Issue |
A&A
Volume 516, June-July 2010
|
|
---|---|---|
Article Number | A54 | |
Number of page(s) | 5 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/200913287 | |
Published online | 24 June 2010 |
Spectral evolution of the extremely fast classical nova V838 Herculis
T. Iijima1 - A. Cassatella2,3
1 - Astronomical Observatory of Padova, Asiago Section, Osservatorio Astrofisico, 36012 Asiago (Vi), Italy
2 -
INAF, Istituto di Fisica dello Spazio Interplanetario, via del Fosso del Cavaliere 100, 00133 Roma, Italy
3 - Departmento de Astrofisica, Facultad de Fisica, Universidad Complutense, 28040 Madrid, Spain
Received 12 September 2009 / Accepted 28 March 2010
Abstract
Spectral evolution of nova V838 Her 1991 was monitored at Asiago
Astrophysical Observatory from 1991 March 31 to July 2. The spectra in the
early decline stage showed emission lines of H I, He I, He II, N II, N III,
C II, C III, C IV, Si II, [S III], [Ne III], and Fe II, where some
identifications are different from those reported in the previous works. The
emission lines of [Ne III] and carbon ions were unusually intense. New
emission peaks appeared at the blue-ward edges of the emission lines of H I
and He I about one week after maximum luminosity. It seems that a new clump
of gas was ejected at that time along the line of sight towards us. The helium
abundance is estimated at N(He) =
.
Key words: stars: individual: V838 Her - novae, cataclysmic variables - ISM: general
1 Introduction
The nova (V838) Her 1991 was discovered independently by Sugano (1991)
and Alcock (1991) on 1991 March 24 as a star of
.
The outstanding property of this nova was the extremely fast decline
rate: two mags in two days, which is one of the fastest novae on record
(Vanlandingham et al. 1996, and references therein). It was
suggested from infrared observations that a little of the dust had existed
before the outburst and a condensation of new dust occurred in less than ten
days from maximum luminosity (Chandrasekhar et al. 1992; Lynch et al. 1992; Woodward et al. 1992; Kidger & Martinez-Roger
1993; Harrison & Stringfellow 1994). This nova is classified
as a neon nova, because strong emission lines of neon ions were detected since
its early decline stage (Matheson et al. 1993; Williams et al. 1994). The central system is known as an eclipsing binary
(Leibowitz et al. 1992; Ingram et al. 1992).
Chemical abundances of this object were estimated using optical and UV
spectra (Vanlandingham et al. 1996, 1997; Schwarz et al. 2007), where overabundances of neon and carbon and a low
abundance of oxygen were remarkable. Recently, Kato et al.
(2009) analysed spectroscopic and photometric UV data and
estimated the interstellar extinction and the distance to the nova as
and
kpc. They proposed in the same
article a model with a massive white dwarf,
,
for the
outburst.
This paper presents the spectral evolution of V838 Her from 1991 March 31 to July 2.
2 Spectral evolution
Spectroscopic observations were carried out on 1991 March 31, April 2, and
July 2 using a Boller & Chivens grating spectrograph mounted on the 182 cm
telescope at the Mount Ekar station of the Astronomical Observatory of Padova.
The spectra were reduced using the standard tasks of the NOAO
IRAF
package at the Asiago Observatory of the University of Padova. The spectral
resolution was
with a grating of 600 lines mm-1, and the spectrophotometric calibrations were made using the
spectra of the standard star Kopff 27 obtained in the same nights. The
interstellar extinction was corrected assuming
E(B-V)=0.53 (Kato et al. 2009). A log of the observations is given in Table 1, where UT is
the universal time at the start of exposure.
Table 1: Log of spectroscopic observations of V838 Her.
2.1 1991 March 31
Figure 1 shows a tracing of our first spectrum obtained on 1991 March 31.
The emission lines of hydrogen Balmer series were prominent. Those of
H
and H8 were blended with [Ne III] 3968 and [Ne III] 3869,
respectively. The heliocentric radial velocity of the line centre of H
was
km s-1, and the FWHM was
km s-1. The corresponding
quantities of H
were
km s-1 and
km s-1. The other
prominent emission lines were of He I, He II, N III, and C III. The red-ward
tail of H
was blended with the emissions of He I 4922 and Fe II 4924.
A weak emission line found around 4248Å was identified as C II 6, 4267.0,
4267.3 (see Sect. 3.1).
![]() |
Figure 1: A spectrum of V838 Her on 1991 March 31. The unit of the ordinate is 10-12 erg cm-2 s-1 Å-1. |
Open with DEXTER |
2.2 1991 April 2
Three spectra obtained on 1991 April 2 covered the spectral range
3900-7130 Å. Figure 2 shows a tracing of the blue spectrum. The
heliocentric radial velocity of the line centre of H
and H
were
km s-1 and
km s-1, respectively, and the line
widths (FWHM) were
km s-1 and
km s-1.
The emission lines of H
and H
showed four peaks on March 31
(Fig. 1), while a new peak was detected at the bluest side of the emissions
on April 2 (Fig. 2). These variations suggest an ejection of a new clump of
gas (see Sect. 4). The emission line at 4248 Å strengthened.
![]() |
Figure 2: A spectrum of V838 Her on 1991 April 2. The unit of the ordinate is 10-12 erg cm-2 s-1 Å-1. |
Open with DEXTER |
Figure 3 shows the spectral range 4950-6050 Å. The strongest emission line in this region was due to He I 5876. The second intense emission line at 5027 Å was identified as C IV 3, 5021 and 5023 (see Sect. 3.2). The emission lines of N II 3, 5680, Fe II 49, 5317, and C IV 1, 5801 and 5812 were also detected. A weak emission line at 5160 Å was identified as Fe II 42, 5169.
![]() |
Figure 3: A spectrum of V838 Her on 1991 April 2. The unit of the ordinate is 10-12 erg cm-2 s-1 Å-1. |
Open with DEXTER |
Figure 4 shows a tracing of the red spectrum. The dotted line demonstrates
the peak profile of H
emission with one tenth scale. The profile of
H
was nearly the same as that
of H
(Fig. 2). The heliocentric radial velocity was
km s-1,
and the width (FWHM) was
km s-1. The heliocentric radial velocities
of the five emission peaks were -1940, -930, -180, +630, and
+1650 km s-1 with errors of
km s-1. These velocities were somewhat
different from those observed by Matheson et al. (1993) on 1991 May 5,
i.e., -1880, -946, -122, +746, and +1790 km s-1. The peak at
-930 km s-1 was the highest in our spectrum (Fig. 4), while the peak at
+747 km s-1 was the highest on 1991 May 5 (Matheson et al. 1993).
As seen later, the peak at -930 km s-1 became the highest again in H
on 1991 July 2 (Fig. 5). These variations of the individual peaks in
radial velocity and in relative intensity suggest a very complicated
structure of the ejecta.
We identified the emission complex around 6353 Å as a blend of Si II 2, 6347, 6371, and [S III] 3F, 6310 (see Sect. 3.3). The last component became dominant in the nebular stage (Matheson et al. 1993; Williams et al. 1994; Vanlandingham et al. 1996), but it was still a minor component in this spectrum. The other prominent emission lines were of He I.
![]() |
Figure 4:
A spectrum of V838 Her on 1991 April 2. The unit of the ordinate
is 10-12 erg cm-2 s-1 Å-1, and the profile of H |
Open with DEXTER |
![]() |
Figure 5: A spectrum of V838 Her on 1991 July 2. The unit of the ordinate is 10-12 erg cm-2 s-1 Å-1. |
Open with DEXTER |
2.3 1991 July 2
Our last spectrum, which covered the blue region 3950-5100 Å, was
obtained on 1991 July 2 (Fig. 5). The strongest emission line in the
spectrum was [Ne III] 3968. The broad emission complex at
Å
was a blend of He II 4686, [Ne IV] 4720, and probably [Ar IV] 4711 and 4740.
The castellated structure of H
was virtually the same as that of April
2 (Fig. 2), which suggests that there was no further ejection of a
clump of gas since the last observation. The other prominent emission lines
were of [O III] 5007, 4959, H
+[O III] 4363, and H
+[S II]
4076. These spectral features were nearly the same as those observed on 1991
June 12 by Williams et al. (1994). High intensities of the
emission lines of neon ions and the weakness of [O III] lines were notable,
which are consistent with the overabundance of neon and the low abundance
of oxygen of this object (Schwarz et al. 2007).
3 Identifications of emission lines
Probably because of the overabundance of carbon (Schwarz et al. 2007), some emission lines unfamiliar in the spectra of usual classical novae were detected in our spectra. We discuss the identifications of these lines in this section.
3.1 Emission line at 4248 Å
A weak emission line was seen at 4248 Å in the spectrum on March 31 (Fig. 1). This emission line grew much in intensity on April 2 (Fig. 2). In our preliminary report (Iijima 1991), we tentatively identified it as N II 47, 4242. However, this identification is unlikely, because the line profile was different from that of N II 3, 5680 (Fig. 3). A prominent emission line with a nearby wavelength in the spectra of novae is [Fe II] 21F, 4244 (Meinel et al. 1968). The identification as [Fe II] 21F, 4244, however, is also unlikely, because emission lines of [Fe II] are usually observed in slow novae (McLaughlin 1960). CP Pup is the unique fast nova which showed [Fe II] lines (Sanford 1945). However, the other [Fe II] lines detected in the spectra of CP Pup, e.g., [Fe II] 21F, 4276.8 or [Fe II] 7F, 4287.4 (Sanford 1945), were not detected in this object.
As mentioned above, the emission line at 4248 Å grew much in intensity
between 1991 March 31 and April 2. At the same time, the blue-ward parts of
the emission lines of H I and He I also grew in intensity (Figs. 1, 2).
Thus we assumed that the emission line at 4248 Å was due to a
blue-ward part of an emission line, namely the peak at 4240 Å in the
spectrum on April 2 was blue-shifted by -1920 km s-1 as for the bluest peaks
of the H I and He I lines. Under this assumption, we estimated the
laboratory wavelength of the emission line as Å, which coincides
with that of C II 6, 4267.0, 4267.3 (Moore 1959). These lines are
prominent in the spectra of some novae (e.g., Sanford 1945; Iijima
& Nakanishi 2008). Therefore we identified the emission
line at 4248 Å as the blue-ward part of C II 6, 4267.0, 4267.3. The
FWHM of the emission feature was about 2000 km s-1, which was less than
half that of H I lines (Sect. 2.2). This result seems to support our
hypothesis, namely, only the blue-ward parts of the emission lines were
seen. However, the reason why the red-ward parts were not seen
is still an open question.
3.2 Emission line at 5027 Å
The second intense emission line in Fig. 3 is seen at 5027 Å. The blend of He I 5016 and Fe II 5018 is prominent around this wavelength in the spectra of usual classical novae in the early decline stage. Thus Williams et al. (1994) identified this emission as He I 5016. However, if this had been the case, the other emission line of He I 4922 should have been detected with nearly the same intensity, but such an emission was not seen in our spectrum (Fig. 2). In addition to this, the profile of the emission at 5027 Å was different from those of the other He I lines, but resembled that of the highly ionized emission line He II 4686 (Fig. 2). Because the emission lines of carbon ions were unusually intense in these spectra, we identified the emission at 5027 Å as C IV 3, 5021 and 5023 (Moore 1959). Probably there were also He I 5016, Fe II 42, 5018, and C II 35, 5047.2 as minor components in the emission.
3.3 Emission complex at 6353 Å
An emission complex with multiple peaks is seen around 6353 Å (Fig. 4).
Williams et al. (1994) identified this emission complex as N II 46,
6346. This identification is problematic however, because there is no line
with the wavelength of 6346 Å in the group of multiplet 46 of N II (Moore
1959). Probably they identified the emission complex as a blend of
N II 46, 6340.7 and 6357.0 (Moore 1959), whose weighted mean
wavelength is nearly 6346 Å. The resolution of our spectra was high enough
to separate the two lines of N II multiplet 46. As seen in Fig. 3, the
emission line of N II 3, 5680 showed prominent peaks at its blue and red
edges. The heliocentric radial velocities of these peaks were
km s-1 and
km s-1, respectively. We assumed that the
profiles of the other N II emission lines were similar to that of N II 3,
5680. Thus if the emission complex had been due to N II 46, 6340.7 and 6357.0,
there should have been emission peaks at 6302, 6318, 6381, and 6397 Å. The
corresponding emission peaks, however, were not detected. It seems that the
emission complex were not due to the N II lines, but were more probably
identified as a blend of Si II 2, 6347, 6371, and [S III] 3F, 6310.
4 Ejection of a clump of gas
As reported in Sect. 2.2, the profiles of the emission lines of H I and He I
greatly changed between 1991 March 31 and April 2. The emission lines of
H
and H
showed four peaks on March 31 (Fig. 1), which suggested
that there were four major clumps of gas in the ejecta. Probably there was
one more minor clump of gas in the ejecta, because a step was seen on the
blue-ward wall of each emission, and weak emission peaks were seen on the
blue-ward edges of the emissions of H
and H
.
The emissions
of the last clump strengthened much and appeared as the bluest emission peaks
of H I and He I lines by April 2 (Fig. 2). The blue-shifts of the new emission
peaks with respect to the line centre were about -1920 km s-1. Because the new
peaks were found at the bluest part of the emissions, the clump of gas seems
to have been ejected along the line of sight towards us.
Optical and infrared light curves showed that fading in V band and brightening in infrared K and L bands began about ten days after maximum luminosity (Woodward et al. 1992; Harrison & Stringfellow 1994). These phenomena suggest a dust condensation at that time in the circumstellar ambient. Woodward et al. (1992) suggested that a dust condensation occurred on 1991 March 31 or April 1 in an optically thick clump of gas that happened to lie along the line of sight. Our spectra suggest that a clump of gas was ejected along the line of sight towards us, probably several days earlier than March 31. At the present time we do not know whether the clump of gas proposed by Woodward et al. (1992) was identical with that suggested in our observations. If this was the case, the dust condensation should have occurred in a few days after the ejection of the clump of gas. Detailed studies, beyond the scope of this work, are required to understand whether such a rapid dust condensation is possible.
5 Helium abundance
Intensities of prominent emission lines relative to H
= 100 are
measured in the spectra obtained on 1991 April 2, where the interstellar
extinction is corrected by
E(B-V)=0.53 (Kato et al. 2009). The
results are given in Table 2. The observational errors in the intensities are
about 10%, and the values of larger errors are denoted by a colon.
Table 2: Relative intensities of prominent emission lines of V838 Her on 1991 April 2.
Using the intensities of the emission lines of He I 4471, and He II 4686
relative to H,
the helium abundance in the ejecta were estimated.
The other He I lines were not used, because He I 5876 was blended with C III
20, 5871.6 and 5894.1, He I 6678 was blended with H
,
and He I 7065
was blended with C II 20, 7115. The formula to derive the helium abundance
is presented by Iijima (2006). The effect of the collisional
excitation of the He I line was corrected using the formula of Peimbert and
Torres-Peimbert (1987) at
K and
cm-3. We estimated the abundances as
and
,
namely
.
Our result is slightly lower
than those obtained by Schwarz et al. (2007), i.e.,
.
The bluest peaks were higher than the others in the He I lines (Figs. 2-4), while they were not the highest peaks in the H I lines (Figs. 2, 4). These results, however, do not necessarily mean a higher helium abundance in the new ejecta. When the electron density in nebulosity is high, the collisional excitation plays an important role in He I lines (Clegg 1987; Peimbert & Torres-Peimbert 1987), and its efficiency depends on the electron temperature. If the electron temperature in the new ejecta were higher than those in the other parts of the ejecta by 1000 degrees, the high emission peaks of the He I lines would be possible even with the same helium abundance. At the present time, we are not able to estimate the electron temperatures of the individual clump of gas precisely. Thus we can not decide whether the high emission peaks in the He I lines were due to a higher helium abundance or a higher electron temperature.
6 Concluding remarks
Because of the very fast fading, we observed only three nights of this nova. However, even with these scarce data, we noticed some interesting phenomena which were not reported in the previous works.The variations of the profiles of the emission lines of H I and He I between 1991 March 31 and April 2 suggest that a clump of gas was ejected along the line of sight towards us. The dust condensation started at nearly the same time in a clump of gas, which happened to lie along the line of sight (Woodward et al. 1992). If these two clumps of gas were identical, the dust should have condensed in a few days after the ejection of the clump.
New identifications are proposed for some emission lines (Sect. 3). The emission lines of carbon ions, which were unfamiliar in the spectra of usual classical novae, were detected in this object. It will be necessary to take into account the unusually intense emission lines of carbon ions in the analyses of the chemical abundance of this object, because for example some He I lines were probably blended with those of carbon ions.
AcknowledgementsWe are grateful to Profs. M. Kato and I. Hachisu for the useful discussions. Thanks are also due to Prof. R. Barbon for the careful reading of the manuscript and useful suggestions.
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Footnotes
- ...
IRAF
- IRAF is distributed by NOAO for Research in Astronomy, Inc. under cooperative agreement with the National Science Foundation.
All Tables
Table 1: Log of spectroscopic observations of V838 Her.
Table 2: Relative intensities of prominent emission lines of V838 Her on 1991 April 2.
All Figures
![]() |
Figure 1: A spectrum of V838 Her on 1991 March 31. The unit of the ordinate is 10-12 erg cm-2 s-1 Å-1. |
Open with DEXTER | |
In the text |
![]() |
Figure 2: A spectrum of V838 Her on 1991 April 2. The unit of the ordinate is 10-12 erg cm-2 s-1 Å-1. |
Open with DEXTER | |
In the text |
![]() |
Figure 3: A spectrum of V838 Her on 1991 April 2. The unit of the ordinate is 10-12 erg cm-2 s-1 Å-1. |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
A spectrum of V838 Her on 1991 April 2. The unit of the ordinate
is 10-12 erg cm-2 s-1 Å-1, and the profile of H |
Open with DEXTER | |
In the text |
![]() |
Figure 5: A spectrum of V838 Her on 1991 July 2. The unit of the ordinate is 10-12 erg cm-2 s-1 Å-1. |
Open with DEXTER | |
In the text |
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