Diffuse Interstellar Bands in Upper Scorpius

Astronomy & Astrophysics manuscript no. aa09746 c ESO 2018 November 7, 2018 Diffuse interstellar bands in Upper Scorpius: Probing variations in the DIB spectrum due to changing environmental conditions⋆ D.A.I. Vos1, N.L.J. Cox2, L. Kaper3, M. Spaans4, and P. Ehrenfreund5 1 Radboud University Nijmegen, Toernooiveld 1, Postbus 9010, 6500 GL, Nijmegen, The Netherlands 2 Instituut voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200D, bus 2401, 3001, Leuven, Belgium 3 Astronomical Institute ”Anton Pannekoek”, Universiteit van Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands 4 Kapteyn Astronomical Institute, Rijksuniversiteit Groningen, Postbus 800, 9700 AV Groningen, The Netherlands 5 Astrobiology Group, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, 2300 RA, Leiden, The Netherlands Received 7 March 2008; Accepted 28 July 2011 ABSTRACT Aims. We study the effects of local environmental conditions affecting the diffuse interstellar band (DIB) carriers within the Upper Scorpius subgroup of the Sco OB2 association. The aim is to reveal how the still unidentified DIB carriers respond to different physical conditions prevailing in interstellar clouds, in order to shed light on the origin of the DIB carriers. Methods. We obtained optical spectra with FEROS on the ESO 1.52m telescope at La Silla, Chile, and measured the equivalent widths of five DIBs (at 5780, 5797, 6196, 6379, and 6613 Å) as well as those of absorption lines of di-atomic molecules (CH, CH+, CN) and atoms (K i, Ca i) towards 89 targets in the direction of Upper Scorpius. We construct a simple radiative transfer and chemical network model of the diffuse interstellar medium (ISM) sheet in front of Upp Sco to infer the effective radiation field. Results. By measuring the DIB and molecular spectrum of diffuse clouds towards 89 sightlines in the Upper Scorpius region, we have obtained a valuable statistical dataset that provides information on the physical conditions that influence the band strengths of the DIBs. Both the interstellar radiation field strength, IUV, and the molecular hydrogen fraction, fH2 , have been derived for 55 + sightlines probing the Upp Sco ISM. We discuss the relations between DIB strengths, CH and CH line strengths, E(B V), IUV, and − fH2 . The ratio between the 5780 and 5797 Å DIBs reveals a (spatial) dependence on the local environment in terms of cloud density and exposure to the interstellar radiation field, reflecting the molecular nature of these DIB carriers. Key words. Astrochemistry – ISM – ISM: lines and bands – ISM: molecules – ISM: dust, extinction – ISM: clouds – ISM: individual objects: Upper Scorpius 1. Introduction In order to understand the chemical and physical properties of the DIB carrier(s) it is important to study their be- The diffuse interstellar medium contains compounds of uniden- haviour in different interstellar environments, both in our own tified origin that absorb in the UV-visual to near-infrared spec- galaxy and beyond. Studies of DIBs in the Magellanic Clouds tral range. More than 300 different diffuse interstellar bands (Ehrenfreundet al. 2002; Cox et al. 2006, 2007; Welty et al. (DIBs) are currently identified (Herbig 1995; Hobbs et al. 2008). 2006), M31 (Cordineretal. 2008a,b) and beyond (e.g. Many possible carriers have been proposed, ranging from Heckman & Lehnert 2000; York et al. 2006; Sollerman et al. grain impurities and exotic molecules to H2. In the past two 2005; Cox & Patat 2008) illustrate that DIB carrier abundances decades the field has converged towards larger carbonaceous (per amount of dust and gas) can be similar to galactic values. molecules, like the fullerenes and polycyclic aromatic hydro- However, these studies have also revealed systematic differences carbons (PAHs), which have electronic transitions in the opti- in these extragalactic environments. cal (see for example Salama et al. 1999, Ruiterkamp et al. 2005; Kokkin& Schmidt 2006; Zhouet al. 2006 and Salama et al. A large amount of published information is available re- arXiv:1108.1083v2 [astro-ph.GA] 9 Aug 2011 2011). New diffuse bands have been detected in one line-of- garding DIBs in many sightlines probing the Galaxy (e.g. sight which appear to match with naphthalene and anthracene Herbig 1993; Chlewicki et al. 1986; Krełowskiet al. 1999; cations (Iglesias-Groth et al. 2008, 2010) and the weak 5450 Å Thorburn et al. 2003; Galazutdinov et al. 2004; Weselak et al. DIB is found to match with an absorption band arising from 2004, 2008b; Friedman et al. 2011), yielding relations of DIB a hydrocarbon plasma created in the laboratory (Linnartz et al. properties with respect to each other and to other diffuse ISM 2010). Linear-C3H2 has been put forwardas a carrier of the 5450 gas & dust tracers. Most of these studies focused on DIBs prob- and 4881 Å DIBs by Maier et al. (2011). These assignments are, ing various galactic environments, and provided average results however, tentative and disputed (Galazutdinov et al. 2011). for the Milky Way. Studies dealing with a particular region usu- ally only include a very limited number of sightlines. An excep- tion is the study of the Orion region by Jenniskens et al. (1994) Send offprint requests to: Nick Cox, e-mail: which entails 22 lines of sight. Another multi-object study, by [email protected] van Loon et al. (2009), used the distant globular cluster ω Cen to ⋆ Based on observations collected at the European Southern probe fluctuations of Ca ii, Na i and the λλ5780 and 5797 DIBs Observatory, Paranal, Chile (ESO program 63.H-0456) in the diffuse - low reddening - foreground ISM. This study re- 2 Vos, Cox, Kaper, Spaans & Ehrenfreund: Diffuse interstellar bands in Upper Scorpius Fig. 1. The positions (black and white dots) of the 89 mainly B-type members of Upp Sco are shown (with HD numbers) on top of a 100 µm infrared dust map of this region (Schlegel et al. 1998). The north-east arrows are 3◦ in length. The ρ Oph cloud can be identified by the bright filamentary emission located just left to the center of the figure. The dust emission (on a logarithmic grey scale) is proportional to the reddening E(B V) of sightlines penetrating these clouds; the lowest intensities correspond to E(B V) 0.02 mag (black) and highest intensities to −2 mag (white). The well-known targets σ Sco (HD 147165, in the ρ Oph cloud) and− ζ∼Oph (HD 149757, top left) are included in our≥ study. vealed small-scale structure - on parsec scales - in the warm neu- (2008) suggested a mean distance of 139 6pc forthe distanceof tral and weakly ionised medium of the Disc-Halo interface. The the Ophiuchus molecular cloud, which they± placed within 11 pc observed low 5797/5780 DIB ratio was found to be consistent of the centroid of the Upper Scorpius subgroup. with the relative high UV radiation levels typically inferred for Filamentary - interstellar - material connected to the the extra-planar warm medium. ρ Ophiuchus cloud complex is observed towards Upper Scorpius Nearby OB associations host many bright early-type stars (deGeus 1992). The densest part of this complex is the confined in a relatively small area of the sky. These stars have ρ Oph dark cloud, a site of ongoing low-mass star formation only few stellar lines in the optical spectrum contaminating the (Grasdalen et al. 1973; Greene & Young 1992; Wilking et al. interstellar spectrum. Thus, these associations provide a setup 1997; Preibisch & Zinnecker 2001) that is exposed to the ra- that is perfectly suited to study the effect of varying local condi- diation fields and stellar winds produced by nearby early- tions on the DIB spectrum. One of these associations, Scorpius type stars. A detailed review on the stellar population and 3 OB2, is a young (5 20 Myr), low-density ( 0.1 M pc− ) star formation history of the Sco OB2 association is given by grouping of stars divided− in three subgroups≈ (de Zeeuw⊙ et al. Preibisch & Mamajek (2008) and Wilking et al. (2008). 1999; Kouwenhoven et al. 2005). Scorpius (Upp Sco) region is The advantages of studying the properties of DIBs in the the subgroup near the Ophiuchus star forming region and the ρ Upper Scorpius region are numerous. It is in close vicinity and it Oph cloud at a distance of 145 2 pc (de Zeeuw et al. 1999). has been extensively studied in the past. Detailed information is Combining 2MASS extinction maps± with Hipparcos and Tycho available on both the stellar content (spectral types, photometry, parallaxes, Lombardi et al. (2008) found a distance of 119 6 pc distances, kinematics, etc.) and the conditionsof the surrounding for the ρ Ophiuchi cloud (with the core at 128 8 pc). Mamajek± interstellar medium (dust emission and absorption, IR-to-far-UV ± Vos, Cox, Kaper, Spaans & Ehrenfreund: Diffuse interstellar bands in Upper Scorpius 3 extinction curves, UV emission, molecular content, etc.). It ex- hibits a significant variation in local environmental conditions 1 which should translate into changing properties of the DIBs (if they depend on these conditions) when probing different parts of ρ the Upp Sco region. 0.8 Foreground Oph cloud Previous studies of interstellar gas and dust in the Upp Sco region focused on the ρ Oph cloud and a few other nearby 0.6 bright B stars. Snow et al. (2008) give a concise summary of different studies of the Upp Sco region covering a range of top- E(B-V) [mag] 0.4 ics including UV extinction, atomic and molecular hydrogen, atomic and molecular gas, astrochemistry, and DIBs. For ex- 0.2 ample, H2 observations show that sightlines in this region have both low ( 0.1)and high ( 0.3–0.6) molecular fractions f (e.g. ≤ ∼ H2 Savage et al. 1977).

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