High-velocity absorption components toward the LMC

Abstract

High-resolution (λ/δλ ≅ 6 × 10⁴) spectra of R136 (the central object of 30 Dor) and of 2 LMC supergiants obtained at ESO with the 1.4-m telescope have been analysed in order to study the high-velocity absorption components at υ _LSR ≅ +60 km s⁻¹ and ≅ +130 km s⁻¹ first seen in satellite ultraviolet spectra in the general direction of the LMC. Our data have been integrated with previous literature results of SN1987a and of other 4 LMC supergiants obtained from spectra of similar spectral resolution.

At least 6 (possibly 9) interstellar components are found in the range +40 km s⁻¹≤υ _LSR≤+150 km s⁻¹, suggesting that the IUE features at ≅ +60 km s⁻¹ and ≅ +130 km s⁻¹ are in fact splitted in several components when observed at higher resolution. Two components originally discovered toward SN1987a (υ _LSR ≅ +42 km s⁻¹ and +107 km s⁻¹), are detected for the first time in the CaII interstellar spectrum of R136. A component at υ _LSR ≅ +49 km s⁻¹ shows significant variations both in CaII and in NaI along the different lines of sight. A relatively broad component at υ _LSR ≅ +58 km s⁻¹ is present in CaII toward R136, but is generally undetected in NaI in the other lines of sight. Also the components in the range +100 km s⁻¹≤υ _LSR≤+150 km s⁻¹ are detected in CaII, but not in NaI. Two weak absorptions, detected at +125 km s⁻¹ and +141 km s⁻¹ toward R136, may be associated with the ≅ +130 km s⁻¹ component characteristic of the ultraviolet spectra.

The almost perfect velocity coincidence of the +107 km s⁻¹, +42 km s⁻¹, +49 km s⁻¹ and +57 km s⁻¹ components along different lines of sight suggests a common origin of the absorbing gas and a location close to our Galaxy. In spite of the low detection limit in our spectra (W_λ ≃ 5 mÅ), the lack of detection of NaI is compatible with the presence of foreground gas clouds with hydrogen column density N(HI) ≤ 2 × 10¹⁹ cm⁻², or even greater. The fact that we can detect more easily CaII absorptions than the corresponding NaI absorptions indicates a very low value of CaII depletion.