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We have performed optical imaging observations of the dark cloud L1251 at multiple wavelengths, B, V, R, and I, using the 105 cm Schmidt telescope at the Kiso Observatory, Japan. The cloud has a cometary shape with a dense head showing star formation activity and a relatively diffuse tail without any signs of star formation. We derived extinction maps of A_B and A_V with a star count method, and also revealed the color excess (E_{B-V}, E_{V-R}, and E_{V-I}) distributions. On the basis of the color excess measurements we derived the distribution of the ratio of total to selective extinction R_V over the cloud using an empirical relation between R_V and A_lambda/A_V reported by Cardelli et al. In the tail of the cloud, R_V has a uniform value of ~3.2, close to that often found in the diffuse interstellar medium (~3.1), while higher values of R_V=4-6 are found in the dense head. Since R_V is closely related to the size of dust grains, the high R_V-values are most likely to represent the growth of dust grains in the dense star-forming head of the cloud.
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The optical and near-infrared (OIR) polarization of starlight is typically understood to arise from the dichroic extinction of that light by dust grains whose axes are aligned with respect to a local magnetic-field. The size distribution of the aligned-grain population can be constrained by measurements of the wavelength dependence of the polarization. The leading physical model for producing the alignment is radiative alignment-torques (RAT), which predicts that the most efficiently aligned grains are those with sizes larger than the wavelengths of light composing the local radiation field. Therefore, for a given grain-size distribution, the wavelength at which the polarization reaches a maximum ($lambda_mathrm{max}$) should correlate with the characteristic reddening along the line of sight between the dust grains and the illumination source. A correlation between $lambda_mathrm{max}$ and reddening has been previously established for extinctions up to $A_Vapprox4$ mag. We extend the study of this relationship to a larger sample of stars in the Taurus cloud complex, including extinctions $A_V>10$ mag. We confirm the earlier results for $A_V<4$ mag, but find that the $lambda_mathrm{max}$ vs. $A_V$ relationship bifurcates above $A_Vapprox4$ mag, with part of the sample continuing the previously observed relationship and the remaining part exhibiting a significantly steeper rise. We propose that the data exhibiting the steep rise represent lines-of-sight towards high density clumps, where grain coagulation has taken place. We present RAT-based modeling supporting these hypotheses. These results indicate that multi-band OIR polarimetry is a powerful tool for tracing grain growth in molecular clouds, independent of uncertainties in the dust temperature and emissivity.
We demonstrate a facile method to produce crystallographically textured, macroporous materials using a combination of modified ice templating and templated grain growth (TGG). The process is demonstrated on alumina and the lead-free piezoelectric material sodium potassium niobate. The method provides macroporous materials with aligned, lamellar ceramic walls which are made up of crystallographically aligned grains. Each method showed that the ceramic walls present a long-range order over the entire sample dimensions and have crystallographic texture as a result of the TGG process. We also present a modification of the March-Dollase equation to better characterize the overall texture of materials with textured but slightly misaligned walls. The controlled crystallographic and morphologic orientation at two different length scales demonstrated here can be the basis of multifunctional materials.
We searched for shocked carbon chain chemistry (SCCC) sources with C$_3$S abundances surpassing those of HC$_5$N towards the dark cloud L1251, using the Effelsberg telescope at K-band (18 -- 26,GHz). L1251-1 and L1251-3 are identified as the most promising SCCC sources. The two sources harbor young stellar objects. We conducted mapping observations towards L1251-A, the western tail of L1251, at $lambda$ $sim$3,mm with the PMO 13.7 m and the NRO 45 m telescopes in lines of C$_2$H, N$_2$H$^+$, CS, HCO$^+$, SO, HC$_3$N and C$^{18}$O as well as in CO 3--2 using the JCMT. The spectral data were combined with archival data including Spitzer and Herschel continuum maps for further analysis. Filamentary sub-structures labeled as F1 to F6 were extracted in L1251, with F1 being associated with L1251-A hosting L1251-1. The peak positions of dense gas traced by HCO$^+$ are misaligned relative to those of the dust clumps. Episodic outflows are common in this region. The twisted morphology of F1 and velocity distribution along L1251-A may originate from stellar feedback. SCCC in L1251-1 may have been caused by outflow activities originated from the infrared source IRS1. The signposts of ongoing SCCC and the broadened line widths of C$_3$S and C$_4$H in L1251-1 as well as the distribution of HC$_3$N are also related to outflow activities in this region. L1251-1 (IRS1) together with the previously identified SCCC source IRS3 demonstrate that L1251-A is an excellent region to study shocked carbon chain chemistry.
Data from the Five College Radio Astronomy Observatory CO Mapping Survey of the Taurus molecular cloud are combined with extinction data for a sample of 292 background field stars to investigate the uptake of CO from the gas to icy grain mantles on dust within the cloud. On the assumption that the reservoir of CO in the ices is well represented by the combined abundances of solid CO and solid CO2 (which forms by oxidation of CO on the dust), we find that the total column density (gas + solid) correlates tightly with visual extinction (Av) over the range 5 < Av < 30 mag, i.e., up to the highest extinctions covered by our sample. The mean depletion of gas-phase CO increases monotonically from negligible levels for Av < 5 to approximately 30 percent at Av = 10 and 60 percent at Av = 30. As these results refer to line-of-sight averages, they must be considered lower limits to the actual depletion at loci deep within the cloud, which may approach 100 percent. We show that it is plausible for such high levels of depletion to be reached in dense cores on timescales of order 0.6 Myr, comparable with their expected lifetimes. Dispersal of cores during star formation may be effective in maintaining observable levels of gaseous CO on the longer timescales estimated for the age of the cloud.
Grain growth by accretion of gas-phase metals is a common assumption in models of dust evolution, but in dense gas, where the timescale is short enough for accretion to be effective, material is accreted in the form of ice mantles rather than adding to the refractory grain mass. It has been suggested that negatively-charged small grains in the diffuse interstellar medium (ISM) can accrete efficiently due to the Coulomb attraction of positively-charged ions, avoiding this issue. We show that this inevitably results in the growth of the small-grain radii until they become positively charged, at which point further growth is effectively halted. The resulting gas-phase depletions under diffuse ISM conditions are significantly overestimated when a constant grain size distribution is assumed. While observed depletions can be reproduced by changing the initial size distribution or assuming highly efficient grain shattering, both options result in unrealistic levels of far-ultraviolet extinction. We suggest that the observed elemental depletions in the diffuse ISM are better explained by higher initial depletions, combined with inefficient dust destruction by supernovae at moderate ($n_{rm H} sim 30 {rm , cm^{-3}}$) densities, rather than by higher accretion efficiences.
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