2011A&A...529A..28A

Abundances of the Mg isotopes ²⁴Mg, ²⁵Mg, and ²⁶Mg can be used to test models of chemical enrichment of interstellar/intergalactic gas clouds. Additionally, because the position of the MgII λλ2796, 2803Å lines is often taken as a reference in computations of possible changes of the fine-structure constant α, it should be clarified to which extent these lines are affected by isotopic shifts. We use a high-resolution spectrum (pixel size ≃1.3km/s) of the quasar HE0001-2340 observed with the UVES/VLT to measure Mg isotope abundances in the intervening absorption-line systems at high redshifts. Line profiles are prepared taking into account possible shifts between the individual exposures. In the line-fitting procedure, the lines of each ion are treated independently. Because of the unique composition of the selected systems - the presence of several transitions of the same ion - we can test the local accuracy of the wavelength scale calibration, which is the main source of errors in the sub-pixel line position measurements. In the system at z_abs=0.45, which is probably a fragment of the outflow caused by SN Ia explosion of high-metallicity white dwarf(s), we measured velocity shifts of MgII and MgI lines with respect to other lines (FeI, FeII, CaI, CaII): ΔV_MgII=-0.44±0.05km/s, and ΔV_MgI=-0.17±0.17km/s. This translates into the isotopic ratio ²⁴Mg:²⁵Mg:²⁶Mg=(19±11):(22±13):(59±6) with a strong relative overabundance of heavy Mg isotopes, (²⁵Mg+²⁶Mg)/²⁴Mg=4, as compared to the solar ratio ²⁴Mg:²⁵Mg:²⁶Mg=79:10:11, and (²⁵Mg+²⁶Mg)/²⁴Mg=0.3. In the systems at z_abs=1.58 and z_abs=1.65 enriched by AGB-stars we find only upper limits on the content of heavy Mg isotopes (²⁵Mg+ ²⁶Mg)/²⁴Mg≲0.7 and (²⁵Mg+²⁶Mg)/²⁴Mg≲2.6, respectively. At z_abs=1.58, we also put a strong constraint on a putative variation of α: Δα/α=(-1.5 ±2.6)x10^–6, which is one of the most stringent limits obtained from optical spectra of QSOs. We reveal that the wavelength calibration in the range above 7500Å is subject to systematic wavelength-dependent drifts.