The origin of micrograins

Using ultraviolet and infrared techniques, researchers investigated the origins of the tiny (approx. 10A) grains whose presence in the interstellar medium (ISM) is inferred from near-infrared photometry (Sellgren, Werner, and Dinerstein 1983; Sellgren 1984). The authors consider two possibilities: (1) that the grains are formed by condensation in stellar atmospheres; or (2) that they are formed by fragmentation of larger grains in interstellar shocks. They searched for evidence of very small grains in circumstellar environments by analyzing ultraviolet extinction curves in binaries containing hot companions, and by searching for the 3.3-micron emission feature in similar systems. The ultraviolet extinction curve analysis could be applied only to oxygen-rich systems, where small carbonaceous grains would not be expected, so these results provide only indirect information. Researchers find a deficiency of grains smaller than 800A in oxygen-rich systems, consistent with theoretical models of grain condensation which suggest that grains grow to large sizes before injection into the interstellar medium. More direct information on carbonaceous micrograins was obtained from the search for the 3.3-micron feature in carbon-rich binaries with hot companions, whose ultraviolet flux should excite the tiny grains to emit in the infrared. No 3.3-micron feature was found, suggesting that the micrograins are absent in these systems. In addition to the negative search for micrograins in circumstellar environments, researchers have also studied the possible association of these grains with shocks in the diffuse interstellar medium. Using Infrared Astronomy Satellite (IRAS) colors as indicators of the presence or absence of the small grains (e.g., Ryter, Puget, and Perault 1987 and references cited therein), researchers systematically searched for them in regions (reflection nebulae) expected to have sufficient ultraviolet flux to make them glow in the infrared. They found that the distribution is not uniform. The researchers propose that production of micrograins by fragmentation of larger grains in shocks could explain this uneven distribution

V453 Oph: a s-process enriched, but carbon-deficient RV Tauri star of low intrinsic metallicity

This paper reports the detection of a heavy element enriched RV Tauri variable with an abundance pattern that differs significantly from a standard s-process enriched object: V453 Oph. Based on optical high-resolution spectra, we determined that this object of low intrinsic metallicity ([Fe/H] = -2.2) has a mild, but significant, enrichment ([s/Fe] ~ +0.5) of heavy elements for which the distribution points to slow neutron capture nucleosynthesis. This result is strengthened by a comparative analysis to the non-enriched RV Tauri star DS Aqr ([s/Fe] = 0.0). Although V453 Oph is the first RV Tauri star showing a strong s-process signature, it is NOT accompanied by C enhancement, challenging our current nucleosynthetic models of post-AGB stars that predict a simultaneous enrichment in C and s-process elements. The low N abundance excludes CN cycling as being responsible for the low C abundance. We explore three different scenarios to explain the heavy element distribution in this evolved object: an enrichment of the parental cloud, an accretion scenario in which the chemical patterns were acquired by mass transfer in a binary system and an intrinsic enrichment by dredge-up.Comment: Accepted for publication in A&

Detections of Diffuse Interstellar Bands in the SDSS Low-resolution Spectra

Diffuse interstellar bands (DIBs) have been discovered for almost a century, but their nature remains one of the most challenging problems in astronomical spectroscopy. Most recent work to identify and investigate the properties and carriers of DIBs concentrates on high-resolution spectroscopy of selected sight-lines. In this paper, we report detections of DIBs in the Sloan Digital Sky Survey (SDSS) low-resolution spectra of a large sample of Galactic stars. Using a template subtraction method, we have successfully identified the DIBs $\lambda$$\lambda$5780, 6283 in the SDSS spectra of a sample of about 2,000 stars and measured their strengths and radial velocities. The sample is by far the largest ever assembled. The targets span a large range of reddening, E(B-V) ~ 0.2 -- 1.0, and are distributed over a large sky area and involve a wide range of stellar parameters (effective temperature, surface gravity and metallicity), confirming that the carriers of DIBs are ubiquitous in the diffuse interstellar medium (ISM). The sample is used to investigate relations between strengths of DIBs and magnitudes of line-of-sight extinction, yielding results (i.e., EW(5780)= 0.61 x E(B-V) and EW(6283) = 1.26 x E(B-V)) consistent with previous studies. DIB features have also been detected in the commissioning spectra of the Guoshoujing Telescope (LAMOST) of resolving power similar to that of SDSS. Detections of DIBs towards hundreds of thousands of stars are expected from the on-going and up-coming large scale spectroscopic surveys such as RAVE, SDSS III and LAMOST, particularly from the LAMOST Digital Sky Survey of the Galactic Anti-center (DSS-GAC). Such a huge database will provide an unprecedented opportunity to study the demographical distribution and nature of DIBs as well as using DIBs to probe the distribution and properties of the ISM and the dust extinction.Comment: 10 pages, 5 figures, accepted for publication in MNRA

Diffuse interstellar bands of unprecedented strength in the line of sight towards high-mass X-ray binary <ASTROBJ>4U 1907+09</ASTROBJ>

Contains fulltext : 32411.pdf (publisher's version ) (Open Access)High-resolution VLT/UVES spectra of the strongly reddened O supergiant companion to the X-ray pulsar 4U 1907+09 provide a unique opportunity to study the nature of the diffuse interstellar bands (DIBs) at unprecedented strength. We detect about 180 known DIBs, of which about 25 were listed as tentative and are now confirmed. A dozen new DIB candidates longwards of 6900 Å are identified. We show that the observed 5797 Å DIB strength is in line with the Galactic correlation with reddening, whereas the 5780 Å DIB strength is relatively weak. This indicates the contribution of denser regions, where the UV penetration is reduced. The presence of dense cloud cores is supported by the detection of C2 rotational transitions. Members of one DIB family (5797, 6379 Å and 6196, 6613 Å) behave coherently, although one can make a distinction between the two correlated pairs. The broadened profiles of narrow DIBs are shown to be consistent with the premise that each of the main clouds in the line of sight discerned in the interstellar K I profile is contributing proportionally to the DIB profile. We complement and extend the relation of DIB strength with reddening {E}(B-V), as well as with neutral hydrogen column density N(H I), respectively, using strongly reddened sightlines towards another four distant HMXBs. The 5780 Å DIB, and tentatively also the 5797 and 6613 Å DIBs, are better correlated to the gas tracer H I than to the dust tracer {E}(B-V). The resulting relationship can be applied to any line of sight to obtain an estimate of the H I column density. In the search for the nature of the DIB carrier, this strongly reddened line of sight is a complementary addition to single cloud line of sight studies