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Interstellar Extinction in Orion. Variation of the Strength of the UV

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Interstellar Extinction in Orion. Variation of the Strength of the UV MNRAS 000, 1–9 (2017) Preprint 9 July 2018 Compiled using MNRAS LATEX style file v3.0 Interstellar extinction in Orion. Variation of the strength of the UV bump across the complex. Leire Beitia-Antero,1⋆ Ana I. G´omez de Castro,1† 1AEGORA Research Group, Universidad Complutense de Madrid, Facultad de CC. Matem´aticas, Plaza de Ciencias 3, E-28040 Madrid Accepted XXX. Received YYY; in original form ZZZ ABSTRACT There is growing observational evidence of dust coagulation in the dense filaments within molecular clouds. Infrared observations show that the dust grains size distri- bution gets shallower and the relative fraction of small to large dust grains decreases as the local density increases. Ultraviolet (UV) observations show that the strength of the 2175 A˚ feature, the so-called UV bump, also decreases with cloud density. In this work, we apply the technique developed for the Taurus study to the Orion molecular cloud and confirm that the UV bump decreases over the densest cores of the cloud as well as in the heavily UV irradiated λ Orionis shell. The study has been extended to the Rosette cloud with uncertain results given the distance (1.3 kpc). Key words: ISM: dust, extinction 1 INTRODUCTION feature of the extinction curve, the so-called UV bump. The UV bump is, by far, the strongest spectral feature in the ex- The Interstellar Medium (ISM) is a complex ensemble of tinction curve but its source remains uncertain (see e.g. the gas, large molecules and dust coupled through the action of review by Draine (2003)). Though small graphite grains were the magnetic fields permeating the Galaxy. In some parts proposed initially as the main source of the bump, the base- of the Galaxy, molecular clouds are formed due to galactic line today are PAHs (Weingartner & Draine (2001)), that stresses (Elmegreen (1990); Franco et al. (2002)) or eventual share with a graphite sheet a similar structure in terms of accretion from the intergalactic medium (Mirabel (1982); the distribution of the carbon atoms. PAHs are required Wang et al. (2004)). In the outskirts of these clouds the dust to reproduce the observed infrared emission (Leger & Puget size distribution varies due to density gradients that deter- (1984)) and they somewhat represent the extension of the mine the characteristics of the stars that will be formed in- dust grain size distribution into the molecular domain. side them (see e.g. Dzyurkevich et al. (2017)). Particularly, small dust grains are relevant because they act as main car- The relative fraction of the smallest components of dust riers of the field and set the fundamental scales for ambipo- grains in the ISM can be modified through coagulation into larger particles in regions of high density or through de- arXiv:1704.06059v1 [astro-ph.GA] 20 Apr 2017 lar diffusion and magnetic waves propagation (Pilipp et al. (1987)), favouring the coupling of the cloud with the ambi- struction by a strong ultraviolet radiation field. In a pre- ent field up to cut-off wavelengths of 0.1pc (Nakano (1998)). vious work, we developed a method to explore the former The distribution of large dust grains has been stud- process applying the star counts method in the near ul- ied, for instance, by The Herschel Space Telescope. It has traviolet (NUV) (G´omez de Castro et al. (2015), hereafter All Sky Survey provided measurements of this population in the nearest GdC2015), using the (AIS) carried out by molecular complexes such as Taurus (Ysard et al. (2013)) the Galaxy Evolution Explorer (GALEX) The method was and Orion (Roy et al. (2013); Stutz & Kainulainen (2015)). successfully applied to the Taurus molecular complex and On the other hand, the most sensitive way to detect varia- we found evidence of a decreasing UV bump strength as tions in the distribution of small dust grains (<0.05 microns) approaching to the densest parts of the clouds, in excellent and large molecules (such as the Polycyclic Aromatic Hydro- agreement with Spitzer-based results (Flagey et al. (2009)). carbons or PAHs) is to measure the strength of the 2175 A˚ In this work, we apply the same method to the Orion molecular cloud to detect possible dust coagulation processes and to explore the impact of the strong UV background in 2 ⋆ E-mail: [email protected] the PAHs survival. Orion spans more than 700deg in the sky † E-mail: [email protected] and it contains three main molecular structures: the Orion © 2017 The Authors 2 L. Beitia-Antero and A. I. G´omez de Castro A (d = 371 ± 10pc) and Orion B (d = 398 ± 12pc) clouds pixel sizes: 30arcmin, 15arcmin and 10arcmin (see Section 4 and the λ Orionis ring (d = 445 ± 50pc) (distances from for a discussion of the results). Lombardi et al. (2011)). In Section 2, the method is shortly described and in Sections 3 and 4, it is applied to the Orion star forming region. Evidence of UV bump suppression close to the dense filaments is found, as well as deviations from 3 DATA PROCESSING the ISM law around the heavily irradiated area of λ Orio- nis. Finally, in Section 5, the work has been extended to the 3.1 Data nearby (in projection) Rosette cloud. Ultraviolet spectro- scopic observations are available for some Rosette stars that The GALEX mission was intended to survey the ultra- have been used to test the statistical quality of the results. violet sky avoiding the Galactic Plane, as well as re- A brief summary is provided in Section 6. gions with bright sources that could damage the detectors (Martin et al. (2005)). GALEX images (or tiles) were ob- tained in two UV bands: Far Ultraviolet (FUV) band and Near Ultraviolet (NUV) band, with a circular field of view 2 METHOD DESCRIPTION of 1.2°. The GALEX AIS was completed in 2007 and cov- The method is based on the extensively used extinction ers ∼ 26, 000deg2. Point-like sources were extracted from the model derived by Fitzpatrick & Massa (2007) (hereafter images to generate the GALEX catalogue (Bianchi et al. FM07). GdC2015 show that the strength of the bump can be (2012)) that is used as a baseline for this work; only NUV expressed in terms of extinction in the GALEX NUV band, data will be used due to statistical reasons since GALEX de- ANUV, and in the K infrared band, AK, as, tected ∼ 10 times more sources in NUV band than in FUV band. = ANUV + Abump (0.106 ± 0.008) (2.0 ± 0.3) (1) In the Orion region, the survey consists of 359 im- AK ages (or tiles): ∼ 406deg2, including a small fraction of where Abump is the strength of the bump in the extinction the Orion A cloud and part of the λ Orionis ring. To curve (FM07). This result simply indicates that the ISM remove spurious sources, the NUV sources in the cat- extinction law admits a fairly simple parametric approxima- alogue were cross-correlated with the 2MASS catalogue tion from the near infrared to near ultraviolet except for the (see G´omez de Castro et al. (2011); Bianchi et al. (2011) strength of the bump. for details). Only NUV sources with 2MASS counterpart AK has been evaluated for the nearby molecular in a search radius of 3arcmin were considered bona-fidae clouds by Froebrich et al. (2007) (hereafter F07) based sources for the application of the star counts method on data from the 2MASS survey (Skrutskie et al. (2006)). (G´omez de Castro et al. (2015)). The selection of the NUV ANUV may be calculated by using the star counts method sources in the Orion region was obtained in the same man- (GdC2015) that has been extensively used in astronomy ner (S´anchez et al. (2014), hereafter S2014); the final sample (Bok & Cordwell (1973)). For example, this method was ap- amounts to a total of 289,968 NUV sources. Their spatial dis- plied by Lombardi et al. (2011) to estimate the distances tribution is shown in Figure 1 together with F07’s A -map = V to the main components of the Orion complex (d(Ori A) as a reference. Also overplotted are the 111 Young Stellar = = 371 ± 10pc, d(Ori B) 398 ± 12pc, d(λ Ori) 445 ± 50pc). Objects candidates determined by S2014; concentrations are In its most usual application, star counts in a given observed towards the Orion A cloud and the λ Orionis ring. field are compared with the predictions for an unextincted, The sensitivity of the star counts method depends on nearby field and the extinction is given by, the vast majority of the stars being background to the cloud under study. As the Orion cloud is at ∼ 400 pc from the = 1 ∗ Sun, most sources are expected to be background. The re- ANUV log(NNUV/NNUV) (2) bNUV lease of the Gaia DR1 catalogue (Gaia Collaboration et al. ∗ (2016)) has allowed an assessment of the fraction of fore- where NNUV are the observed counts, N are the ex- NUV ground stars. Unfortunately, only 13,562 stars have been pected counts from a non-extinguished field, and bNUV = found in the GAIA - GALEX cross-match; i.e. about 4.7% d log N(mNUV)/dmNUV (with mNUV, the apparent magnitude at wavelength NUV) is a measure of the slope of the lumi- of the sample. 33% of these sources (4,444) are foreground nosity function in the area under study. As the NUV lumi- stars; this factor is an upper limit since GAIA’s first release nosity function has not been determined for the Galaxy yet, is not complete neither in brightness (it is balanced to nearby we have derived it for the Orion region using GALEX data.
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