Issue |
A&A
Volume 420, Number 2, June III 2004
|
|
---|---|---|
Page(s) | 571 - 588 | |
Section | Stellar structure and evolution | |
DOI | https://doi.org/10.1051/0004-6361:20034102 | |
Published online | 28 May 2004 |
A study of the expanding envelope of Nova V1974 Cyg 1992 based on IUE high resolution spectroscopy*
1
Istituto di Astrofisica Spaziale e Fisica Cosmica, CNR, Area di Ricerca Tor Vergata, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
2
Dipartimento di Fisica E. Amaldi, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
3
Astronomical Institute and SRON Laboratory for Space Research, Utrecht University, Princetonplein 5, 3584CC, Utrecht, The Netherlands
4
Dipartimento di Fisica, Università degli Studi La Sapienza, Piazzale A. Moro 2, 00185 Roma, Italy
5
XMM Observatory, Villafranca Satellite Tracking Station, PO Box 50727, 28080 Madrid, Spain
Corresponding author: A. Cassatella, cassatella@fis.uniroma3.it
Received:
23
July
2003
Accepted:
12
February
2004
We have carried out a detailed analysis of the
archival high resolution spectra of the classical nova V1974 Cyg 1992.
The main UV resonance lines show P Cygni profiles in the first days,
which change into symmetric pure emission lines, and then slowly
become fainter and narrower. Lines of higher ionization species reach
their peak luminosity later than those of low ionization. This can be
explained by a fast wind which is optically thick in the early days,
when the pseudo-photosphere is located inside the wind. As the mass loss
decreases, the radius of the pseudo-photosphere schrinks. This has
three effects that explain the observed changes: (1) the deeper
accelerating layers of the wind become visible where the emission
lines are formed by collisional excitation and/or recombination; (2)
as the mass loss rate decreases the emission comes from deeper regions of
the wind where the velocities are smaller; (3) the effective
temperature and the degree of ionization increase. In addition to the
P Cygni and emission lines, we could identify two shortward shifted
absorption systems which originate in two separate expanding shells,
outside the wind layers where the emission lines are formed. The
velocity of both shells increase with time. The outer main shell,
containing most of the matter ejected at the outburst, produces the
so-called “principal absorption line system”, and the inner faster
moving second shell produces the so-called “diffuse-enhanced
absorption line system”. The acceleration of the two shells is the
result of increasing line-radiation pressure due to the UV-brightening
of the star as the effective radius decreases. Around day 60 the
second shell has overtaken the slower moving principal system shell,
and merged with it. This explains: the sudden disappearance of the
diffuse line system near that date, the upward jump of
=
240 km s-1 in velocity of the principal system and the first
detection of hard X-ray emission on day 63. This velocity jump
indicates that the main shell is ≈4 times more massive than
the second shell. The deceleration suffered by the diffuse-enhanced
system after the shock provides a shock temperature
≈ 1.6 keV, in fairly good agreement with the temperature of
the observed hard X-ray emission. The UV observations are
interpreted through an empirical model in which the pre-nova slow
wind phase is followed by the ejection of two shells, where the
principal and the diffuse-enhanced absorption systems are formed, and
by a phase of fast continuous lower density wind. Our empirical
expansion velocity law for the principal system, together with
Hα interferometric observations of the angular radius on day 10 are used to determine the distance to the nova, which is found to
be 2.9 ± 0.2 kpc, in agreement with HST imaging and with the
absolute magnitude versus rate of decline relationship.
Key words: stars: novae, cataclysmic variables / techniques: spectroscopic / ultraviolet: stars / X-rays: stars
© ESO, 2004
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