A project to analyze ultraviolet spectra of novae obtained at or near maximum optical light is presented. These spectra are characterized by a relatively cool continuum with superimposed permitted emission lines from ions such as Fe II, Mg II, and Si II. Spectra obtained late in the outburst show only emission lines from highly ionized species and in many cases these are forbidden lines. The ultraviolet data will be used with calculations of spherical, expanding, stellar atmospheres for novae to determine elemental abundances by spectral line synthesis. This method is extremely sensitive to the abundances and completely independent of the nebular analyses usually used to obtain novae abundances

Baschek, B.

Hauschildt, P.

Hestand, J.

Shaviv, G.

Spies, W.

Starrfield, S.

Stryker, L. L.

Wehrse, R.

English

NASA Technical Reports Server

N89 - 107 0 61,9
,I_b-'I_D_NG PAOE BLANg NOT FILMED
THE ANALYSIS OF SPECTRA OF NOVAE TAKEN NEAR MAXIMUM
L. L. Stryker, J. Hestand, and S. Starrfield
Department of Physics, Arizona State University
R. Wehrse, P. Hauschildt, W. Spies, B. Baschek
Institute of Theoretical Astronomy
University of Heidelberg
G. Shaviv
Department of Physics, The Technion
ABSTRACT
We have recently begun a project to analyze ultraviolet
spectra of novae obtained at or near maximum optical
light. These spectra are characterized by a relatively
cool continuum with superimposed permitted emission
lines from ions such as Fe II, Mg II, and Si II. In
contrast, the spectra obtained late in the outburst
show only emission lines from highly ionized species
and in many cases these are forbidden lines. These
ultraviolet data will be used in combination with
recent calculations of spherical, expanding, stellar
atmospheres for novae to determine elemental abundan-
ces by spectral line synthesis. This method is
extremely sensitive to the abundances and completely
independent of the nebular analyses usually used to
obtain novae abundances.
1. Introduction
The initial eruption of a nova is very rapid, with the
major part of the rise to visual maximum taking place
in a day or less. During the rising branch of the light
curve, the nova basically consists of an optically thick,
uniformly expanding shell. As the expansion is very
rapid and the luminosity fairly constant, the effective
temperature smoothly declines and reaches a minimum
of 4000K-7000K at visual maximum (e.g., Gallagher and
Ney 1976; Gallagher and Starrfield 1978). At this
time the material, which is mostly hydrogen (except in
some very unusual novae), begins to recor.bine and the
pseudo-photosphere begins to move inward in mass. In
fact, as has been emphasized by Gallagher and
Starrfield (1978), the optical maximum is purely an
opacity effect caused by the large decrease in opacity
when hydrogen recombines.
It is only during this time that pure absorption line or
P-Cygni profiles are seen and are accessible to analysis
(Williams et al. 1981; Starrfield 1987). In fast,
optically luminous, novae the primary shell will become
optically thin in a few days; thereafter, the optical
spectral region is dominated by bremsstrahlung and
hydrogen bound-free emission (Gallagher and Ney
1976; Ennis et al. 1977; Martin 1987). However,
because the ultraviolet opacity is higher, the ultra-
violet region of the spectrum remains optically thick
for a much longer period and the continuum can be
present for days to weeks. The cause of the enhanced
ultraviolet opacity is the presence of thousands of
lines from the astrophysically abundant elements such
as iron and magnesium and other elements with similar
atomic structure. In other astrophysical environments
this is called the "iron forest".
In a slow (low optical luminosity) nova, such as Nova
Vul 1984 #:2, continuous mass loss, via a radiation
pressure driven wind, maintains a low temperature
photosphere for several months or years (Geisel et al.
1970; Bath 1978; Ney and Hatfield 1978). Because the
radius at which the pseudo-photosphere is formed
determines the "effective" temperature of the layers,
at luminosities of 104L®, photospheric radii of 10f2cm
will produce continuum energy distributions charac-
teristic of F (or later) supergiants. That is exactly
what is observed for novae.
Ultraviolet data have now been obtained for more than
a dozen novae with the IUE satellite. The data
obtained late in the outburst have been analyzed and
published for many of them (Starrfield and Snijders
1987; Starrfield 1988) while the spectra obtained near
maximum optical light have never been analyzed and
are seldom shown. Nevertheless, these data are an
important source of information about the expanding
layers at the beginning of the explosion and now that
the techniques have been developed to analyze these
data we expect to see this situation change rapidly.
2. Physical Model and Assumptions
In order to proceed with the analysis, some of us have
developed a program to calculate model stellar atmo-
spheres (Shaviv, Wehrse, and Wagoner 1984; Spies etal.
1987). These models are calculated with the following
assumptions: l) spherical symmetry (not plane parallel),
2) the density follows a power law with an exponent
around 2 or 3 (in contrast to the supernova models
that have a density exponent approaching 7 to 10), 3)
LTE (including scattering), 4) radiative equilibrium, 5)
and the expansion velocity is proportional to the
radius. All of the relevant opacity sources are
included so that we will be able to calculate the
effects of the iron forest (c.f., Baschek and Johansson
1986) and then determine the expansion opacity (Karp
etal. 1977).
The original motivation for the development of this
code was to analyze the spectra of SN II and to try
A Decade of UV Astronomy with IUE, Proc. Celebratory Symposium, GSFC, Greenbelt, USA, 12-15 Aprd 1988, ESA SP-281, Vol. 1 (June 1988)
https://ntrs.nasa.gov/search.jsp?R=19890001335 2020-03-20T05:58:18+00:00Z
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Figure 1. Comparison of a theoretical energy distribu-
tion for a line-blanketed, spherical, expanding model
atmosphere with the appropriate black-body energy
distribution. This model had T = 10,000K and
E(B-V)=0.5. The outer radius ewas 1012cm and the
density exponent was 2.
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Figure 2b. The LWP image of Nova Vul 1984 #1 taken
just after the spectrum shown in Figure 2a. Note that
each spectrum is scaled to the strengths of the
emission lines in the given spectral region.
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Figure 2a. The SWP image of Nova Vul 1984 #1 taken
about 15 days after discovery. This slow nova was
still at maximum light. The cool continuum is obvious
and this spectrum looks completely unlike spectra
obtained later in the outburst• Lye is present and so
strong that the other lines would not have been
visible.
ANALYSISOFSPECTRAOFNOVAENEARMAXIMUM 151
and understand the cause of the large ultraviolet
opacity in these objects. In fact, it has been shown
that the code can be applied to this problem with
excellent agreement between theory and observations
(Spies etal. 1987).
In Figure 1 we show the results of the initial attempt
to calculate a model for a nova atmosphere near
maximum. The model atmosphere had a radius of
1012cm, a density law exponent of 2, an effective
temperature of 10,000K, E(B-V)=0.5, and included 30,000
lines. The upper line is the equivalent black body
curve and shows the large amount of ultraviolet
opacity present in this model. It is important to note
that even with the large number of blends, there are
only a few lines apparent in the spectrum and in order
to analyze a nova we will have to do spectral line
synthesis.
We are also in the process of developing a method to
facilitate our line identifications for novae spectra
using a technique similar to that outlined for super-
novae spectra by Branch (1987). We use the Saha and
Boltzmann equations and the line list of Kurucz and
Petrymann (1975) to compute the strongest expected
lines in each wavelength interval.
3. IUE Observations of Novae
We have obtained early spectra of numerous nova and
in this section we show a few representative LWP and
SWP spectra. Figure 2a and 2b shows spectra of Nova
Vul 1984 #1 taken on day 245 of 1984 when the nova
was at 7.0m; Figure 3a and 3b shows spectra of Nova
Vul 1984 #2 taken on day I of 1985 when this nova
was at 6.3m. Finally, in Figure 4a and 4b we show
spectra of Nova And 1986 obtained on day 343 of 1986
when this nova was at 8.8m. This series of spectra
emphasize our statements about the structure of the
atmosphere at maximum optical light. The cool
continuum is obvious and the strongest features are
probably caused by Si I and Fe I. Many of the other
emission lines are probably blends of lines from
elements such as iron, nitrogen, carbon, oxygen, and
silicon. Unfortunately, these spectra are too complex
for proper line identification and we have also obtained
some high dispersion spectra in order to improve our
wavelengths [see Sion et al. 1987]. In fact, as can be
seen from Figure I, it is likely that most of the lines
that we see are blends.
We also note that there is a general similarity of the
early ultraviolet nova spectra with those of SN 1987A
obtained on the first day of its outburst. Of course,
the lines are much broader in SNI987A and it is much
harder to identify the elements leading to the specific
blends. In fact, we should not be surprised at the
resemblance since, in both cases, we are examining
material with a great deal of hydrogen and helium.
One motivating factor in this study is to apply the
codes designed for studying the supernova to a study
of the novae since there is so much more data on
novae. In addition, once we have identified the
features in the nova spectra, it will then be possible
to take the nova spectra, broaden them, use this data
to assist in the analysis of the SN spectra, and
ultimately determine which elements are contributing to
the broad blends in SN 1987A.
4. Summary and Discussion
We have begun an analysis of the early spectra of
novae in an attempt to develope a completely new
method (as applied to novae) to determine the
elemental abundances in these exciting objects. We
shall use a new stellar atmosphere code that includes
the effects of sphericity, expansion, and numerous lines
to calculate model atmospheres for novae at maximum.
This same code is being used on comparable studies of
SN 1987A.
This will allow us to determine the abundances for
novae in a completely independent way from the
commonly used nebular studies of the emission lines
seen much later in the outburst when the density has
dropped to where photoionization and recombination
are the only important factors effecting the line
strengths.
This study would not have been possible without the
rapid response of the IUE Observatory to our requests
for Target-of-Opportunity observations of novae and
we are grateful to the IUE observatory for their
continued support of nova observations. The data were
reduced with the facilities of the RDAF at the
University of Colorado which are supported by NASA
Grant NAS5-28731 and T. Armitage's help is gratefully
acknowledged. We also acknowledge partial support for
this research from NASA, NSF, and the DOE through
grants to our various institutions.
References
Baschek, B., and Johansson, S. 1986, in Proceedings Qf
IAU Coll. # 94: Physics of Formation of Fe II
Lines Outside LTE, ed. R. Viotti (Reidel), in
press.
Bath, G.T. 1978, M.N.R.A.S., 182, 35.
Branch, D. 1987, ADJ. Lett., 320, LI21.
Ennis, D., Becklin, E.E., Beckwith, S., Elias, J., Gately,
I., Matthews, K., Neugebauer, G., and Willner, S.P.
1977, Ao. J., 214, 33.
Gallagher, J.S., and Ney, E.P. 1976, Ao. J. Lett., 204,
L35.
Gallagher, J.S., and Starrfield, S. 1978, Ann. Rev. Astr.
Ao., 16, 171.
Geisel, S.L., Kleinmann, D.E., and Low, F.J. 1970, ADJ.
Lett., 161, LI01.
Karp, A.H, Lasher, G., Chan, K.L., and Salpeter, E. E.
1977, ADJ., 214, 161.
Kurucz, R.I., and Peytremann, E. 1975, SAO Special
Report Number 362.
Martin. P.G. 1988, in The Classical Nova. ed. M. Bode
and N. Evans, (Wiley, New York), in press.
Ney, E.P., and Hatfield, B. 1978, AD. J. Lett., 219,
LIII.
Shaviv, G., Wehrse, R., and Wagoner, R. V. 1984,
ADJ., 289, 198.
Sion, E. M., Starrfield, S., Van Steenberg, M. E.,
Sparks, W. M., Truran, J. W., and Williams, R.E.
1986, Astronomical Journal, 92, 1145-1149.
Spies, W., Hauschildt, P., Wehrse, R., Baschek, B., and
Shaviv, G. 1987, in Proceedings of the Fourth
Rin_ber_ Castle Workshoo on Nuclear Astro-
ohvsics, ed. W. Hillebrandt (Garching), in press.
Starrfield, S. 1988, in The Classical Nova. ed. M. Bode
and N. Evans (Wiley, NY) in press.
Starrfield, S. 1987, in New Insights in Astrophysics:
Ei=ht Years of Ultraviolet Astronomy with IUE.
ed. E. Rolfe, ESA press,(Nordwijk) SP-263, 239,
1986.
Starrfield, S. and Snijders, M. A. J. in Exolorin_ the
Universe with the IUE Satellite. ed. Y. Kondo
(Dordrecht: Reidel), 377, 1987.
Williams, R.E., Sparks, W.M., Gallagher, J.S., Ney, E.P.,
Starrfield, S.G., and Truran, J.W. 1981, Ao. J.,
251, 221.
152 LLSTRYKERTAL
1400 1500 1600 1700 1800 1900 2000
WAVELENGTIi (_)
7
220O 2400 2600 2000 3000 3200
WAVELLNG rll (A)
Figure 3a. The SWP image of Nova Vul 1984 #2
obtained on January 1, 1985 just a few days after
discovery. Most of the same lines are present as in
Nova Vul 1984 #1 but the relative strengths differ.
This may be caused by either a real abundance
difference or because of differences in the atmospheric
conditions.
Figure 3b. The LWP image of Nova Vul 1984 #2
obtained at the same time as the SWP image in Figure
3a. Note the strong P Cygni profile of Mg II 2800A.
Note the difference in scaling from Figure 3a to 3b.
&
o
i_r_racrlvE RrDu_IoN sYsrt_ pLOt OF S2_0_, I_S • _-_
1400 1500 1600 1700 1800 1900 2000
WAVELENGTH (_.]
2200 2400 2000 2900 3000 3200
WAVELLNG Ht (A)
Figure 4a. The SWP image of Nova Andromeda 1986
obtained in December 1986 about 5 days after
discovery. Again, note the striking similarity in the
lines present in all three SWP spectra.
Figure 4b. The LWP image of Nova Andromeda 1986
obtained just after the SWP image. In this spectrum it
appears that Mg II 2800A is completely in absorption.
In fact, Mg II was never very strong in this nova.
Note the difference in scaling from Figure 4a to 4b.


The analysis of spectra of novae taken near maximum

The analysis of spectra of novae taken near maximum

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