Binaries discovered by the SPY project I. HE 1047-0436: a subdwarf B + white dwarf system

Abstract

In the course of our search for double degenerate binaries as potential progenitors of type Ia supernovae with the UVES spectrograph at the ESO VLT (ESO SN Ia Progenitor surveY - SPY) we discovered that the sdB star HE 1047-0436 is radial velocity variable. The orbital period of 1.213253d, a semi-amplitude of 94km/s, and a minimum mass of the invisible companion of 0.44Msol are derived from the analysis of the radial velocity curve. We use an upper limit on the projected rotational velocity of the sdB star to constrain the system inclination and the companion mass to M>0.71Msol, bringing the total mass of the system closer to the Chandrasekhar limit. However, the system will merge due to loss of angular momentum via gravitational wave radiation only after several Hubble times. Atmospheric parameters and metal abundances are also derived. The resulting values are typical for sdB stars. Comment: A&A Letters, accepted, 4 pages, 5 figures

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arXiv:astro-ph/0109042v1 4 Sep 2001
Astronomy & Astrophysics manuscript no.
(will be inserted by hand later)
Binaries discovered by the SPY project I. HE1047–0436: a
subdwarf B + white dwarf system⋆ ⋆⋆
R. Napiwotzki1, H. Edelmann1, U. Heber1, C. Karl1, H. Drechsel1, E.-M. Pauli1, N. Christlieb2
1Dr. Remeis-Sternwarte, Astronomisches Institut der Universit¨ at Erlangen-N¨ urnberg, Sternwartstr. 7, D-96049
Bamberg, Germany; e-mail: napiwotzki@sternwarte.uni-erlangen.de
2Hamburger Sternwarte, Universit¨ at Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
Abstract. In the course of our search for double degenerate binaries as potential progenitors of type Ia supernovae
with the UVES spectrograph at the ESO VLT (ESO SNIa Progenitor surveY – SPY) we discovered that the sdB
star HE1047−0436 is radial velocity variable. The orbital period of 1.213253 d, a semi-amplitude of 94kms−1, and
a minimum mass of the invisible companion of 0.44M⊙ are derived from the analysis of the radial velocity curve.
We use an upper limit on the projected rotational velocity of the sdB star to constrain the system inclination
and the companion mass to M > 0.71M⊙, bringing the total mass of the system closer to the Chandrasekhar
limit. However, the system will merge due to loss of angular momentum via gravitational wave radiation only
after several Hubble times. Atmospheric parameters and metal abundances are also derived. The resulting values
are typical for sdB stars.
Key words. Stars: early-type – binaries: spectroscopic – Stars: fundamental parameters – white dwarfs
1. Introduction
Type Ia supernovae (SNe Ia) play a prominent role in
the study of cosmic evolution. In particular they are re-
garded as one of the best standard candles to determine
the cosmological parameters H◦, Ω and Λ (e.g. Riess et
al. 1998, Leibundgut 2001). What kind of stars produce
SN Ia events remains largely a mystery (e.g. Livio 2000).
There is general consensus that the event is due to the
thermonuclear explosion of a white dwarf (WD) when a
critical mass is reached, but the nature of the progenitor
system remains unclear.
One of the two viable scenarios is the so-called double
degenerate (DD) scenario (Iben & Tutukov 1984). The
DD model considers a binary with the sum of the masses
of the WD components larger than the Chandrasekhar
mass, which merges in less than a Hubble time due the
loss of angular momentum via gravitational wave radia-
tion. Previous radial velocity (RV) surveys have discov-
ered about 15 DDs with P < 6.d3 (see Marsh 2000). None
Send offprint requests to: R. Napiwotzki
⋆Based on data obtained at the Paranal Observatory of the
European Southern Observatory for program No. 165.H-0588
and 266.D-5658
⋆⋆Based on observations collected at the German-Spanish
Astronomical Center (DSAZ), Calar Alto, operated by the
Max-Planck-Institut f¨ ur Astronomie Heidelberg jointly with
the Spanish National Commission for Astronomy
of these systems seems massive enough to qualify as a
SNIa precursor. In order to perform a definite test of the
DD scenario we have embarked on a large spectroscopic
survey of 1500 WDs using the UVES spectrograph at the
ESO VLT UT2 (Kueyen) to search for RV variable WDs
and pre-WDs (ESO SNIa Progenitor surveY – SPY).
Recently, subluminous B stars (sdB) with WD com-
panions have been proposed as potential SNeIa pro-
genitors by Maxted et al. (2000) who discovered that
KPD1930+2752 is a sdB+WD system. Its total mass ex-
ceeds the critical mass and the components will merge
within a Hubble time which makes KPD1930+2752 the
best candidate for a SNIa progenitor known today (al-
though this interpretation has been questioned recently,
Ergma et al. 2001).
SdB stars are pre-white dwarfs of low mass (≈ 0.5M⊙)
still burning helium in the core, which will evolve directly
to the WD stage omitting a second red giant phase (Heber
1986). Only seven sdB+WD systems with known periods
are available in the literature. Except for KPD1930+2752
their total masses do not exceed the Chandrasekhar limit.
Herewereporton
HE1047−0436 (Section 2). The RV curve is derived and
the nature of the companion is discussed in Section 3.
A spectroscopic analysis and further constraints on the
system inclination and companion mass are presented in
Section 4.
follow-upspectroscopyof

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2Napiwotzki et al.: Binaries discovered by the SPY project. I. HE1047−0436
Fig.1. Power spectrum of the HE1047−0436 measure-
ments. The inset shows details of the region around the
main peak
2. Observations and Data Analysis
HE1047−0436 (α2000= 10h50m26.s9, δ2000= −4◦52′36′′,
Bpg = 14.m7) was discovered by the Hamburg ESO sur-
vey (HES; Wisotzki et al. 1996, Christlieb et al. 2001) as
a potential hot WD and, therefore, was included in our
survey. The UVES spectra showed that it is in fact a sdB
star (Christlieb et al. 2001) and a rather large RV shift of
160km/s was found which made the star a prime target
for further study.
14 high resolution Echelle spectra of HE1047-0436have
been secured with VLT-UVES between March 7 and 18,
2001. The nominal resolution with the 2.1′′slit used by
us amounts to
δλ= 19000. Additional long slit spectra
of somewhat lower resolution (1˚ A) have been obtained at
the Calar Alto observatory using the TWIN spectrograph
on March 11 and 12, 2001. Details on the observational set
up of the UVES instrument and the data reduction can
be found in Koester et al. (2001). The TWIN instrument
and the reduction strategy are described in Napiwotzki &
Sch¨ onberner (1995).
λ
3. Radial velocity curve and the nature of the
invisible companion
Radial velocities from all UVES and TWIN spectra were
derived by cross correlating parts of the blue spectra that
display numerous sharp metal absorption lines besides he-
lium and Balmer lines to a synthetic spectrum calculated
from an LTE model atmosphere (see below). The mea-
surements are accurate to ±3km s−1for the UVES data
and somewhat less precise (±12km s−1) for the TWIN
data because of the lower spectral resolution of the latter
instrument.
Since this system is single-lined the analysis is straight-
forward. A periodogram analysis was performed and the
resulting power spectrum is shown in Fig. 1 and the best fit
RV curve in Fig. 2. The discovery spectra taken at April
Fig.2. Measured RVs as a function of orbital phase
and fitted sine curve for HE1047−0436. Filled triangles
indicate 2001 UVES observations, open triangles both
2000 UVES discovery observations, and filled circles 2001
TWIN measurements
21 and May 17, 2000 are included in Fig. 1 and Fig. 2.
Since these spectra were taken about one year before the
2001 data a very accurate period could be determined:
P = 29h7m5sand a semi-amplitude of 94kms−1. The sys-
tem velocity is γ0= 25kms−1(this has to be corrected by
a gravitational redshift of 1.9kms−1to derive the real sys-
tem velocity). Accordingly the ephemeris for the time T0
defined as the conjunction time at which the star moves
from the blue side to the red side of the RV curve (cf.
Fig. 2) is
Hel.JD(T0) = 24451975.03228+ 1.213253× E
Two aliases exist, which differ by ±6m, but they can be
ruled out on a high confidence level (cf. inset of Fig. 1).
Since HE1047−0436 is a single-lined binary we can
only use the mass function to constrain the mass of
the invisible companion. From a comparison with evo-
lutionary calculations (Dorman et al. 1993) we adopted
M = 0.474M⊙ for the sdB primary. The mass function
yields a lower limit of 0.44M⊙for the invisible companion
(for an inclination i = 90◦). Therefore we conclude that
it is a WD with a C/O core. A main sequence companion
with such a high mass can be ruled out, because it would
leave a detectable fingerprint in our high-resolution spec-
tra. In the next section we will show how the measurement
of the projected rotational velocity of the sdB constrains
the inclination i and the WD mass.
4. Spectroscopic analysis
The stars in the HE1047−0436 binary are only separated
by ≈ 5R⊙(slightly depending on i). Thus the system must
have gone through phases of heavy interaction, probably
common envelope evolution, during the red giant stages
of the progenitors. Thus we checked if the sdB parameters
and chemical abundances deviate from normal sdBs.

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Napiwotzki et al.: Binaries discovered by the SPY project. I. HE1047−04363
Fig.3. Fit of the hydrogen and helium lines of the coadded
spectrum of HE1047−0436
Fig.4. Parameters derived from individual UVES and
TWIN spectra (open circles and squares) and the coad-
ded spectra (filled symbols)
In order to improve the S/N ratio the UVES spec-
tra were RV corrected and then coadded. The coadded as
well as the individual UVES and spectra TWIN have been
analyzed to derive atmospheric parameters using NLTE
model atmospheres (Napiwotzki 1997) and a χ2proce-
dure described by Napiwotzki et al. (1999). An LTE metal
abundance analysis of the coadded UVES spectrum was
performed (see Heber et al. 2000 for details of the models
and the fit procedure).
Matching the synthetic Balmer and He I line pro-
files to the observations resulted in effective tempera-
ture, gravity and He abundance with very small fitting
errors, i.e. Teff = 30242 ± 39K; logg = 5.66 ± 0.008 and
Table 1. Metal abundances for HE1047−0436 compared
to Feige36 (Edelmann et al. 1999) and to solar composi-
tion (Grevesse & Sauval 1998). n is the number of spectral
lines per ion
ion
CII
CIII
NII
NIII
OII
MgII
AlIII
SiIII
SiIV
SII
SIII
ArII
FeIII
n
4
3
ǫ(HE1047−0436)
8.03±0.12
7.94±0.11
8.01±0.19
8.40
7.80±0.11
6.78
5.60±0.03
6.82±0.23
6.60
7.57±0.23
7.48±0.33
7.17±0.22
<6.8
ǫ(Feige36)
ǫ(sun)
8.52
7.18±0.20
7.67±0.1249
1
23
1
3
7
1
14
11
7
7.92
7.94±0.17
6.58
5.75±0.06
6.93±0.18
8.83
7.58
6.47
7.55
7.71±0.43
7.34±0.47
7.09±0.10
7.20±0.26
7.33
6.40
7.50
lognHe
of 0.47M⊙the radius can be calculated from the gravity:
R = 0.17 ± 0.02R⊙.
Since more than a dozen individual UVES spectra are
available systematic errors can be estimated. Individual
results (with one exception) differ by no more than ±500K
and ±0.05dex in logg from that derived from the coadded
spectrum (see Fig.4). Another important accuracy check
can be obtained from the long slit spectra. The fit of the
coadded TWIN spectrum resulted in a moderately higher
effective temperature (about 1000K) than from the UVES
spectrum, whereas the gravity is in very close agreement
(cf. Fig. 4).
Lines of several heavy elements can be identified in
the UVES spectra (cf. Table 1). The large number of
NII lines allows to determine the microturbulent velo-
city which turns out to be zero. The resulting abundances
are summarized in Table 1 and compared to the sdB star
Feige36 (Edelmann et al. 1999) which has atmospheric
parameters (Teff = 29700K, logg = 5.9, lognHe
very similar to HE1047−0436 and to solar composition.
In order to match the ionization equilibria of C, N, Si, and
S it would be necessary to lower Teff to 29000K. This is
probably caused by the neglect of NLTE effects. However,
the abundances are almost unaltered (by less than 0.1 dex)
by such a change in Teff.
Thecomparison ofthe
HE1047−0436 and Feige36 in Table 1 shows that
the abundance patterns are quite similar. Only iron is
somewhat underabundant in HE1047−0436 (only an
upper limit could be derived). We conclude that the
phases of close binary interaction didn’t produce any
detectable peculiarities of the sdB star.
A closer inspection of the metal lines showed that these
are much narrower than expected from the nominal reso-
lution (R ≈ 19000). The obvious reason is that the seeing
during all exposures (0.4′′...1.2′′) was smaller than the
slit width (2.1′′). This enables us to derive meaningful up-
nH= −2.632± 0.026 (Fig.3). Adopting the sdB mass
nH= −2.1)
metalabundancesof

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4 Napiwotzki et al.: Binaries discovered by the SPY project. I. HE1047−0436
Fig.5. Observed spectrum (coaddition of the seven spec-
tra obtained during best seeing) compared to synthetic
spectra with v sini = 0 (solid line) and v sini = 10kms−1
(dashed line)
per limits on the projected rotational velocity v sini of
the sdB. Since we were interested in minimizing the in-
strumental broadening we coadded only the seven spectra
taken during periods of best seeing (0.4′′...0.8′′) for this
task. We adopted a conservative value of the spectral res-
olution corresponding to the mean of the three best seeing
values (0.5′′).
Following the procedure described in Heber et al.
(1997) we took synthetic line profiles, convolved them with
rotational profiles, and measured the fit quality (Fig. 5).
We limited this to lines of the ions NII and OII because
for these the quality of the atomic data is highest and
these lines are best reproduced by the model spectrum
calculated with the abundances of Table 1. A 3σ upper
limit on v sini of 4.7kms−1results.
Since the synchronization time between orbit and ro-
tation of the sdB is less than 700,000years (Tassoul &
Tassoul 1992), it is very likely that orbital and rotational
period are equal. This corresponds to a rotational velocity
of the sdB of 7.2 ± 0.8kms−1. Our upper limit on v sini
thus transforms to an upper limit on the inclination angle
of this system of 48◦. From the mass function we then de-
rive a lower limit on the mass of the invisible companion
of 0.71M⊙. Thus the total mass of the binary is 1.2M⊙
or even larger, close to the Chandrasekhar limit. A more
definite determination would need dedicated observations
with a narrow slit and a well defined spectral resolution.
5. Discussion and Conclusions
We measured accurate RVs for the sdB star He1047−0436
from high resolution spectra and derived an orbital period
of 1.213253d and a semi-amplitude of 94kms−1. The mass
of the invisible companion is shown to be at least 0.44M⊙
indicating that the companion is likely a C/O core WD. If
we make the reasonable assumption, that the sdB rotation
is synchronized with the orbital period, we can use an up-
per limit on v sini to compute an upper limit on the incli-
nation angle and a lower limit on the WD mass of 0.71M⊙.
Six of the seven known sdB+WD binaries (Maxted et al.
2001) have considerably shorter periods (0.09d to 0.57d)
than HE1047−0436. Only PG1538+269 has a longer or-
bital period (2.501d, Saffer et al. 1998). Even the mini-
mum mass we derive for the WD companion ranks third
among the known systems.
From a quantitative spectral analysis precise atmo-
spheric parameters and metal abundances were derived.
Intercomparison with an analysis of long slit spectra
demonstrated that the atmospheric parameters derived
from the Echelle spectra are highly reliable and system-
atic errors are small. The resulting atmospheric parame-
ters and metal abundances are typical for sdB stars.
The effective temperature of HE1047−0436 lies within
the range where short period (2–10min.) non-radial
pulsating sdB stars were discovered (O’Donoghue et
al. 1999). Photometric monitoring of HE1047−0436 for
about 20min did not reveal any variations above a level
of 2mmag (Østensen, priv. com.)
Acknowledgements. We express our gratitude to the ESO
staff, for providing invaluable help and conducting the ser-
vice observations and pipeline reductions, which have made
this work possible. C. K., H. E., and E.-M. P. gratefully ac-
knowledge financial support by the DFG (grants Na365/2-1
and He1354/30-1). This project was supported by DFG travel
grant Na365/3-1.
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