A&A 459, 875-883 (2006)
DOI: 10.1051/0004-6361:20065741
A. Ederoclite1,2,3 - E. Mason2 - M. Della Valle4,5 - R. Gilmozzi2 - R. E. Williams6 - L. Germany2 - I. Saviane2 - F. Matteucci3 - B. E. Schaefer7 - F. Walter8 - R. J. Rudy9 - D. Lynch9 - S. Mazuk9 - C. C. Venturini9 - R. C. Puetter10 - R. B. Perry11 - W. Liller12 - A. Rotter13
1 - Vrije Universiteit Brussel, 2 Pleinlaan, Brussels, Belgium
2 -
ESO - European Southern Observatory, Alonso de Cordova 3107,
Casilla 91001, Santiago, Chile
3 -
Department of Astronomy, University of Trieste, via Tiepolo 11,
Trieste, Italy
4 -
INAF - Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5,
Firenze, Italia
5 -
Kavli Institute for Theoretical Physics, UC Santa Barbara,
California, 93106, USA
6 -
Space Telescope Science Institute 3700 San Martin Drive,
Baltimore, MD, USA
7 -
Department of Physics and Astronomy, Louisiana State
University, Baton Rouge, Louisiana, 70803, USA
8 -
Department of Physics and Astronomy, SUNY, Stony Brook, NY,
11794-3800, USA
9 -
Aerospace Corporation, PO Box 92957, Los Angeles, CA
90009-2957, USA
10 -
University of California San Diego, 9500 Gilman Dr., La
Jolla, CA, USA
11 -
NASA Langley Research Center, 100 NASA Road, Hampton, VA,
USA
12 -
Isaac Newton Institute, Casilla 8-9, Correo 9, Santiago,
Chile
13 -
Department of Astronomy and Astrophysics, Penn State
University, 525 Davey Lab, University Park, PA, USA
Received 1 June 2006 / Accepted 28 July 2006
Abstract
Aims. We present optical and near-infrared spectral evolution of the Galactic nova V5114 Sgr (2004) during few months after the outburst.
Methods. We use multi-band photometry and line intensities derived from spectroscopy to put constrains on the distance and the physical conditions of the ejecta of V5114 Sgr.
Results. The nova showed a fast decline (
days) and spectral features of Fe II spectroscopic class. It reached
MV = -8.7
0.2 mag at maximum light, from which we derive a distance of 7700
700 kpc and a distance from the galactic plane of about 800 pc. Hydrogen and oxygen mass of the ejecta are measured from emission lines, leading to
10-6 and
,
respectively. We compute the filling factor of the ejecta to be in the range 0.1-10-3. We found the value of the filling factor to decrease with time. The same is also observed in other novae, then giving support to the idea that nova shells are not homogeneously filled in, rather being the material clumped in relatively higher density blobs less affected by the general expanding motion of the ejecta.
Key words: stars: novae, cataclysmic variables - stars: individual: V5114 Sgr
Nova Sgr 2004 (V5114 Sgr) was independently discovered by Nishimura (2004)
and Liller (2004) on Mar. 15.8 UT and Mar. 17.3 UT, respectively.
West (2004) provided precise
coordinates RA = 1819
32
29,
Dec = -28$^$36
35
7
(gal. coord. l=3
9 b=-6
3).
Early spectroscopy on Mar. 18.3 UT by Della Valle et al. (2004)
confirmed this object to be a classical nova caught near maximum light.
Here, we present photometric and spectroscopic observations taken at
McDonald Observatory, Cerro Tololo Inter-American Observatory (CTIO),
ESO-La Silla, and Lick Observatory. The spectra from ESO-La Silla have
been obtained as part of the Target of Opportunity campaign for the
observation of Classical Novae in the Galaxy and in the Magellanic
Clouds. The paper is organized as follows: in Sect. 2 we present the
analysis of the photometric and spectroscopic data, in Sects. 3 and 4 we analyze the evolution of the light curve and spectra of V5114 Sgr, in Sect. 5 we discuss the effects of interstellar
absorption. In Sect. 6 we derive the physical parameters of the nova
ejecta. Summary and conclusions are given in Sect. 7.
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Figure 1:
Upper left panel: V band light curve. Filled symbols
represent our data points, empty symbols represent IAUC data.
Vertical dotted lines represent maximum light, t2, and t3.
Data points are affected by errors
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Optical photometry has been carried out during 8 nights in March and April 2004 with the 0.8-m telescope at McDonald Observatory. Another observing run has been carried out during 14 nights in June and July with the Small and Moderate Aperture Research Telescope System (SMARTS) 1.0-m telescope at CTIO. Optical and infrared photometry has been also carried out with the ANDICAM dual-channel imager on the SMARTS 1.3-m at CTIO during 25 nights from March to August. One night of observations has been carried out with a 0.3-m telescope equipped with an SBIG ST7E CCD camera, in Exmouth, Australia. A log of photometric observations is given in Table A.1.
Spectra at maximum and during the early decline have been obtained with FEROS
(Kaufer et al. 1999)
with a resolution
48 000 and spectral range 4000-9000 Å.
Spectrophotometric standard stars have not been observed each night, and
in this case the spectra have been corrected with
an "average response curve''. This procedure
can introduce an uncertainty on the flux measurement up to 50%.
Flux-determination is affected by undetermined uncertainty because FEROS is
a fiber-fed spectrograph that was not equipped with an atmospheric distortion
corrector at the time of these observations. Indetermination is due to the
fact that the observations are carried out guiding on the V-band image of the star that is
differently displaced (due to atmospheric refraction) in the other bands.
During our analysis, fluxes were corrected in order to match the observed
magnitudes.
An independent spectroscopic follow up has been carried out with the RC spectrograph on the SMARTS 1.5-m telescope at CTIO. A spectrophotometric standard star (either LTT 4364 or Feige 110) has been observed each night to remove the instrumental signature. Standard reduction has been carried out with an author's written IDL routine.
An IR spectrum was taken June 22, 2004 UT at Lick Observatory using the Aerospace Corporation's Near-Infrared and Visible Imaging Spectrograph (NIRIS). The standard star used was HR 6836.
All spectra have been analyzed with the onedspec
package in
IRAF
. Line fluxes have been measured by the integration
of the line profile and not by Gaussian fitting. Full width at half
maximum (FWHM) of lines have been measured also via Gaussian fitting but
show no significant difference from direct measure.
A complete log of our spectroscopic observation is reported in
Table A.2.
Table 1: Observed photometric properties for V5114 Sgr.
The optical and near-infrared light curves of V5114 Sgr are shown in
Fig. 1. The light curves have been derived using both our photometric data and
photometry available in the literature (IAUC 8306, 8307, 8310).
The V light curve shows that V5114 Sgr reached V = 8.0 mag on Mar. 17.17 UT
(MJD = 53 081.556).
West (2004) noted that nothing was visible at the same position in
the red Digitized Sky Survey.
After considering that the DSS limiting magnitude is 21 mag, we can infer that the
outburst amplitude was
13 mag which is consistent with values observed for
other novae with about the same rate of decline (see Warner 1995).
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Figure 2: Spectral energy distribution of V5114 Sgr one day after maximum. The magnitudes in the various bands have been corrected for interstellar extinction. |
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The nova decreased by two magnitudes from maximum in t2=11 days
and by three magnitudes in
t3=21 days. Adopting the maximum magnitude versus rate of
decline (MMRD) relation by Della Valle & Livio (1995),
V5114 Sgr achieved an absolute V magnitude at maximum light of
MV=-8.7
0.2 mag. Photometric properties are summarized in Table 1.
The typical photometric errors are smaller than 0.04 mag in all bands but in
mag (J-H) color changes its slope after
day 40. This could be in
principle due to formation of dust in the ejecta but an inspection of
the B and V light curves rules out "DQ Herculis'' behavior, therefore
the reddened color is likely due to to variations in intensity of
emission lines in the NIR.
The spectral energy distribution (SED) one day after maximum appears
to be well fitted by a blackbody at T=9600 K as shown in Fig. 2.
Spectroscopic observations started immediately after discovery. Line identification for FEROS spectra (see Figs. 3 and 4) is given in Table 2 while in Table 3 we show line identification for spectra taken at CTIO (Williams et al., in preparation).
The first spectrum (phase +1)
was dominated by Balmer, Fe II and O I emission lines. This behavior characterized
the nova as a typical "Fe II'' type object, according to the
Cerro Tololo classification (Williams et al. 1991, 1994).
P-Cyg profiles were clearly visible in Balmer lines as well as in
Fe II, O I, and Na I lines. P-Cyg profiles were double,
thus suggesting the presence of two expanding systems with velocities
(obtained by averaging of measurements of Balmer lines) of 1400
50 km s-1 and 850
30 km s-1.
Eight days after maximum the spectrum was still dominated by low ionization species. The double P-Cyg profiles were still clearly visible and the velocities (as derived from both the P-Cyg profiles and FWHM) were increasing. The emission lines started developing a flat topped profile.
On April 9 (phase +23), we observe the
4640 Å emission band together with N II and
N III, although Fe II emission lines were still present.
The O I 8446 emission line was more intense than the
H
one and showed a flatter profile.
By April 18 (phase +32) P-Cyg profiles had disappeared and Fe II emission lines were fading and forbidden and high excitation lines strengthened.
The intensity of [O III]
indicated that V5114 Sgr entered the auroral phase, described in
Williams et al. (1991, 1994). Fluxes of
Balmer lines, that had decreased very slowly until this moment,
started to decrease faster (see Fig. 7). The FWHM of
Balmer lines reached a plateau (2000
100 km s-1).
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Figure 3: Dereddened spectra of V5114 Sgr (blue part). Fluxes are in logarithmic scale to show the less intense lines. |
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Figure 4: Dereddened spectra of V5114 Sgr (red part). Fluxes are in logarithmic scale to show the less intense lines. |
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Figure 5:
Dereddened spectra of V5114 Sgr taken at CTIO and covering the
region 3500-5200 ![]() |
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Figure 6: Dereddened IR spectrum of V5114 Sgr observed with NIRIS at Lick Observatory on June 22, 2004 UT. Upper panel shows the "blue'' part of the spectrum and the lower panel shows the "red'' part. |
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The O I 8446 and He I
5876 lines show flat topped profiles while the
hydrogen lines have a clearly asymmetric profile (the red side being
more prominent than the blue one). At this stage the O I
8446
emission line reached its maximum intensity.
By May 13 (phase +57) the hydrogen lines turn to flat topped profiles
(like oxygen) while nitrogen lines were still rounded. It has been
noted in the past (see Payne-Gaposchkin 1957) that
different line profiles observed at the same stage indicate that the
emission lines originate in different layers of the ejecta.
Flat topped profiles originate from optically thin spherical shells while
rounded profiles are related to optically thick winds.
The NIR part of the spectrum (observed only on June 22, see Fig. 6,
line identification given in Table 5) showed prominent
Paschen and Brackett lines as well as oxygen and nitrogen lines.
Common but unknown lines (1.10, 1.19, 1.55 and 2.10 m) were
present in this spectrum (see Venturini et al. 2004). Tentative identifications for these lines with van Hoof's line list
are given in Table 6.
Few suggested identifications have already been suggested by Rudy et al.
(2002) during analysis of lines of V723 Cas.
In September the spectrum was dominated by O I
4959-5007 lines.
All lines showed saddle-shaped profiles.
The O I
8446 line had almost disappeared.
H
was clearly strongly blended with N II.
The overall evolution of expansion velocities measured from P-Cyg
absorption is shown in Fig. 8. Cassatella et al.
(2004) have shown that the evolution of P-Cyg profiles as
measured in the UV of Nova Cyg 1992 can be modelled with an exponential
law
,
where v(t) is
the velocity t days after maximum, v0 is the expansion velocity
at maximum light,
is the asymptotic velocity and
is
a time scale similar to t3. The best fit to the data gives
50 km s-1 and 1500
50 km s-1,
v0 = 1300
50 km s-1 and 800
50 km s-1 and
1 day. Both
and velocity measurements
based on the FWHM of emission lines during the nebular stage are
important quantities that should be considered in view of future
observations aimed at determining the distance to V5114 Sgr, via
nebular expansion parallax.
Table 5: Reddening-corrected near infra red line fluxes (in erg s-1 cm-2) observed with NIRIS at Lick Observatory on June 22, 2004, UT.
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Figure 7:
Upper panel: evolution of Balmer lines fluxes.
Triangles represent H![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
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We have used several methods to estimate the reddening of V5114 Sgr. This piece of information is crucial to measure the distance to the object. The results of our various estimates of the interstellar reddening are summarised in Table 7.
Table 6: Tentative identification of previously unidentified NIR lines.
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Figure 8: Velocities of the ejecta. Measures are derived as average of P-Cyg profiles of Balmer lines. Squares refer to the faster absorption system and triangles to the slower one. Filled symbols refer to FEROS observations and empty sybols to CTIO observations. Continuous lines are exponential description of the evolution (see text). |
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Table 7: Measured values for reddening of V5114 Sgr.
We could not use the commonly used NaD lines because they are saturated.
Nevertheless it has
been shown by Munari & Zwitter (1997) that alternatively to
these lines, the K I 7699 line can be used.
The advantage to use this line is that it has a less steep curve
of growth, thus giving the opportunity to measure higher reddenings.
After applying this method to V5114 Sgr, we found
EB-V = 0.45 mag.
Colors at maximum are a widely used empirical indicator of
interstellar reddening. Van Den Bergh & Younger (1987) showed
that novae at maximum have an intrinsic B-V of 0.23
0.06 mag. The same authors showed also that novae, two magnitudes below maximum, have an intrinsic
B-V = -0.02
0.04 mag. As from Table 1, V5114 Sgr had
mag and
(B-V)t2 = 0.38 mag. This leads to
EB-V = 0.43
0.06 mag
and
EB-V = 0.40
0.04 mag, respectively.
From Schlegel et al. (1998) dust maps of the Milky Way,
we have derived
EB-V = 0.58 mag.
Relations based on emission-line ratios are considered the best ones
(see Williams 1994), unfortunately they can be used only
at very late stages when the nova is in the optically thin phase. As an example, if we compute the reddening through O I
8446 and O I
13164
(following Rudy et al. 1991) we find
EB-V=1.2 mag, which is very different
from the reddening estimates derived with other methods (see Table 7).
Williams (1994) suggested that if the
He II
4686, 10124 and the
H I
4861, 10049 are optically thin, then they can be used
to measure the absorption to the nova via:
After assuming the maximum magnitude obtained in Sect. 3 with the MMRD
(
MV = -8.7
0.2 mag) and the average absorption derived above, we derive a distance to V5114 Sgr of 9000
900 pc. A complementary estimate of the distance has been obtained
using the Buscombe-DeVaucouleurs relation (all novae show the same
magnitude 15 days after maximum) as from Capaccioli et al. (1989). This second estimate leads to 6000
1200 pc.
After taking the average weight, we get d = 7700
700 pc.
For l=3
9 and b=-6
3, we find
the nova to be located 770
70 pc above the galactic plane
Analysis of dereddened line fluxes is the only way to derive physical
parameters of nova ejecta (e.g. masses and temperatures).
Since ejecta are still evolving toward the nebular stage, the
line ratios are not the ones expected from atomic transition
probabilities. For example, looking at Fig. 7 the
H/H
ratio converges toward the theoretical value only
after phase
100.
A different explanation (also at later stages) is invoked for the case of
[O I]
6300, 6364. This is a well known example of lines
that does not respect the theoretical ratio
3:1.
Williams (1994) interpreted this as due to large optical
depth in the 6300 Å line, and showed that
the optical depth of that line can be derived from
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(2) |
The knowledge of the optical depth and electron temperature allows us to estimate
the mass of oxygen in the ejecta using the 6300 Å line:
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(3) |
Electron densities can be determined adopting the temperatures computed
above and [O III] line ratios as in Osterbrock (1989)
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(4) |
Hydrogen mass can be derived following Mustel & Boyarchuk
(1970). The H line is given by
Solving Eq. (5) for ,
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(7) |
![]() |
(8) |
The derived physical parameters are summarized in Tables 8 and 9.
V5114 Sgr is an Fe II nova that occurred in the bulge of the Milky Way. The rate of decline characterizes V5114 Sgr as a borderline
object between the fast (t2 < 12 days) and slow (t2 > 12 days)
classes (Della Valle & Livio 1998). These authors have
shown that He/N and Fe IIb (i.e. Fe II broad) novae belong
to the fast class and are preferentially concentrated towards the galactic
disc, i.e. at small z above the galactic plane (z<200 pc), while
Fe II novae belong to both slow and fast classes and are observed
both in the disk and in the bulge, extending up to 1 kpc.
V5114 Sgr located at about 0.8 kpc above the galactic plane in the
direction of the galactic bulge is an Fe II nova which does not
represent an exception to this scenario (Della Valle et al. 1992).
Table 8:
Physical parameters for V5114 Sgr: O I
line ratios, optical depth in
(
), electron
temperature, O I mass, [O III] line ratios and electron densities.
Spectroscopic observations showed a dramatic change in the overall appearance of the spectrum of the nova at about 30 days after maximum light (in coincidence with the entrance in the "auroral phase''): permitted lines start fading at a different rate, P-Cyg profiles disappear, and velocities reach a "plateau'' phase and U-V and J-K colors increase. The lack of detection of [Ne III] line hints for a "standard'' evolution of the nebular spectrum (see Williams et al. 1994).
The very high values of optical depth in the O I
suggest
very high densities for the zones of the ejecta where these lines are
formed. We have derived the filling factor in the range
7.1
10-2
to 7.9
10-4. Comparing these values with other values
reported in the literature (see Table 10), two facts emerge:
a) the filling factors in nova ejecta are definitely smaller than 1,
likely close to 0.1, during the early stages; b) these values decrease
by 1-3 orders of magnitude with time. This fact indicates that the volume of the expanding shell (computed with Eq. (6)) increases with time more rapidly than the volume actually
occupied by most of the ejected material. In other words, the decreasing
trend exhibited by the filling factor suggests that the ejected matter tends
to remain clumped in sub-structures having higher density than the average
density characterizing the expanding shell. This is consistent with the
[O I]
6300, 6364 ratio < 2 (Table 8), which suggests that [O I] lines should be formed in very dense small blobs of neutral material embedded within the ionized shell (see paragraph 6).
However, recently Williams & Mason (2006) proposed an alternative
interpretation of this behavior, assuming that the [O I] lines arise in
regions of high magnetic field, and their intensity and profile are
modified by Quadratic Zeeman Effect.
Table 9:
Physical parameters for V5114 Sgr:
and the hydrogen mass.
Table 10: Comparison between filling factors in literature.
Computed oxygen and hydrogen masses are in the ranges 1.9
10-5-2.4
and 3.0
10-5-1.1
.
This high mass ratio is close to the upper limit
for classical novae shown in Warner (1995).
Acknowledgements
The authors are indebted to Pierluigi Selvelli and Chris Sterken for their critical reading of the manuscript. They also thank the anonymous referee and Steve Shore, who helped improving the presentation.
This work has been partly supported by "IAP P5/36'' Interuniversity Attraction Poles Programme of the Belgian Federal Office for Scientific, Technical and Cultural Affairs.
Table 2: V5114 Sgr reddening-corrected emission line fluxes (in erg s-1 cm-2) from FEROS spectra.
Table 3:
V5114 Sgr reddening-corrected emission line fluxes (in erg s-1 cm-2)
from CTIO spectra with wavelength range larger than 2000 and including the H
region.
Table 4: V5114 Sgr reddening-corrected emission line fluxes (in erg s-1 cm-2) from CTIO spectra shown in Fig. 5.
Table A.1: Measured magnitudes during photometric campaigns (described in text).
Table A.2: Log of the spectroscopic observations for V5114 Sgr.