The diffuse interstellar band around 8620 Å

II. Kinematics and distance of the DIB carrier^★

¹ University Côte d’Azur, Observatory of the Côte d’Azur, CNRS, Lagrange Laboratory, Observatory Bd, CS 34229, 06304 Nice cedex 4, France
e-mail: he.zhao@oca.eu, mathias.schultheis@oca.eu
² Instituto de Astrofísica, Facultad de Física, Pontificia, Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
³ Millennium Institute of Astrophysics, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna Tenerife, Spain
⁵ Instituto Astrofísica de Canarias, Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain

Received: 19 April 2021
Accepted: 11 August 2021

Abstract

Context. Diffuse interstellar bands (DIBs) are important interstellar absorption features of which the origin is still debated. With the large data sets from modern spectroscopic surveys, background stars are widely used to show how the integrated columns of DIB carriers accumulate from the Sun to great distances. To date, studies on the kinematics of the DIB carriers are still rare.

Aims. We aim to make use of the measurements from the Giraffe Inner Bulge Survey (GIBS) and the Gaia–ESO survey (GES) to study the kinematics and distance of the carrier of DIB λ8620, as well as other properties.

Methods. The DIBs were detected and measured following the same procedures as in Zhao et al. (2021, A&A, 645, A14; hereafter Paper I), assuming a Gaussian profile. The median radial velocities of the DIB carriers in 38 GIBS and GES fields were used to trace their kinematics, and the median distances of the carriers in each field were estimated by the median radial velocities and two applied Galactic rotation models.

Results. We successfully detected and measured DIB λ8620 in 760 of 4117 GES spectra with |b| ≤ 10° and signal-to-noise ratio (S∕N) > 50. Combined with the DIBs measured in GIBS spectra (Paper I), we confirmed a tight relation between EW and E(J − K_S) as well as A_V, with similar fitting coefficients to those found by previous works. With a more accurate sample and the consideration of the solar motion, the rest-frame wavelength of DIB λ8620 was redetermined as 8620.83 Å, with a mean fit error of 0.36 Å. We studied the kinematics of the DIB carriers by tracing their median radial velocities in each field in the local standard of rest (V_LSR) and into the galactocentric frame (V_GC), respectively, as a function of the Galactic longitudes. Based on the median V_LSR and two Galactic rotation models, we obtained valid kinematic distances of the DIB carriers for nine GIBS and ten GES fields. We also found a linear relation between the DIB λ8620 measured in this work and the near-infrared DIB in APOGEE spectra at 1.5273 μm, and we estimated the carrier abundance to be slightly lower compared to the DIB λ15273.

Conclusions. We demonstrate that the DIB carriers can be located much closer to the observer than the background stars based on the following arguments: (i) qualitatively, the carriers occupy in the Galactic longitude–velocity diagram typical rotation velocities of stars in the local Galactic disk, while the background stars in the GIBS survey are mainly located in the Galactic bulge; (ii) quantitatively, all the derived kinematic distances of the DIB carriers are smaller than the median distances to background stars in each field. A linear correlation between DIB λ8620 and DIB λ15273 has been established, showing similar carrier abundances and making them both attractive for future studies of the interstellar environments.