UVES/VLT high resolution spectroscopy of GRB 050730 afterglow: probing the features of the GRB environment^*

¹ INAF - Osservatorio Astronomico di Roma, Via Frascati 33, 00044 Monteporzio Catone, Italy
² Dunsink Observatory, Castleknock, Dublin 15, Ireland
³ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁴ Universita' di Milano Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁵ Astronomic Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK
⁶ School of Physical Sciences and NCPST, Dublin City University, Glasnevin, Dublin 9, Ireland
⁷ CEA Saclay, DSM/DAPNIA/Service d'Astrophysique, Bat. 709, L'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
⁸ JILA, Campus Box 440, University of Colorado, Boulder, CO 80309-0440, USA
⁹ NASA-Goddard Space Flight Center, Greenbelt, Maryland, 20771, USA
¹⁰ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, Pennsylvania 16802, USA
¹¹ ASI Science Data Center, Via G. Galilei, 00044 Frascati (Roma), Italy
¹² INAF - Osservatorio Astronomico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹³ INAF - Roma, Via Fosso del Cavaliere 100, 00133 Roma, Italy
¹⁴ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, Via U. La Malfa 153, 90146 Palermo, Italy
¹⁵ Universita' di Ferrara, Via Paradiso 12, 44100 Ferrara, Italy
¹⁶ UMR 7164, 11 place M. Berthelot, 75231 Paris, France
¹⁷ International School for Advanced Physics, Via Beirut 2-4, 34014 Trieste, Italy
¹⁸ European Southern Observatory, Casilla 19001, Santiago, Chile
¹⁹ PPARC, Polaris House, North Star Avenue, Swindon SN2 15Z, UK
²⁰ INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Bassini, 15, 20133 Milano, Italy
²¹ European Southern Observatory-Vitacura, Casilla 19001, Santiago 19, Chile
²² Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy

Received: 29 September 2006
Accepted: 19 February 2007

Abstract

Aims.The aim of this paper is to study the Gamma Ray Burst (GRB) environment through the analysis of the optical absorption features due to the gas surrounding the GRB.

Methods.To this purpose we analyze high resolution spectroscopic observations (–45 000, corresponding to 14 km s^-1 at 4200 Å and 6.6 km s^-1 at 9000 Å) of the optical afterglow of GRB050730, obtained with UVES@VLT ~4 h after the GRB trigger.

Results.The spectrum shows that the ISM of the GRB host galaxy at is complex, with at least five components contributing to the main absorption system. We detect strong C II*, Si II*, O I* and Fe II* fine structure absorption lines associated to the second and third component.

Conclusions.For the first three components we derive information on the relative distance from the site of the GRB explosion. Component 1, which has the longest wavelength, highest positive velocity shift, does not present any fine structure nor low ionization lines; it only shows very high ionization features, such as C IV and O VI, suggesting that this component is very close to the GRB site. From the analysis of low and high ionization lines and fine structure lines, we find evidences that the distance of component 2 from the site of the GRB explosion is 10–100 times smaller than that of component 3. We evaluated the mean metallicity of the system obtaining values ≈10^-2 of the solar metallicity or less. However, this should not be taken as representative of the circumburst medium, since the main contribution to the hydrogen column density comes from the outer regions of the galaxy while that of the other elements presumably comes from the ISM closer to the GRB site. Furthermore, difficulties in evaluating dust depletion correction can modify significantly these values.
The mean [C/Fe] ratio agrees well with that expected by single star-formation event models. Interestingly the [C/Fe] of component 2 is smaller than that of component 3, in agreement with GRB dust destruction scenarios, if component 2 is closer than component 3 to the GRB site.