Updated extraction of the APOGEE 1.5273 μm diffuse interstellar band: a Planck view on the carrier depletion in dense cores^★,★★

GEPI, Observatoire de Paris, PSL University, CNRS, Place Jules Janssen, 92190 Meudon, France
e-mail: meriem.el-yajouri@obspm.fr

Received: 17 October 2018
Accepted: 31 March 2019

Abstract

Context. Constraining the spatial distribution of diffuse interstellar band (DIB) carriers and their links with gas and dust are mandatory steps in understanding their role in interstellar chemistry.

Aims. The latest SDSS/APOGEE data release, DR14, has provided an increased number of stellar spectra in the H band and associated stellar models using an innovative algorithm known as the Cannon. We took advantage of these novelties to extract the 15 273 Å near-infrared DIB and to study its link with dust extinction and emission.

Methods. We modified our automated fitting methods dedicated to hot stars and used in earlier studies with some adaptations motivated by the change from early- or intermediate-type stars to red giants. A new method has also been developed to quantify the upper limits on DIB strengths. Careful and thorough examinations were carried out of the DIB parameters, the continuum shape, and the quality of the adjustment of the model to the data. We compared our DIB measurements with the stellar extinctions, A_V, from the Starhorse database. We then compared the resulting DIB–extinction ratio with the dust optical depth derived from Planck data, both globally and separately for nearby off-plane cloud complexes.

Results. Our analysis has led to the production of a catalog containing 124 064 new measurements of the 15 273 Å DIB, allowing us to revisit the correlation between DIB strength and dust reddening. The new data clearly reveal that the sky-averaged 15 273 Å DIB strength is linearly correlated with A_V over two orders as reported by earlier studies but leveling off with respect to extinction for highly reddened lines of sight behind dense clouds. The comparison with Planck individual optical depths reveals in a conspicuous way this DIB depletion in the dense cores and shows it applies to all off-plane dense clouds. Using selected targets located beyond the Orion, Taurus, and Cepheus clouds, we derived empirical relationships between the DIB–extinction ratio and the Planck dust optical depth for the three cloud complexes. Their average is similar to the DIB carrier depletion measured in the dark cloud Barnard 68.

Conclusions. APOGEE measurements confirm the ubiquity of the 15 273 Å DIB carrier decrease with respect to dust grains in dense cloud cores, in a manner that can be empirically related to the dust optical depth reached in the cloud. They also show that the ratio between the DIB equivalent width and the extinction A_V for sightlines with τ(353GHz) ≲ 2 × 10⁻⁵ that do not contain dense molecular gas is about four times higher than the constant limit towards which the ratio tends for very long sightlines with many diffuse and dense phases distributed in distance.