extinction A

Preliminary results from a deep R, I and Z band survey of ~ 6 square degrees of the Taurus Dark Cloud region are presented. 186 potential brown dwarfs have been unearthed, with seven having follow-up spectroscopic data. The spectra reveal three mid/late-type M dwarfs, of which two show weak Hα emission. If these objects are members of the TDC region, theoretical models suggest masses in the range 10 – 20 MJ.

Abstract Spectral and luminosity classes, absolute magnitudes, color excesses, interstellar extinctions and distances are determined for 98 stars down to 18 mag in the Barnard 1 dark cloud belonging to the Per OB2 association. The classification of stars is based on their photoelectric photometry in the Vilnius seven-color photometric system. The extinction vs. distance diagram exhibits the presence of two dust layers at 150 and 230 pc distances. The distance of the first cloud, which gives an extinction Ay of 0.3 mag, coincides with the distance of the Taurus dark cloud complex. The second cloud with much larger extinction is about at the same distance as the clouds in the direction of the nearby objects: reflection nebula NGC 1333 and open cluster IG 348.

We calculate the extinction and polarization for aligned dust aggregates consisting of 4, 8, 16, 32, and 64 spherical particles (monomers). We consider a power-law size distribution of monomers and optical constants related to interstellar values. Our calculations match the polarization and extinction by the general diffuse interstellar medium. They also match the linear correlation between the observed shift (increase) of the wavelength of maximum polarization with increasing total to selective extinction (RV). The model naturally incorporates the low values of RV at small wavelength of maximum polarization observed in molecular clouds at high Galactic latitude. A further result of the calculations is a systematic and correlated decrease in polarization efficiency with increasing aggregate size, ruling out very large aggregates if polarization is observed. Our results also support the idea that the observed change of features in molecular clouds such as the Taurus dark clouds is rather the result of a change in alignment than a change in particle size, although the small degree of coagulation that can still be inferred is in agreement with a simple coagulation model. Therefore, small aggregates in a limited size range serve as a good and most natural model structure for interstellar dust particles, with the number of monomers being the only parameter to be adjusted.

ABSTRA C T To better understand the conditions under which ice mantles form on grains in molecular clouds, three globules in the Southern Coalsack have been searched for the presence of H2O ice. Given the total lack of star formation in the Coalsack, it is an ideal site for studying unprocessed icy molecular mantles. In our sample of eight field stars lying behind the Coalsack we detect strong H2O ice absorption in the lines of sight to two stars and possible weak absorption in four others. We estimate H2O ice column densities or upper limits for these lines of sight. Compared to dark clouds such as Taurus, the Coalsack H2O ice column densities are lower than expected given the quiescent nature of the Coalsack region. It is possible that the chemical evolution of the Coalsack may simply be at too early a stage for significant ice mantles to appear on the grains, except perhaps in the densest parts of some of the globules. Alternatively, the presence or absence of ice absorption may be related to the distribution of dust along each line of sight, specifically, the relative contributions of dense globules and a more extended diffuse component. For example, our observations are consistent with an ice threshold extinction similar to that observed in the Taurus dark cloud if extinction amounting to AV , 5 towards Globules 2 and 3 arises in the extended component. Globule 1 appears to have no extended component.

The detection of water ice in interstellar grain mantles by observation of absorbed radiation from field stars through the Taurus molecular cloud complex has revealed that the column density of water ice increases linearly or nearly linearly with visual extinction except at the lowest values (below . Previous explanations of the small or negligible abundance of ice at low extinction have focused on the destruction and desorption of ice by radiation, and have ignored the efficiency of formation of ice. To understand the dependence of the ice column density on AV in more detail and to understand what it tells us about the Taurus dark cloud, we have run gas–grain chemical models under a variety of physical conditions. For higher values of extinction, our models predict that much of the elemental abundance of available oxygen is in the form of ice for evolutionary times greater than about 105yr, and this leads to the observed near linear relationship between ice column density and AV. For lower extinction, our models show that if any significant amount of ice is still present, its existence can be attributed to great cloud age, to our poor understanding of dust grain surfaces, or to cloud clumpiness. The first and third explanations pertain only if photodesorption of ice is inefficient.

I present a comprehensive study of the MBM12 young association (MBM12A). By combining infrared (IR) photometry from the Two-Micron All-Sky Survey (2MASS) survey with new optical imaging and spectroscopy, I have performed a census of the MBM12A membership that is complete to 0.03 Msun (H~15) for a 1.75deg X 1.4deg field encompassing the MBM12 cloud. I find five new members with masses of 0.1-0.4 Msun and a few additional candidates that have not been observed spectroscopically. From an analysis of optical and IR photometry for stars in the direction of MBM12, I identify M dwarfs in the foreground and background of the cloud. By comparing the magnitudes of these stars to those of local field dwarfs, I arrive at a distance modulus 7.2+/-0.5 (275 pc) to the MBM12 cloud; it is not the nearest molecular cloud and is not inside the local bubble of hot ionized gas as had been implied by previous distance estimates of 50-100 pc. I have also used Li strengths and H-R diagrams to constrain the absolute and relative ages of MBM12A and other young populations; these data indicate ages of 2 +3/-1 Myr for MBM12A and 10 Myr for the TW Hya and Eta Cha associations. MBM12A may be a slightly evolved version of the aggregates of young stars within the Taurus dark clouds (~1 Myr) near the age of the IC 348 cluster (~2 Myr).

Vilnius seven-color photometry has been obtained for 238 stars down to ~13 mag in the area of the California Nebula in Perseus. For nearly all of the stars, photometric spectral classes, luminosity classes, absolute magnitudes, interstellar reddenings, extinctions and distances are determined. The "extinction versus distance" diagrams give evidence for the presence of one dust layer at ~160 pc distance in the direction of the California Nebula and its nearest surroundings and of two dust layers at distances of 160 pc and 300 pc north-west of the nebula, in the direction of the dark clouds L 1449 and L 1456. The front layer contributes extinction between 0.3 and 1.3 mag, and the second layer gives about 1 mag of additional extinction. It appears likely that the front dust layer is the extension of the Taurus dark clouds. The second dust layer probably belongs to the complex of dark clouds found in other areas of the southern part of Perseus (in the directions of the open cluster IC 348, the reflection nebula NGC 1333, etc.). Both cloud complexes run more or less parallel to the Galactic plane.

Observations of interstellar linear polarization in the spectral range 0.35-2.2??m are presented for several stars reddened by dust in the Taurus region. Combined with a previously published study by Whittet et al., these results represent the most comprehensive data set available on the spectral dependence of interstellar polarization in this nearby dark cloud (a total of 27 sight lines). Extinction data for these and other reddened stars in Taurus are assembled for the same spectral range, combining published photometry and spectral classifications with photometry from the Two Micron All Sky Survey. The polarization and extinction curves are characterized in terms of the parameters ?max (the wavelength of maximum polarization) and RV (the ratio of total to selective extinction), respectively. The data are used to investigate in detail the question of whether the optical properties of the dust change systematically as a function of environment, considering stars observed through progressively more opaque (and thus progressively denser) regions of the cloud. At low visual extinctions (0 3, real changes in grain properties occur, characterized by observed RV values in the range 3.5-4.0. A simple model for the development of RV with AV suggests that RV may approach values of 4.5 or more in the densest regions of the cloud. The transition between normal extinction and dense cloud extinction occurs at AV ~ 3.2, a value coincident with the threshold extinction above which H2O-ice is detected on grains within the cloud. Changes in RV are thus either a direct consequence of mantle growth or occur under closely similar physical conditions. Dust in Taurus appears to be in a different evolutionary state compared with other nearby dark clouds, such as ? Oph, in which coagulation is the dominant physical process.

The observations made by the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST) of molecular CO in absorbing gas towards X Persei are reported. The two-component statistical equilibrium model incorporating radiative excitation of CO by line emission at the same velocity that originates in nearby molecular clouds has been used to reproduce high-resolution GHRS spectra. Earlier analysis indicates that the cloud has a complex structure and at least a two-component model should be used to obtain accurate results. The spectra obtained from the International Ultraviolet Explorer (IUE) were used to complement GHRS data and constrain the space of possible solutions. The new oscillator strengths recommended by Eidelsberg et al. for A–X bands have been used. The results show that one of the components may be attributed to the Perseus OB2 molecular cloud, and the other component to an extension of the Taurus dark cloud. The total CO column density N(CO)=(1.0±0.2)×1016 cm−2 has been determined. According to the results about 85 per cent of the observed CO belongs to an extension of the Taurus dark cloud. The CO radiation that originates in nearby molecular clouds may be the dominant excitation mechanism of the observed CO. The early results of 13CO line analysis indicate a 13CO/12CO ratio of about 40.

We present a study of the detailed distribution of extinction in a region of the Taurus dark cloud complex. Our study uses new BVR images of the region, spectral classification data for 95 stars, and IRAS Sky Survey Atlas (ISSA) 60 and 100 μm images. We study the extinction of the region in four different ways, and we present the first intercomparison of all these methods, which are as follows: (1) using the color excess of background stars for which spectral types are known, (2) using the ISSA 60 and 100 μm images, (3) using star counts, and (4) using an optical (V and R) version of the average color excess method used by Lada et al. We find that all four methods give generally similar results—with important exceptions. As expected, all the methods show an increase in extinction due to dense dusty regions (i.e., dark clouds and IRAS cores) and a general increase in extinction with increasing declination, due to a larger content of dust in the northern regions of the Taurus dark cloud complex. Some of the discrepancies between the methods are caused by assuming a constant dust temperature for each line of sight in the ISSA extinction maps and not correcting for unexpected changes in the background stellar population (i.e., the presence of a cluster or Galactic gradients in the stellar density and average V-R color). To study the structure in the dust distribution, we compare the ISSA extinction and the extinction measured for individual stars. From the comparison, we conclude that in the relatively low-extinction regions studied, with 0.9<AV<3.0 mag (away from filamentary dark clouds and IRAS cores), there are no fluctuations in the dust column density greater than 45% (at the 99.7% confidence level), on scales smaller than 0.2 pc. We also report the discovery of a previously unknown open cluster of stars behind the Taurus dark cloud near R.A. 4h19m, decl. 27°30' (B1950).

We present near-infrared spectropolarimetric data between 0.9 and 4.2 μm for three prototypes of low-mass young stellar objects (YSOs), L1551 IRS 5, HL Tau, and T Tau in the Taurus dark cloud. These sources are in different classes in the standard spectral classification scheme of low-mass YSOs by Lada. The polarization curves of the observed sources show distinct differences. The class I protostar L1551 IRS 5 shows a flat polarization curve with high polarization through the observed wavelengths. It also shows a polarization excess at the 3.1 μm ice band absorption feature. The flat-spectrum HL Tau, which is thought to be in a transient phase from class I to class II, shows a steep decrease of polarization with increasing wavelengths from 1.0 to 2.5 μm, while it shows a flat polarization curve with high polarization in optical wavelengths and a slowly decreasing slope with small polarization in 3-4 μm. The class II source T Tau displays small polarization no more than 2% through the observed wavelengths; the polarization in the shorter wavelengths from optical to 1.3 μm decreases with increasing wavelengths. T Tau also shows an increasing polarization curve in the longer wavelengths over 1.6 μm, which is most likely to come from the infrared companion T Tau S (Kobayashi et al.). The prominent differences of the observed near-infrared polarization curves can be clearly understood in terms of the standard spectral classification scheme of low-mass YSOs. Thus near-infrared spectropolarimetry could serve as a potentially powerful diagnostic of circumstellar material, complementary to the standard spectral classification.