High-Velocity Absorption Components Toward the LMC

Abstract

AbstractHigh-resolution (λ/δλ ⋍ 6 x 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 VLSR ⋍ +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 ≤ VISR ≤ + 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 (VLSR ⋍ +42 km s-1 and +107 km s-1), are detected for the first time in the Call interstellar spectrum of R136. A component at VLSR ⋍ +49 km s-1 shows significant variations both in Call and in Nal along the different lines of sight. A relatively broad component at VLSR ⋍ +58 km s-1 is present in Call toward R136, but is generally undetected in Nal in the other lines of sight. Also the components in the range +100 km s-1 ≤ VLSR ≤ +150 km s-1 are detected in Call, but not in Nal. 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 (Wλ ⋍ 5 mÅ), the lack of detection of Nal is compatible with the presence of foreground gas clouds with hydrogen column density N(HI) ≤ 2 x 1019 cm-2, or even greater. The fact that we can detect more easily Call absorptions than the corresponding Nal absorptions indicates a very low value of Call depletion.