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Register a new userSpectropolarimetry of Galactic stars with anomalous extinction sightlines
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Highly reddened type Ia Supernovae (SNe Ia) with low total-to-selective visual extinction ratio values, $R_V$, also show peculiar linear polarization wavelength dependencies with peak polarizations at short wavelengths ($lambda_{max} lesssim 0.4 mu m$). It is not clear why sightlines to SNe Ia display such different continuum polarization profiles from interstellar sightlines in the Milky Way with similar $R_V$ values. We investigate polarization profiles of a sample of Galactic stars with low $R_V$ values, along anomalous extinction sightlines, with the aim to find similarities to the polarization profiles that we observe in SN Ia sightlines. We undertook spectropolarimetry of 14 stars, and used archival data for three additional stars, and run dust extinction and polarization simulations to infer a simple dust model that can reproduce the observed extinction and polarization curves. Our sample of Galactic stars with low $R_V$ values and anomalous extinction sightlines displays normal polarization profiles with an average $lambda_{max} sim 0.53 {mu m}$, and is consistent within 3$sigma$ to a larger coherent sample of Galactic stars from literature. Despite the low $R_V$ values of dust towards the stars in our sample, the polarization curves do not show any similarity to the continuum polarization curves observed towards SNe Ia with low $R_V$ values. There is a correlation between the best-fit Serkowski parameters $K$ and $lambda_{max}$, but we did not find any significant correlation between $R_V$ and $lambda_{max}$. Our simulations show that the $K-lambda_{max}$ relationship is an intrinsic property of polarization. Furthermore, we have shown that in order to reproduce polarization curves with normal $lambda_{max}$ and low $R_V$ values, a population of large (a $geq 0.1 mu m$) interstellar silicate grains must be contained in the dusts composition.
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The large majority of extinction sight lines in our Galaxy obey a simple relation depending on one parameter, the total-to-selective extinction coefficient, Rv. Different values of Rv are able to match the whole extinction curve through different environments so characterizing normal extinction curves. In this paper more than sixty curves with large ultraviolet deviations from their best-fit one parameter curve are analyzed. These curves are fitted with dust models to shed light into the properties of the grains, the processes affecting them, and their relations with the environmental characteristics. The extinction curve models are reckoned by following recent prescriptions on grain size distributions able to describe one parameter curves for Rv values from 3.1 to 5.5. Such models, here extended down to Rv=2.0, allow us to compare the resulting properties of our deviating curves with the same as normal curves in a self-consistent framework, and thus to recover the relative trends overcoming the modeling uncertainties. Such curves represent the larger and homogeneous sample of anomalous curves studied so far with dust models. Results show that the ultraviolet deviations are driven by a larger amount of small grains than predicted for lines of sight where extinction depends on one parameter only. Moreover, the dust-to-gas ratios of anomalous curves are lower than the same values for no deviating lines of sight. Shocks and grain-grain collisions should both destroy dust grains, so reducing the amount of the dust trapped into the grains, and modify the size distribution of the dust, so increasing the small-to-large grain size ratio. Therefore, the extinction properties derived should arise along sight lines where shocks and high velocity flows perturb the physical state of the interstellar medium living their signature on the dust properties. (Abridged version)
[ABRIDGED] Context. O stars are excellent tracers of the intervening ISM because of their high luminosity, blue intrinsic SED, and relatively featureless spectra. We are currently conducting GOSSS, which is generating a large sample of O stars with accurate spectral types within several kpc of the Sun. Aims. To obtain a global picture of the properties of dust extinction in the solar neighborhood based on optical-NIR photometry of O stars with accurate spectral types. Methods. We have processed a photometric set with the CHORIZOS code to measure the amount and type of extinction towards 562 O-type stellar systems. We have tested three different families of extinction laws and analyzed our results with the help of additional archival data. Results. The Maiz Apellaniz et al. (2014) family of extinction laws provides a better description of Galactic dust that either the Cardelli et al. (1989) or Fitzpatrick (1999) families, so it should be preferentially used. In many cases O stars and late-type stars experience similar amounts of extinction at similar distances but some O stars are located close to the molecular clouds left over from their births and have larger extinctions than the average for nearby late-type populations. In qualitative terms, O stars experience a more diverse extinction than late-type stars, as some are affected by the small-grain-size, low-R_5495 effect of molecular clouds and others by the large-grain-size, high-R_5495 effect of H II regions. Late-type stars experience a narrower range of grain sizes or R_5495, as their extinction is predominantly caused by the average, diffuse ISM. We propose that the reason for the existence of large-grain-size, high-R_5495 regions in the ISM in the form of H II regions and hot-gas bubbles is the selective destruction of small dust grains by EUV photons and possibly by thermal sputtering by atoms or ions.
Intrigued by the extended red-giant clump (RC) stretching across the colour-magnitude diagram of the stars in a 50x50 pc^2 region of the Large Magellanic Cloud (LMC) containing the clusters NGC 1938 and NGC 1939, we have studied the stellar populations to learn about the properties of the interstellar medium (ISM) in this area. The extended RC is caused by a large and uneven amount of extinction across the field. Its slope reveals anomalous extinction properties, with Av/E(B-V)=4.3, indicating the presence of an additional grey component in the optical contributing about 30% of the total extinction in the field and requiring big grains to be about twice as abundant as in the diffuse ISM. This appears to be consistent with the amount of big grains injected into the surrounding ISM by the about 70 SNII explosions estimated to have occurred during the lifetime of the ~120 Myr old NGC 1938. Although this cluster appears today relatively small and would be hard to detect beyond the distance of M 31, with an estimated initial mass of ~4800 Msun NGC 1938 appears to have seriously altered the extinction properties in a wide area. This has important implications for the interpretation of luminosities and masses of star-forming galaxies, both nearby and in the early universe.
Based on the data obtained from the Spitzer/GLIPMSE Legacy Program and the 2MASS project, we derive the extinction in the four IRAC bands, [3.6], [4.5], [5.8] and [8.0] micron, relative to the 2MASS Ks band (at 2.16 micron) for 131 GLIPMSE fields along the Galactic plane within |l|<65 deg, using red giants and red clump giants as tracers. As a whole, the mean extinction in the IRAC bands (normalized to the 2MASS Ks band), A_[3.6]/A_Ks=0.63, A_[4.5]/A_Ks=0.57, A_[5.8]/A_Ks=0.49, A_[8.0]/A_Ks=0.55, exhibits little variation with wavelength (i.e. the extinction is somewhat flat or gray). This is consistent with previous studies and agrees with that predicted from the standard interstellar grain model for R_V=5.5 by Weingartner & Draine (2001). As far as individual sightline is concerned, however, the wavelength dependence of the mid-infrared interstellar extinction A_{lambda}/A_Ks varies from one sightline to another, suggesting that there may not exist a universal IR extinction law. We, for the first time, demonstrate the existence of systematic variations of extinction with Galactic longitude which appears to correlate with the locations of spiral arms as well as with the variation of the far infrared luminosity of interstellar dust.
We reduced ESOs archival linear spectropolarimetry data (4000-9000AA) of 6 highly polarized and 8 unpolarized standard stars observed between 2010 and 2016, for a total of 70 epochs, with the FOcal Reducer and low dispersion Spectrograph (FORS2) mounted at the Very Large Telescope. We provide very accurate standard stars polarization measurements as a function of wavelength, and test the performance of the spectropolarimetric mode (PMOS) of FORS2. We used the unpolarized stars to test the time stability of the PMOS mode, and found a small ($leq$0.1%), but statistically significant, on-axis instrumental polarization wavelength dependency, possibly caused by the tilted surfaces of the dispersive element. The polarization degree and angle are found to be stable at the level of $leq$0.1% and $leq$0.2 degrees, respectively. We derived the polarization wavelength dependence of the polarized standard stars and found that, in general, the results are consistent with those reported in the literature, e.g. Fossati et al. (2007) who performed a similar analysis using FORS1 data. The re-calibrated data provide a very accurate set of standards that can be very reliably used for technical and scientific purposes. The analysis of the Serkowski parameters revealed a systematic deviation from the width parameter $K$ reported by Whittet et al. (1992). This is most likely explained by incorrect effective wavelengths adopted in that study for the R and I bands.
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