On the Nature of Absorption Features Toward Nearby Stars? S
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A&A 591, A20 (2016) Astronomy DOI: 10.1051/0004-6361/201628482 & c ESO 2016 Astrophysics On the nature of absorption features toward nearby stars? S. Kohl, S. Czesla, and J. H. M. M. Schmitt Hamburger Sternwarte, Universität Hamburg, Gojenbergweg 112, 21029 Hamburg, Germany e-mail: [email protected] Received 10 March 2016 / Accepted 6 April 2016 ABSTRACT Context. Diffuse interstellar absorption bands (DIBs) of largely unknown chemical origin are regularly observed primarily in distant early-type stars. More recently, detections in nearby late-type stars have also been claimed. These stars’ spectra are dominated by stellar absorption lines. Specifically, strong interstellar atomic and DIB absorption has been reported in τ Boo. Aims. We test these claims by studying the strength of interstellar absorption in high-resolution TIGRE spectra of the nearby stars τ Boo, HD 33608, and α CrB. Methods. We focus our analysis on a strong DIB located at 5780.61 Å and on the absorption of interstellar Na. First, we carry out a differential analysis by comparing the spectra of the highly similar F-stars, τ Boo and HD 33608, whose light, however, samples different lines of sight. To obtain absolute values for the DIB absorption, we compare the observed spectra of τ Boo, HD 33608, and α CrB to PHOENIX models and carry out basic spectral modeling based on Voigt line profiles. Results. The intercomparison between τ Boo and HD 33608 reveals that the difference in the line depth is 6.85 ± 1.48 mÅ at the DIB location which is, however, unlikely to be caused by DIB absorption. The comparison between PHOENIX models and observed spectra yields an upper limit of 34.0 ± 0.3 mÅ for any additional interstellar absorption in τ Boo; similar results are obtained for HD 33608 and α CrB. For all objects we derive unrealistically large values for the radial velocity of any presumed interstellar clouds. In τ Boo we find Na D absorption with an equivalent width of 0.65 ± 0.07 mÅ and 2.3 ± 0.1 mÅ in the D2 and D1 lines. For the other Na, absorption of the same magnitude could only be detected in the D2 line. Our comparisons between model and data show that the interstellar absorption toward τ Boo is not abnormally high. Conclusions. We find no significant DIB absorption in any of our target stars. Any differences between modeled and observed spectra are instead attributable to inaccuracies in the stellar atmospheric modeling than to DIB absorption. Key words. ISM: general – stars: late-type – local insterstellar matter – ISM: lines and bands – solar neighborhood – stars: atmospheres 1. Introduction interstellar absorption features have been extensively studied (Herbig 1995). More recently, Hobbs et al.(2009) presented a For a long time, early-type binaries at large distances have detailed characterization of the DIB properties in the early-type been known to show peculiar absorption bands in their spec- supergiant HD 183143. One of the DIBs with the highest equiv- tra. These bands cannot be attributed to stellar lines because alent width is located at 5780.61 Å and, in the specific case of they do not follow the Doppler shift caused by the radial mo- HD 183143, its shape can be modeled adequately by a single tion of the binary components (Merrill 1936). Later, similar ab- sorption bands have also been found in several hundred single Gaussian function with a full-width at half maximum of 2.14 Å stars (Duke 1951). Today, these absorption features are known (Hobbs et al. 2009). as diffuse interstellar bands (DIB), which show widths typically In late-type stars DIBs are more challenging to detect be- ranging from a few mÅ up to several Å. It is assumed that the cause stellar and interstellar lines are blended. Therefore DIB DIBs are a superposition of different components. The nature studies inside our Galaxy remain largely restricted to giants at of the absorbing material is still under debate for most DIBs large distance and high column densities of interstellar matter. (Cami & Cox 2014). However, recently it has been confirmed Comprehensive surveys have been carried out for DIBs that are that three DIBs can be assigned to the absorption spectrum of narrow and have large equivalent widths. Based on several thou- fullerenes (Campbell et al. 2015). There is consensus that the sand spectra, three-dimensional maps showing the distribution of diffuse bands are of interstellar nature and thus the strength of the interstellar medium have been produced, e.g., using the DIBs individual bands depends on the chemical environment along the at 6614 Å (Puspitarini et al. 2015), 8620 Å (Kos et al. 2013) and line of sight, implying a dependence on the stellar distance and 15 272 Å (Zasowski et al. 2015). These maps typically cover dis- the direction of the line of sight (e.g., Hobbs et al. 2008). tances from a few hundred parsec up to several kilo-parsec and Diffuse interstellar bands are most easily observed in bright show that the strength of the DIB absorption generally rises with and distant objects that have few intrinsic spectral lines such distance. Moreover, Friedman et al.(2011) show that for var- as early-type supergiants, for which the equivalent widths of ious DIBs interstellar extinction and hydrogen absorption are correlated. ? The spectra are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via In addition to DIBs, sharp interstellar atomic absorption lines http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/591/A20 can be found. Garcia(1991) gives a compilation of the strength Article published by EDP Sciences A20, page 1 of7 A&A 591, A20 (2016) of interstellar atomic Na and Ca lines in various stellar spectra. Table 1. List of observed objects. These lines are typically much narrower than the stellar lines because they are not affected by the usual stellar line broadening Parameter Value References mechanisms. Like DIBs they are most easily detected in distant τ Boo early-type stars. Until recently, surveys targeting the local interstellar medium Spectral type F6IV 1 were restricted to early-type stars. Lallement et al.(2003) Teff 6450 ± 150 2 mapped the equivalent width of the interstellar Na D lines along log (g) 4.27 ± 0.10 2 different lines of sight. For stars within 70 pc, the authors find [M/H] 0.26 ± 0.08 2 −1 D vrot[km s ] 13.0 3 no interstellar 2 absorption line stronger than 75 mÅ, respec- −1 tively, 60 mÅ for the D line. Similar studies have also been car- vrad[km s ] –16.03 1 1 Parallaxes [mas] 64.03 1 ried out for the DIBs at 5780 Å and 6284 Å by Raimond et al. Epoch of obs. [yr] 2015.01–2015.22 (2012). Out of the 24 objects in their sample with distances a below 200 pc, only two show an equivalent width larger than S=N at 5780 Å 618 a 100 mÅ in the DIB at 5780 Å. S=N at 5892 Å 763 Studies of DIBs in late-type stars are not only hampered Total exposure time [s] 8288 by ubiquitous stellar line blends and the notorious weakness # spectra 20 of the DIBs in the spectra of nearby stars, but also by the HD 33608 presence of telluric lines. This is important because inaccura- Spectral type F5.5IV-V 1 cies in the telluric correction contaminate the spectral signa- Teff 6530 ± 100 2 tures of the DIBs. Using the intermediate dispersion spectro- log (g) 4.08 ± 0.07 2 graph (IDS) with a spectral resolution of ∼2000 mounted at the [M/H] 0.22 ± 0.04 2 2.5 m Isaac Newton Telescope, Farhang et al.(2015) carried out −1 vrot[km s ] 15.5 ± 0.5 4 a search for Na absorption features and DIBs in nearby late- −1 vrad[km s ] 33.90 1 type stars. These authors studied absorption features located at Parallaxes [mas] 33.9 1 5780, 5896, and 5890 Å and conclude that DIBs and interstel- Epoch of obs. [yr] 2015.03–2015.23 lar Na can indeed be distinguished in the spectra of a number S=N at 5780 Åa 188 of nearby late-type stars. Farhang et al.(2015) specifically in- S=N at 5892 Åa 234 τ vestigated the nearby star Boo, a triple system whose primary Total exposure time [s] 2160 τ component hosts a planet; the stellar parameters of Boo are # spectra 4 listed in Table1. As a result of their analysis, Farhang et al. (2015) claim that τ Boo shows abnormally high interstellar ab- α CrB A sorption, and derived substantial equivalent widths for the vari- Spectral type A1IV 5 ous DIBs, viz., 137.08 ± 16.61 mÅ for the DIB at 5780 Å, and Teff 9900 2 407.16 ± 32.95 mÅ and 344.03 ± 32.62 mÅ for the equivalent log (g) 3.89 5 widths of the interstellar Na D2 and D1 lines. [M/H] – −1 These equivalent widths are surprisingly large and therefore vrot[km s ] 128.8 ± 6.2 4 −1 we decided to examine high-resolution spectra of τ Boo and vrad[km s ] 1.7 1 other, similar objects. With these spectra we are in a position Parallaxes [mas] 43.46 1 to revise the strength of the interstellar absorption and thus the Epoch of obs. [yr] 2015.34 outline of the paper is as follows: in Sect. 2.1, we present our S=N at 5780 Åa 569 stellar sample and summarize the stellar parameters and present S=N at 5892 Åa 739 our approach to correct for telluric lines in Sect.