VLT/X-Shooter survey of near-infrared diffuse interstellar bands

Abstract

Context. The unknown identity of the diffuse interstellar band (DIB) carriers poses one of the longest standing unresolved problems in astrophysics. While the presence, properties, and behaviour of hundreds of optical DIBs between 4000 Å and 9000 Å have been well established, information on DIBs in both the ultra-violet and near-infrared (NIR) ranges is limited. Aims. In this paper, we present a spectral survey of the NIR range, from 0.9 µm to 2.5 µm. Our observations were designed to detect new DIBs, confirm previously proposed NIR DIBs, and characterise their behaviour with respect to known line-of-sight properties (including the optical DIBs present in our spectra). Methods. Using the X-Shooter instrument mounted on the ESO Very Large Telescope (VLT) we obtained medium-resolution spectra of eight known DIB targets and one telluric reference star, from 3000 Å to 25 000 Å in one shot.Results. In addition to the known 9577, 9632, 10 780, 11 797, and 13 175 Å NIR DIBs, we confirm 9 out of the 13 NIR DIBs that were presented by Geballe et al. (2011, Nature, 479, 200). Furthermore, we report 11 new NIR DIB candidates. The strengths of the strongest NIR DIBs show a general correlation with reddening, E (B − V) , but with a large scatter. Several NIR DIBs are more strongly correlated with the 5780 Å DIB strength than with E (B − V) ; this is especially the case for the 15 268 Å DIB. The NIR DIBs are strong: the summed equivalent widths of the five strongest NIR DIBs represent a small percent of the total equivalent width of the entire average DIB spectrum (per unit reddening). The NIR DIBs towards the translucent cloud HD 147889 are all weak with respect to the general trend. No direct match was found between observed NIR DIBs and laboratory matrix-isolation spectroscopic data of polycyclic aromatic hydrocarbons (PAHs). Conclusions. The strong correlation between the 5780−15 268 DIB pair implies that (N f ) 5780 /(N f ) 15 268 = 14. However, the reduced strength of the 15 268 Å DIB in HD 147889 rules out a common carrier for these two DIBs. Since the ionisation fraction for small PAHs in this translucent cloud is known to be low compared to diffuse clouds, the weakness of the 15 268 Å DIB suggests that an ionised species could be the carrier of this NIR DIB.

Near-infrared diffuse interstellar bands in APOGEE telluric standard star spectra

“…Our use of bright, often nearby targets may explain that our average width is less affected by cloud velocity dispersion and is closer to the intrinsic width. Gaussian fits to the averaged λλ15617 and 15673 profiles provide FHWM= 4.2 and 3.1 Å, respectively, which corresponds to the maxima in their histograms in Table 3 lists all published DIB detections and candidates in the APOGEE wavelength range (except for the strong λ15273 DIB), based on the earlier works (Geballe et al 2011;Cox et al 2014). For all bands the number of detections is very small and some are quite uncertain.…”

“…Here we focus on the three other, weaker bands. Based on the few available detections (Geballe et al 2011;Cox et al 2014), we expect them to be between two and three times fainter than the band at λ15273. The first part of this paper aims at creating a catalog of equivalent widths and central wavelengths for these three strongestweak DIBs.…”

“…Today, NIR DIB-DIB correlation studies are still limited by the small number of NIR DIB measurements for transitions other than the λ15273 APOGEE main DIB. Cox et al (2014) studied the correlations among three NIR DIBs (λλ11800, 13180, and 15273) and found correlation coefficients of 0.90 and 0.97. On the other hand, their comparisons with optical DIBs led to a wider range of coefficients, from 0.83 to 0.98.…”

Measuring diffuse interstellar bands with cool stars

“…Galazutdinov et al 2000;Hobbs et al 2009). Most of them are seen in the optical, with some additional DIBs clearly identified in the near infrared (Joblin et al 1990;Foing & Ehrenfreund 1994;Cox et al 2014;Hamano et al 2016) and a few proposed candidates in the near-UV (Bhatt & Cami 2015). Also, DIBs seem omnipresent.…”

A CATALOG OF 1.5273 μm DIFFUSE INTERSTELLAR BANDS BASED ON APOGEE HOT TELLURIC CALIBRATORS

“…The first near-infrared (NIR) DIB was discovered by Joblin et al (1990). Currently, ∼25 NIR DIBs have been identified (Foing & Ehrenfreund 1994;Geballe et al 2011;Cox et al 2014;Hamano et al 2015Hamano et al , 2016, including the strong band at 1.318 μm. NIR DIBs are particularly useful since they allow to make use of highly reddened target stars and explore the densest areas of the interstellar medium (ISM).…”