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Register a new userDetection of PAH Absorption and Determination of the Mid-Infrared Diffuse Interstellar Extinction Curve from the Sightline Toward Cyg OB2-12
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The sightline toward the luminous blue hypergiant Cyg OB2-12 is widely used in studying interstellar dust on account of its large extinction ($A_V simeq 10$ mag) and the fact that this extinction appears to be dominated by dust typical of the diffuse interstellar medium. We present a new analysis of archival ISO-SWS and Spitzer IRS observations of Cyg OB2-12 using a model of the emission from the star and its stellar wind to determine the total extinction $A_lambda$ from 2.4--37 $mu$m. In addition to the prominent 9.7 and 18 $mu$m silicate features, we robustly detect absorption features associated with polycyclic aromatic hydrocarbons (PAHs), including the first identification of the 7.7 $mu$m feature in absorption. The 3.3 $mu$m aromatic feature is found to be much broader in absorption than is typically seen in emission. The 3.4 and 6.85 $mu$m aliphatic hydrocarbon features are observed with relative strengths consistent with observation of these features on sightlines toward the Galactic Center. We identify and characterize more than sixty spectral lines in this wavelength range, which may be useful in constraining models of the star and its stellar wind. Based on this analysis, we present an extinction curve $A_lambda/A_{2.2 mu m}$ that extrapolates smoothly to determinations of the mean Galactic extinction curve at shorter wavelengths and to dust opacities inferred from emission at longer wavelengths, providing a new constraint on models of interstellar dust in the mid-infrared.
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We obtained the near-infrared (NIR) high-resolution ($Requivlambda/Deltalambdasim20,000$) spectra of the seven brightest early-type stars in the Cygnus OB2 association for investigating the environmental dependence of diffuse interstellar bands (DIBs). The WINERED spectrograph mounted on the Araki 1.3m telescope in Japan was used to collect data. All 20 of the known DIBs within the wavelength coverage of WINERED ($0.91<lambda<1.36mu$m) were clearly detected along all lines of sight because of their high flux density in the NIR wavelength range and the large extinction. The equivalent widths (EWs) of DIBs were not correlated with the column densities of C$_2$ molecules, which trace the patchy dense component, suggesting that the NIR DIB carriers are distributed mainly in the diffuse component. On the basis of the correlations among the NIR DIBs both for stars in Cyg OB2 and stars observed previously, $lambdalambda$10780, 10792, 11797, 12623, and 13175 are found to constitute a family, in which the DIBs are correlated well over the wide EW range. In contrast, the EW of $lambda$10504 is found to remain almost constant over the stars in Cyg OB2. The extinction estimated from the average EW of $lambda$10504 ($A_Vsim3.6$mag) roughly corresponds to the lower limit of the extinction distribution of OB stars in Cyg OB2. This suggests that $lambda$10504 is absorbed only by the foreground clouds, implying that the carrier of $lambda$10504 is completely destroyed in Cyg OB2, probably by the strong UV radiation field. The different behaviors of the DIBs may be caused by different properties of the DIB carriers.
A precise measure of the mid-infrared interstellar extinction law is crucial to the investigation of the properties of interstellar dust, especially of the grains in the large size end. Based on the stellar parameters derived from the SDSS-III/APOGEE spectroscopic survey, we select a large sample of G- and K-type giants as the tracers of the Galactic mid-infrared extinction. We calculate the intrinsic stellar color excesses from the stellar effective temperatures and use them to determine the mid-infrared extinction for a given line of sight. For the entire sky of the Milky Way surveyed by APOGEE, we derive the extinction (relative to the K$_{rm S}$ band at wavelength $lambda=2.16mu$m) for the four WISE bands at 3.4, 4.6, 12 and 22$mu$m, the four Spitzer/IRAC bands at 3.6, 4.5, 5.8 and 8$mu$m, the Spitzer/MIPS24 band at 23.7$mu$m and for the first time, the AKARI/S9W band at 8.23$mu$m. Our results agree with previous works in that the extinction curve is flat in the ~3--8$mu$m wavelength range and is generally consistent with the $R_V=5.5$ model curve except our determination exceeds the model prediction in the WISE/W4 band. Although some previous works found that the mid-IR extinction law appears to vary with the extinction depth $A_{rm{K_S}}$, no noticeable variation has been found in this work. The uncertainties are analyzed in terms of the bootstrap resampling method and Monte-Carlo simulation and are found to be rather small.
We combine VI photometry from OGLE-III with VVV and 2MASS measurements of E(J-K_{s}) to resolve the longstanding problem of the non-standard optical extinction toward the Galactic bulge. We show that the extinction is well-fit by the relation A_{I} = 0.7465*E(V-I) + 1.3700*E(J-K_{s}), or, equivalently, A_{I} = 1.217*E(V-I)(1+1.126*(E(J-K_{s})/E(V-I)-0.3433)). The optical and near-IR reddening law toward the inner Galaxy approximately follows an R_{V} approx 2.5 extinction curve with a dispersion {sigma}_{R_{V}} approx 0.2, consistent with extragalactic investigations of the hosts of type Ia SNe. Differential reddening is shown to be significant on scales as small as as our mean field size of 6, with the 1{sigma} dispersion in reddening averaging 9% of total reddening for our fields. The intrinsic luminosity parameters of the Galactic bulge red clump (RC) are derived to be (M_{I,RC}, sigma_{I,RC,0}, (V-I)_{RC,0}, sigma_{(V-I)_{RC}}, (J-K_{s})_{RC,0}) = (-0.12, 0.09, 1.06, 0.121, 0.66). Our measurements of the RC brightness, brightness dispersion and number counts allow us to estimate several Galactic bulge structural parameters. We estimate a distance to the Galactic center of 8.20 kpc, resolving previous discrepancies in distance determinations to the bulge based on I-band observations. We measure an upper bound on the tilt {alpha} approx 40{deg}. between the bars major axis and the Sun-Galactic center line of sight, though our brightness peaks are consistent with predictions of an N-body model oriented at {alpha} approx 25{deg}. The number of RC stars suggests a total stellar mass for the Galactic bulge of 2.0*10^{10} M_{odot}, if one assumes a Salpeter IMF.
Two star-forming regions are studied: the young embedded open cluster vdB 130 and the protocluster neighbourhood observed in the head and tail of the cometary molecular cloud located in the wall of the expanding supershell surrounding the Cyg OB1 association. The GAIA DR2 catalogue is employed to verify the stellar composition of the vdB 130 cluster whose members were earlier selected using the UCAC4 catalogue. The new sample of vdB 130 members contains 68 stars with close proper motions (within 1 mas yr$^{-1}$) and close trigonometric parallaxes (ranging from 0.50 to 0.70 mas). The relative parallax error is shown to increase with distance to objects and depend on their magnitude. At a distance of 1.5-2 kpc it is of about 3-7 per cent and 20-30 per cent for bright and faint stars, respectively. The cluster is not older than ~10 Myr. New spectroscopic and photometric observations carried out on Russian telescopes are combined with GAIA DR2 to search for optical components in the protocluster region - a new starburst. An analysis of 20 stars in the vicinity of the protocluster revealed no concentration of either proper motions or parallaxes. According to spectroscopic, photometric, and trigonometric estimates, the distances to these stars range from 0.4 to 2.5 kpc, and colour excess is shown to increase with a distance D (kpc) in accordance with the law: $E(B-V)simeq 0.6times D$ mag.
I revisit the Cepheid-distance determination to the nearby spiral galaxy M101 (Pinwheel Galaxy) of Shappee & Stanek (2011), in light of several recent investigations questioning the shape of the interstellar extinction curve at $lambda approx 8,000$ AA (i.e. I-band). I find that the relatively steep extinction ratio $A_{I}/E(V-I)=1.1450$ (Fitzpatrick & Massa 2007) is slightly favoured relative to $A_{I}/E(V-I)=1.2899$ (Fitzpatrick 1999) and significantly favoured relative the historically canonical value of $A_{I}/E(V-I)=1.4695$ (Cardelli et al. 1989). The steeper extinction curves, with lower values of $A_{I}/E(V-I)$, yield fits with reduced scatter, metallicity-dependences to the dereddened Cepheid luminosities that are closer to values inferred in the local group, and that are less sensitive to the choice of reddening cut imposed in the sample selection. The increase in distance modulus to M101 when using the preferred extinction curve is ${Delta}{mu} sim 0.06$ mag, resulting in an estimate of the distance modulus to M101 relative to the LMC of $ {Delta}mu_{rm{LMC}} approx 10.72 pm 0.03$ (stat). The best-fit metallicity-dependence is $dM_{I}/drm{[O/H]} approx (-0.38 pm 0.14$ (stat)) mag dex$^{-1}$.
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