A&A 405, 1153-1156 (2003)
DOI: 10.1051/0004-6361:20030715
G. Hébrard 1 - N. F. Allard 1,2 - J. F. Kielkopf 3 - P. Chayer 4 - J. Dupuis 4 - J. W. Kruk 4 - I. Hubeny 5
1 - Institut d'Astrophysique de Paris, CNRS, 98bis boulevard
Arago, 75014 Paris, France
2 -
Observatoire de Paris-Meudon, LERMA, 92195 Meudon Principal
Cedex, France
3 -
Department of Physics, University of Louisville,
Louisville, KY 40292, USA
4 -
Department of Physics and Astronomy, Johns Hopkins University,
Baltimore, MD 21218, USA
5 -
NOAO, 950 North Cherry Avenue, Tucson, AZ 85726, USA
Received 1 April 2003 / Accepted 7 May 2003
Abstract
We present new theoretical calculations of the line profile of
Lyman
that include transitions in which a photon is absorbed
by a neutral hydrogen atom while it interacts with a proton. Models
show that two absorption features located near 992 Å and 996 Å
are due to H-H+ collisions. These quasi-molecular satellites are
similar to those that were identified in the wings of Lyman
and Lyman
lines of hydrogen-rich white dwarfs. We compute
synthetic spectra that take account of these new theoretical profiles
and compare them to the spectra of four DA white dwarfs that were
observed with FUSE. The models predict the absorption features that
are observed in the wing of Lyman
near 995 Å, and confirm
that these features are quasi-molecular satellites.
Key words: line: profiles - radiation mechanisms: general - stars: atmospheres - white dwarfs - ultraviolet: stars
Quasi-molecular satellites are absorption features due to transitions that take place during close collisions of a radiating hydrogen atom with a perturbing atom or proton. These spectral features, which are present in the red wings of the Lyman series lines, provide an important source of opacity in the atmospheres of hydrogen-rich white dwarfs.
The IUE spectra of the DA white dwarf 40 Eri B were the first
ones which revealed a strong, unexpected absorption feature near 1400 Å (Greenstein 1980). Absorption features near 1600 Å were
thereafter detected in spectra of cooler DA white dwarfs (Holm et al. 1985). These features remained unexplained until Koester et al. (1985) and Nelan & Wegner (1985) simultaneously identified them
as quasi-molecular satellites of the Lyman
line. The H2 and H2+ satellites at 1600 Å and 1400 Å were thereafter observed
in other DA white dwarfs, as well as in laboratory plasmas (Kielkopf
& Allard 1995). The H2+ satellite absorption features at 1058 Å and 1076 Å were first identified in the spectrum of the
DA white dwarf Wolf 1346, as observed with HUT (Koester et al. 1996). ORFEUS (Koester et al. 1998) and FUSE (Wolff et al. 2001; Hébrard et al. 2002b) observations allow these
Lyman
satellites to be observed in other targets.
New calculations for the absorption profiles of Lyman
line of
atomic hydrogen perturbed by protons have been used to predict
synthetic spectra of hot hydrogen-rich white dwarfs and allow the
identification of a strong feature near 995 Å. This feature was
present in the HUT spectrum of Wolf 1346 (Koester et al. 1996) but it is only in Koester et al. (1998) that a similar
feature present in ORFEUS spectra of WD 1031-114 and
WD 0644+375 was suspected to be a new satellite in the wing of
Lyman
.
It was also distinctly detected in FUSE spectra of
CD
(Wolff et al. 2001) and of
Sirius B (Holberg 2002).
These last two recent observations provided motivation to investigate
the theory of the Lyman
line profile using accurate
theoretical molecular potentials, instead of using the simple Stark
broadening approximation. Our new calculations of Lyman
satellites are summarized in Sect. 2. We present in Sect. 3 a
comparison between synthetic spectra including Lyman
satellites and the FUSE spectra of four DA white dwarfs. The
results are discussed in Sect. 4.
The theoretical approach is based on the theory of pressure broadening due to Baranger (1958), developed in an "adiabatic representation'' that does not exclude degeneracy of atomic levels. A detailed description of our semi-classical approach as applied to the shape of the Lyman lines has been given by Allard et al. (1994) and Allard et al. (1999).
The Lyman
profile and its satellites are calculated for
physical conditions encountered in the atmospheres of white dwarfs.
Because the density of the hydrogen atoms is low (1015 to 1017 cm-3), we computed the line profile by using the low
density approximation as described by Allard et al. (1994). This
approximation uses the expansion of the autocorrelation function in
powers of density. We also used accurate theoretical potentials
describing the binary interactions of one hydrogen atom with a ionized
hydrogen atom. The H2+ molecular potentials are available for the
H (
)
states (Madsen & Peek 1971). We computed all those
related to n=4 using a code kindly provided by J. Peek.
Table 1: Targets.
Our theoretical approach allows the variation of the radiative dipole
moment during a collision. But through lack of appropriate data for
Lyman ,
we assumed that the dipole moment for the transitions
is constant. Such approximation has been applied to the calculations
of the quasi-molecular satellites observed in the IUE and HST spectra of white dwarfs (see, e.g., Koester & Allard 1993;
Koester et al. 1994; Bergeron et al. 1995). In the last section we
discuss the possible effects of a variable dipole moment on the line
profile calculations.
An H2+ correlation diagram was constructed for Lyman (Allard et al. 2003), to which 14 transitions contribute. Two of
these transitions,
and
,
yield satellites in
the red portion of Lyman
at 992 Å and 996 Å.
Figure 1 shows the profiles of both individual satellites
and their effect on the total profile of Lyman
.
We obtain a
blend of the line satellites with a maximum near 992 Å. The profile
gives a shape similar to the 995 Å feature observed in white dwarf
spectra.
![]() |
Figure 1:
The 995 Å-region Lyman ![]() ![]() |
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Atmosphere models have been calculated using the programs TLUSTY
(Hubeny 1988; Hubeny & Lanz 1992, 1995). We considered LTE model
atmospheres with pure hydrogen composition that explicitly include the
Lyman
and Lyman
quasi-molecular opacities. The
resulting spectra were computed by using the spectral synthesis code
SYNSPEC that incorporates the quasi-molecular satellites of
Lyman
,
Lyman
,
and Lyman
.
Models predict that
the Lyman
satellites are visible roughly for effective
temperatures between 15 000 K and 30 000 K. We verified that
non-LTE effects are unimportant in the range of effective temperatures
considered in this study.
We used observations of four DA white dwarfs performed with FUSE
(Moos et al. 2000). The spectra were obtained between 2000 and 2002
and were retrieved from the FUSE public database (see Table 1). The
one-dimensional spectra were extracted from the two-dimensional
detector images and calibrated using the CalFUSE pipeline. When
possible, we did not use exposures with strong airglow emission. We
also excluded regions of the spectra near the edges of the detector
segments, which provide poor quality data. The eight FUSE detector
segments of the different exposures were co-added together and
projected on a 0.16 Å-pixel base, so pixels about 25 times larger
than the original FUSE detector pixels. This degradation of the
FUSE spectral resolution (typically
for this kind of target with the
slit LWRS or MDRS; see Hébrard et al. 2002a or Wood et al. 2002) has
no effect on the shapes of the large stellar features which we study
and allows the signal-to-noise ratio to be increased.
In Fig. 2 we compare the FUSE spectra with the models that we
obtained for these targets. The effective temperatures are within the
intermediate temperature range for the visibility of both the
Lyman
and Lyman
satellites. The agreement indicates,
by the coincidence of position and approximate shape, that the large
feature near 995 Å is indeed a satellite of Lyman
due to
the blend of two close features at 992 Å and 996 Å.
![]() |
Figure 2:
A comparison of the FUSE spectra of the white dwarfs
and the theoretical models.
Top to bottom: Wolf 1346, EG 102, BPM 6502, and
CD
![]() ![]() ![]() |
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The first target plotted in Fig. 2 is Wolf 1346. The comparison
for the FUSE spectrum shows that the far UV is well fitted with a
synthetic spectrum computed with our new profile calculations for
and
K. These atmospheric parameters
are close to those determined from optical spectra (7.9; 20 000) and
adopted by Koester et al. (1996) to fit the HUT spectrum of that
white dwarf. The Lyman
satellite, already observed in the
HUT spectrum, is well-reproduced by our synthetic spectrum.
The next two objects, EG 102 and BPM 6502, are
slightly hotter white dwarfs, with smaller gravities. We used the
effective temperatures and gravities determined from optical spectra
(Finley et al. 1997; Bragaglia et al. 1995). The predicted spectra
for EG 102, with
K and
,
and for
BPM 6502, with
K and
,
fit quite
well the FUSE spectra. In the case of BPM 6502, all the
FUSE exposures present strong airglow emissions, so it was not
possible to remove them. These two stars are not normal DA white
dwarfs: EG 102 presents metallic lines (Holberg et al. 1997)
and BPM 6502 is in a post common-envelope binary system
(Kawka et al. 2000, 2002). These particularities seem however to have
no effects on the quality of the fits.
The hottest object we present is CD
.
The FUSE
observation that we used is the same as the one presented by Wolff et al. (2001), but our data reduction was performed with a more recent
version of CalFUSE. Our best fit was obtained using
K and
.
The values of the atmospheric
parameters are slightly smaller than the ones determined from optical
spectra (Vauclair et al. 1997) to get a better agreement with the
Lyman
satellite.
CD
is near the upper limit of the expected
visibility range of the Lyman
satellites. Nevertheless,
Lyman
and Lyman
quasi-molecular satellites have been
observed in the FUSE spectrum of the ultra-massive white dwarf
PG 1658+441 (
K,
)
as reported
by Dupuis et al. (2001). We speculate that the presence of
quasi-molecular satellites in this star is related to its high surface
gravity and we will investigate the question in more details in Dupuis
et al. (2003). It is interesting to note that Sirius B (
K,
;
Holberg et al. 1998), which also has a
relatively high surface gravity, shows the quasi-molecular satellites
(Holberg 2002).
Finally, the four spectra show a kink in the profile around 1037 Å
that is well reproduced by our models (Fig. 2). This feature is
probably the first identification of the H2+ satellite of
Lyman ,
as predicted in Fig. 7 of Allard et al. (1998a)
The good agreement between the FUSE spectra and our calculations allowed the line satellites near 995 Å to be identified, although there are still some systematic discrepancies between the predicted and observed profiles. The shape of the profile in the region of the satellites is sensitive to the relative strength of these two main features. It is therefore important to get an accurate quantitative determination of the satellite amplitudes. Accurate theoretical molecular potentials have to be used to describe the interaction between the radiator and the perturber as we did in the work described here. Another important factor, not yet included, is the variation of the dipole moment during the collision. Allard et al. (1998a,b, 1999) have shown that the strengths of line satellites are dependent on values of the electric-dipole moments at the internuclear separation responsible for the satellites. Large enhancements in the amplitudes of the satellites may occur whenever the dipole moment increases through the region of internuclear distance where the satellites are formed. The theoretical shape in this region of the profile is then dependent on the dipole moment of the transition. Some of the differences between the observed and the synthetic spectra may be due to our constant dipole moment approximation. That will be improved in future calculations.
As a summary we present in Fig. 3 the sum of the profiles
of Lyman ,
Lyman
,
and Lyman
perturbed by
collisions with neutral hydrogen and protons for two different
densities of neutral hydrogen. We can note that beside the
quasi-molecular lines already detected, there is another predicted
H2 satellite at 1150 Å (Allard et al. 2000), in the red wing of
Lyman
,
which should be observed in cool DA white dwarfs in the
range of effective temperatures (11 000 to 13 000 K), where the variable
ZZ Ceti objects are found (Hébrard et al. 2002b). This line
satellite has never been observed in stars or laboratory plasmas.
Future FUSE observations and ongoing study of the FUV spectrum of
dense hydrogen plasmas may allow this feature to be detected.
![]() |
Figure 3:
Total profile of Lyman ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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Acknowledgements
This work is based on data obtained by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins University. Financial support to U. S. participants has been provided by NASA contract NAS5-32985. French participants are supported by CNES. The molecular potential computation code was kindly provided by J. Peek; we would like to thank him.