Detection of Photometric Variations in the sdBV Star JL 166

Abstract

We report the discovery of oscillations in the hot subdwarf B star JL 166 from time-series photometry using the Goodman Spectrograph on the 4.1-m Southern Astrophysical Research Telescope. Previous spectroscopic and photometric observations place the star near the hot end of the empirical sdB instability strip and imply the presence of a cool companion. Amplitude spectra of the stellar light curve reveal at least 10 independent pulsation modes with periods ranging from 97 to 178 s and amplitudes from 0.9 to 4 mma. We adopt atmospheric parameters of T_eff = 34350 K and log g = 5.75 from a model atmosphere analysis of our time-averaged, medium-resolution spectrum. Comment: 18 pages, 6 figures, accepted for publication in the Astronomical Journal

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arXiv:0906.2566v1 [astro-ph.SR] 14 Jun 2009
Detection of Photometric Variations in the sdBV Star JL 1661
B.N. Barlow2, B.H. Dunlap2, A.E. Lynas-Gray3, & J.C. Clemens2
ABSTRACT
We report the discovery of oscillations in the hot subdwarf B star JL 166 from
time-series photometry using the Goodman Spectrograph on the 4.1-m Southern
Astrophysical Research Telescope. Previous spectroscopic and photometric ob-
servations place the star near the hot end of the empirical sdB instability strip
and imply the presence of a cool companion. Amplitude spectra of the stellar
light curve reveal at least 10 independent pulsation modes with periods ranging
from 97 to 178 s and amplitudes from 0.9 to 4 mma. We adopt atmospheric pa-
rameters of Teff= 34350 K and log g = 5.75 from a model atmosphere analysis
of our time-averaged, medium-resolution spectrum.
Subject headings: stars: subdwarfs – stars: oscillations – stars: individual: JL
166
1. Introduction
Hot subdwarf B (sdB) stars are a class of objects identified with models of extended
horizontal branch stars and have temperatures ranging from 22000 to 40000 K and log g
values from 5.0 to 6.0 dex. They dominate surveys of faint blue objects at high galactic
latitudes and are often cited as the main source of the UV excess observed in giant elliptical
galaxies. Although their origins and evolutionary tracks are still debated, the sdB stars are
believed to be evolved, lower-mass (˜0.5M⊙) stars with a He-burning core surrounded by a
thin H layer. Models of sdB stars show they will evolve directly to the white dwarf cooling
1Based on observations at the SOAR Telescope, a collaboration between CPNq-Brazil, NOAO, UNC, and
MSU.
2Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA;
bbarlow@physics.unc.edu, bhdunlap@physics.unc.edu, clemens@physics.unc.edu
3Department
aelg@astro.ox.ac.uk
of Physics,University ofOxford,Keble Road, OxfordOX13RH,England;

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sequence after core He exhaustion. Their optical spectra are dominated by Balmer lines and
sometimes also display He lines.
Charpinet et al. (1996, 1997) predicted the existence of a class of pulsating sdB stars
based on the presence of p-modes in their model stars at the right temperatures and surface
gravities. The oscillations they found were driven by a κ-mechanism associated with an opac-
ity bump from the ionization of Fe. Contemporaneous with this prediction, Kilkenny et al.
(1997) reported the first detection of p-mode oscillations in the sdB star EC 14026-2647,
opening up the possibility of using asteroseismological methods to probe the interiors of
these stars. Since this discovery, more than 60 pulsating sdB stars (sdBVs) have been ob-
served with either p- or g-mode oscillations. The rapid p-mode pulsators4oscillate with
periods between 80 and 600 s and amplitudes around 10 mma. The g-mode pulsators5have
periods ranging from 1 to 2 hrs and amplitudes comparable to the p-mode pulsators. In
the log g-Teffplane, the p-mode oscillators cluster together in an instability strip with Teff
from 28000 to 35000 K and log g between 5.2 and 6.1 dex. The slow pulsators typically have
cooler temperatures and lower surface gravities with Teff between 23000 and 30000 K and
log g near 5.4 dex. In the region where the red edge of the p-mode instability strip overlaps
the blue edge of the g-mode pulsator strip, three sdBVs have been discovered that show both
p- and g-mode oscillations (Schuh et al. 2006; Baran et al. 2006; Lutz et al. 2009).
We report the discovery of p-mode oscillations in JL 166, a hot subdwarf B star that was
first listed in the catalogue of Jaidee & Lyng˚ a (1969). In their survey, Jaidee & Lyng˚ a (1969)
used the Schmidt telescope of the Uppsala Southern Station at Mount Stromlo to search for
faint violet stars in Southern galactic latitudes. 296 stars were catalogued in the survey;
their colors were assigned “decidedly violet” or “possibly violet” designations, while their
brightnesses were estimated as “bright,” “intermediate,” or “faint.” JL 166 was classified
as “decidedly violet” with “intermediate” brightness, and further observations with a 100
cm reflector at Siding Spring Observatory led to B and V measurements of 15.00 and 15.23,
respectively (Jaidee & Lyng˚ a 1969). More recent 2MASS observations (Skrutskie et al. 2006)
found J, H, and K magnitudes that imply the presence of a cool companion. The only
published spectral data for JL 166 (Heber 1986) revealed strong Balmer lines, He I lines,
and the 4686˚ A He II line, and resulted in an sdOB spectral classification. Heber (1986)
reported temperature and gravity values of 35500 K and 5.8 dex, respectively, which place
JL 166 near the blue edge of the empirical sdB instability strip (see Fig. 1 of Charpinet
2001).
4referred to as V361 Hya or EC 14026 stars
5also known as PG1716, V1093 Her, or Betsy stars

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We first learned of JL 166 after searching the online Subdwarf Database (Østensen 2006)
for sdB stars in the instability strip. Noticing JL 166 fell in the p-mode instability strip,
we observed the star to look for variations in the stellar luminosity. With the Goodman
Spectrograph on the 4.1 m Southern Astrophysical Research (SOAR) telescope, we have
detected at least 10 independent frequencies in the light curve with periods ranging from 97
to 178 s and amplitudes from 0.9 to 4 mma.
Heber (2009) reviews what is currently known about sdB stars, a poorly understood late-
stage of stellar evolution. In particular, Heber (2009) highlights the diversity and emphasizes
(his §14) the need for more determinations of Teff and log g. Further studies, such as the
one on JL 166 presented here, would in due course permit a better, informed study of sdB
star evolution and their contribution to the ultraviolet upturn seen in the spectra of giant
elliptical galaxies.
2. Photometry
We obtained time-series photometry for JL 166 with the Goodman Spectrograph on the
4.1-m SOAR telescope. In imaging mode, the camera-collimator combination re-images the
SOAR telescope focal plane with a focal reduction of three times, resulting in a plate scale
of 0.15 arcsec pixel−1. The camera houses a 4k x 4k Fairchild back-illuminated CCD with
electronics and dewar provided by Spectral Instruments, Inc. Using optics of fused silica,
NaCl, and CaF2, the entire system is optimized for high throughput from 320 to 850 nm.
See Clemens et al. (2004) for further details on the spectrograph.
We observed JL 166 on two engineering nights in 2008 September and October, obtaining
uninterrupted runs of over 2 hours on each night. Each run was obtained through a red-
blocking S8612 filter, which has a bandpass of 300 to 700 nm. To minimize the processing
time between exposures yet keep the readnoise low, we read out a small subsection of the
CCD (approximately 500 x 500 pixels binned 2x2) and used a readout speed of 100 kHz.
Each exposure had an integration time of 10 s, resulting in a cycle time of ˜13 s. Table 1
summarizes the details of our photometric observations.
In order to obtain light curves from the raw images, we extracted our photometry using
the external IRAF package CCD HSP developed by Antonio Kanaan, which employs the
aperture photometry preferred by O’Donoghue et al. (2000). Aperture widths were chosen
to maximize the signal-to-noise ratio in the light curves and were approximately 1.7 times
the seeing width. To subtract the sky from each stellar aperture, we used sky annuli with
widths of 1.5 arcsec that started approximately 4.5 arcsec from the centers of the stars.

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We divided our light curves by those of a constant comparison star to remove small-scale
variations in the sky transparency. The comparison star used is located approximately 02′
39.′′4 southwest of JL 166 and was the only one present in our frames with an adequate signal-
to-noise ratio. Atmospheric extinction effects were corrected by fitting and normalizing the
curves with parabolas. We note that this normalization may remove real variations in the
stellar brightness on the order of our run length. Due to superb telescope guiding, we did
not flat-field or bias-subtract any of the frames. The reduced light curves for JL 166 are
presented in Figure 1.
We analyzed our reduced light curves by combining Fourier analysis and least-squares
fits in a standard manner using Period04 (see Lenz & Breger (2005) for program details).
As our two observing runs were separated by a month, the light curves were analyzed on a
night-by-night basis. The amplitude spectra are displayed in Figure 2. We used a prewhiten-
ing technique to perform our temporal analysis, fitting the largest signal in the amplitude
spectrum, subtracting the fit from the data, and re-calculating the Fourier transform of the
residual light curve. We repeated this process until the candidate peaks had confidence levels
below 99%, as given by the statistical test proposed by Koen (1990). After each iteration
we applied a non-linear, least-squares fitting routine to simultaneously fit the periods, am-
plitudes, and phases of the isolated frequencies. Figure 3 shows the amplitude spectra at
various stages of this iterative process. The extracted frequencies are printed in Table 2 and
are each labeled fj, where j is ordered in terms of decreasing average amplitude. The errors
shown are derived from the least-squares fits; the actual errors may be larger by a factor of
three (see Montgomery & O’Donoghue 1999).
The multiperiodic nature of JL 166 is apparent from the amplitude spectrum of the
complete light curve. We report 10 independent oscillations with periods ranging from 97 s
to 178 s and amplitudes from 0.9 to 4 mma; the least significant of these has a false alarm
probability near 10−6. Some of the reported frequencies were only detected on one of the
two nights. Amplitude variability is detected in many sdB stars observed over an extended
period; see for example Reed et al. (2007). Whether the non-detection of some of the modes
on one of the two nights is due to the amplitudes decreasing below detection limits is unclear
since the amplitudes of the majority of the modes varied between the observation sets. Most
of the power in the spectrum is concentrated between 6800 and 7600 µHz. The highest-
frequency variation (13461 µHz) is a combination frequency of the dominant peak, f1, and
another lower-amplitude frequency, f9. Interestingly, neither f9 nor the cross-frequency is
present in the October data.
We note that additional oscillation modes may be present in the amplitude spectrum.
The mean noise level in the 4 to 14 mHz range is greater than that on either side of this

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band. This effective plateau in the amplitude spectrum is visible both before and after
prewhitening and suggests the presence of additional frequencies that cannot be identified
in our data.
3.Spectroscopy
To improve upon the spectroscopy of Heber (1986), we obtained four medium-resolution
spectra of JL 166 with a combined integration time of 1800 s using the Goodman Spectro-
graph on 2008 September 19. We used a 1.03 arcsecond slit and the 600 mm−1VPH grating
(0.65˚ A pixel−1dispersion) to cover a spectral range from 3500 to 6300˚ A with a resolution
of 4.5˚ A. Our spectral images were binned 2x2, resulting in 3.4 pixels per resolution element
and a signal-to-noise ratio of approximately 70, measured at the continuum near 4200˚ A.
Table 3 summarizes the details of our spectroscopic observations.
We reduced our spectra with IRAF using standard bias-subtraction and wavelength-
calibration procedures. A standard flat-field correction could not be applied due to a lack of
proper flat frames. We corrected this potential issue by applying a flux calibration using a
spectrophotometric standard star (EG 21) whose spectrum was aligned on the same pixels as
that of the JL 166 spectra. The reduced spectral frames were averaged to produce the single,
time-averaged spectrum for JL 166 shown in Figure 4. As expected, our mean spectrum is
dominated by Balmer lines, He I lines, and the 4686˚ A He II line, confirming the sdOB
classification assigned by Heber (1986). We derive a redshift of 79 ± 27 km s−1from these
lines, also in agreement with the results of Heber (1986).
Although typical indications of a low-mass main sequence companion (Ca I triplet, Mg
I triplet, G band) are absent from the spectrum, Na D absorption lines (5890˚ A and 5896
˚ A) are present and confirm the existence of a cool companion (see inset of Figure 4). An
over-subtraction of the Na D sky emission features is unlikely as the removal of the OI sky
emission line at 5577˚ A left no residuals. Moreover, the Na features in our spectra have
redshifts comparable to those computed from the H and He features, and, consequently, we
believe they are real. Contamination from interstellar Na absorption cannot be completely
ruled out but is unlikely due to the high galactic latitude (-69.8◦) of JL 166. As no other
spectral features typical of a cool companion were observed, we cannot classify the companion
using our data. The optical-IR color-color plots of Stark et al. (2004) suggest the companion
is an M-type star contributing less than 5% of the total flux (in the V-band) from the system.
To determine the atmospheric parameters of JL 166, we employed the model grid of
synthetic optical spectra described in Han et al. (2007). The grid is defined in terms of