Issue |
A&A
Volume 507, Number 3, December I 2009
|
|
---|---|---|
Page(s) | 1621 - 1624 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361/200912984 | |
Published online | 01 October 2009 |
A&A 507, 1621-1624 (2009)
Helium lines in RR Lyrae spectra
(Research Note)
G. W. Preston
Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA
Received 24 July 2009 / Accepted 23 September 2009
Abstract
Context. During a comparative study of atmospheric phenomena
that occur during the pulsation cycles of 10 RRab stars I
unexpectedly found helium lines in emission and/or absorption in all
10 stars during rising light. The progression of events in the
time-evolution of the helium spectrum differs in detail from star to
star, but 9 of the 10 stars share a number of general
characteristics, illustrated approximately by the behavior of
RV Oct described in this Note.
Aims. My long-term aim is to provide a comprehensive empirical
description of atmospheric phenomena that take place during the
pulsation cycles of RRab stars. The short-term goal of this paper is to
inform readers that measurements of helium lines place new constraints
on shock-wave models for RRab stars.
Methods. Results presented here are based on measurements and
analysis of observations made with the echelle spectrograph of the du
Pont 2.5-m telescope at Las Campanas Observatory.
Results. A general pattern of behavior emerges albeit with significant star-to-star variation. He I 5376 (D3)
appears first as a pure emission feature, unlike the Balmer lines for
which the emission profile is always overlain initially by strong
redward-displaced absorption. The phase of maximum D3 emission
coincides approximately with that of H
,
but the duration of conspicuous D3 emission (in excess of the
local continuum) is less than 15 mn, approximately half the
duration of similarly defined H
emission. The emission phase is followed by the appearance, first, of
redward-displaced absorption, and shortly thereafter by a second
violet-displaced absorption component that strengthens with advancing
phase. Both absorption components gradually weaken and disappear after
radial-velocity minimum, which lags luminosity maximum by no more than
0.01P as discussed in Sect. 3. However, bona fide He I
5876 emission
flux in excess of the local continuum flux reappears near maximum light
for some stars in the sample and persists for about 45 mn. The
radial velocities of the initial and final He I
5876
emission features differ little from the time-average value of the
metallic-line radial velocities for RV Octantis. However, the
absorption features appear to distort the location of the emission
centroid, when they are strong.
Key words: stars: variables: RR Lyr - shock waves
1 introduction
In this Research Note I report the appearance of helium lines in emission
and/or absorption in the spectra of 10 RRab stars during numerous episodes
of rising light that have occurred since April 2006. He I 5876
(hereafter D3) emission has long been known to occur in the cepheid-like
variable stars of Population II during rising light: in W Vir, first
observed by Sanford in the 1940s, as reported by Wallerstein (1959), and in the RV Tau stars U Mon (Preston 1964) and AC Her (Baird 1982). Recently,
Schmidt et al. (2004)
summarized their spectral survey of cepheids, reporting the presence of
D3 emission in a total of nine Population II cepheids. No
helium absorption components have been reported in these stars, but it
seems possible that this could be a consequence of inadequate phase
coverage and/or inadequate spectral resolution.
The ten RRab stars in which helium lines have been found were chosen for study initially to learn how the pulsation characteristics of the peculiar RRab star TY Gru (Preston et al. 2006) compare to those of a random sample of RRab stars with periods near 0.57 d. Accordingly, a program of observations of RRab stars was begun in 2006 with the echelle spectrograph of the du Pont telescope at Las Campanas Observatory.
TY Gru is a Blazhko star, and perchance 5 of the 10 stars selected for study are also Blazhko variables. The Blazhko phenomenon introduces unavoidable complications in the gathering of data and in the inter-comparisons of stars. On the other hand Blazhko stars offer the opportunity to study changes in the behavior of helium lines in individual stars as velocity amplitudes wax and wane during their Blazhko cycles.
2 The observational data
Table 1: RRab stars in whose spectra He I lines appear during rising light.
In addition to TY Gruis the 10 stars used for comparison are listed in order
of increasing pulsation period in Table 1. The [Fe/H] values in the 3rd column were taken from Layden (1994). The V and B magnitudes in the 4th and 5th columns of Table 1 were taken from the SIMBAD database. The numbers of
observations accumulated to date for each star (6th column) reflect the
complexity of each star's behavior, position in the sky, lunar interference,
and weather.
The observations, all made with the
arcsec entrance aperture of the echelle spectrograph of the du Pont 2.5-m telescope at Las Campanas
Observatory, cover the spectral range
3500-9000 Å
with resolution
at the Mg I b lines (
5180 Å), and
are more or less uniformly distributed with respect to pulsation phase. Time
resolution of 3 to 10 mn with
at
4300 Å is
achieved for the 10 comparison stars in Table 1.
Spectra for measurement and analysis are created by use of IRAF
reduction packages for bias subtraction, flat field division, one
dimensional extraction, and for wavelength calibration provided by
frequent observation (at least one per hour at each star position) of a
Thorium-Argon lamp.
Particular attention has been paid to the phases of rising light (
); thus, the phase density of the observations during
rising light is generally high enough to detect phenomena that occur on a
time scale of 0.01d for all of the stars in Table 1 except 14th-magnitude TY Gruis, for which time-resolution can only be preserved at the expense of S/N.
In spite of its faint apparent magnitude D3 emission in the
spectrum of TY Gru is well-marked on two occasions. A detailed
inter-comparison of the spectroscopic behavior of these eleven stars is
underway even as observations continue.
Table 2: Properties of He I lines measured in du Pont echelle spectra of RV Oct.
![]() |
Figure 1:
Profile variations of D3 and the sodium D lines ( left panel) and H |
Open with DEXTER |
3 Variation of the D3 line profile with pulsation phase
The best example of helium emission/absorption phenomena in my sample is
illustrated in Fig. 1
by the montage of spectra of RV Oct, obtained during rising light
on JD 2454194.5, which traces the progression of changes in the
profiles and wavelength displacements of D3, the sodium D
lines, and H
with phase, indicated above each spectrum in the left panel. The
first seven spectra were observed at approximately 7 mn intervals. To
increase S/N as the He features weakened near velocity minimum I co-added
two sets of 4 successive observations to produce the bottom two spectra, for
which average phases are 0.998, and 0.043, respectively. Phases were
calculated by use of the ephemeris

The zero point of phase is my estimated heliocentric Julian Date of RV minimum, a good surrogate for light maximum. No photometry accompanies my spectroscopic observations. However, from inspection of the best data for 16 RRab stars provided by Cacciari et al. (1987), Jones et al. (1988), Liu & Janes (1989), and Skillen et al. (1993), I estimate that RV minimum probably lags visual luminosity maximum by a small amount, no greater on average than 0.01 P.
The following sequence of events is observed. Beginning at phase 0.920
emission at D3 appears suddenly during rising light and lasts for
about 15 mn. The largest observed value of peak emission in excess
of local normalized continuum is 0.25, which must be a lower limit on the
true maximum, because the time scale for duration of the emission is
comparable to the integration times of the observations, which range from 4
to 15 mn. During the 20 mn or so following phase 0.927 the pure
emission profile is replaced, successively, by emission plus red-shifted
absorption, and briefly by red-shifted absorption alone. At phase 0.953 a
blue-shifted absorption component is present and it strengthens relative to
the red-shifted component until phase 0.962. Thereafter, both absorption
components gradually weaken with advancing phase past maximum light. At
phase 0.998 near maximum light D3 emission above the local continuum
reappears weakly between the absorption components and persists for another
45 mn as discussed in Sect. 4. Thus, for RV Oct D3 is detectable
for 0.15 P, or about 2 h, i.e., the He I spectrum is not an evanescent
phenomenon.
Among the Blazhko stars in Table 1 (see Kolenberg 2005 for a review of the Blazhko effect) I have detected D3 emission only during those pulsation
cycles for which the metal-line radial-velocity amplitude is greater than
about 60 km s-1, and there are large star-to-star differences among the
remaining five stars. For example, I have only detected weak D3 absorption,
not clearly resolved into two components in Z Mic and in BS Aps during its
low-amplitude Blazhko cycles. Finally, emission is not confined to D3; I
have found emission lines for the five transitions listed in Table 2,
which contains in successive columns the average values and standard
deviations for four quantities derived from spectra of RV Oct:
radial velocity, emission equivalent width in units of normalized local
continuum, FWHM values returned by the IRAF package splot, and
peak intensity also measured with splot in units of normalized local
continuum. The last column lists the number of observations
used to form each average.
Radial velocity (hereafter RV) measurements for RV Oct are plotted in Fig. 2. The RVs in the top two panels of Fig. 2 are merely Doppler
transformations of wavelengths of local extrema in the complex profiles of
D3 and H,
measured with the IRAF package splot. By extrema I mean
minima in the absorption troughs or local maxima in the apparent emission
features between absorption components. When maxima between the double
absorption components lie below the local continuum, there is no way to know
whether true emission is present or not, so the reality of these ``emission
peaks'' is problematical. Most or perhaps all of these extrema are produced
in complicated combinations of emitting and absorbing layers moving with
respect to each other, so the velocities calculated from them do not in
general indicate true atmospheric motions, as discussed below.
![]() |
Figure 2:
Radial velocities for RV Oct on JD 2454194.5 calculated from
wavelengths of extrema in the absorption/emission profiles as discussed
in the text: ( upper left) for D3; ( upper right) for H |
Open with DEXTER |
The horizontal lines in all four panels of Fig. 2 represent the
time-average velocity, 141.3 km s-1, of the smooth curve drawn through
the metallic-line velocities (derived from the IRAF package fxcor) plotted
in the lower-right panel of Fig. 2. These were obtained by
cross-correlation of flattened, normalized spectra, constructed from 13 echelle orders covering the region
4000-4600 Å
with the high S/N template spectrum of CS 22874-009 used to conduct the blue
metal-poor star binary survey of Preston & Sneden (2000). RVs derived
from wavelengths of the Na I lines, measured with IRAF package splot, are
plotted in the lower-left panel of Fig. 2. They are in excellent agreement
with the metallic-line fxcor velocities.
H
emission begins to strengthen in the top spectrum (
=
0.913) of Fig. 1. Helium emission appears, barely visible, in the second spectrum (
)
in which the stellar Na I D lines are
noticeably broader (
FWHM = 0.70 Å) than those in the immediately
preceding (
FWHM = 0.66 Å) and following (
FWHM = 0.64 Å) spectra. D3 emission reaches maximum strength sometime during the 3rd and 4th
exposures (
and 0.935), as does the H
emission.
The D3 emission line in the first of these, apparently uncontaminated by
red-shifted absorption, yields an RV that is closely equal to the
time-average RV derived from the metal-lines, which I use as a rough
approximation for the center of mass velocity of the star (Bono et al.
1994). Thus, when it first
appears, the D3 emission centroid lies near the center-of-mass velocity
of the star, as surmised long ago (Preston et al. 1965, p. 131) for the hydrogen emission in RR Lyrae. So, it appears that at
midrising light we observe a shock that is approximately stationary with
respect to the center of mass of the star, consistent with the pictures
presented by Gillet & Crowe (1988) for X Ari, and most recently by Chadid et al. (2008)
for S Arae: the shock initially appears to move inward, stop, and
then advance outward on a timescale of minutes. For whatever
reasons I have failed to detect these systematic in and out
motions of the emitting gas.
The appearance of red-shifted absorption in the fifth panel (
)
``pushes'' the measured emission initially to spurious more negative
velocities in the top left panel of Fig. 2. This RV trend is quickly
reversed when a second rapidly strengthening violet absorption appears in
the 6th and 7th spectra (
and 0.962). Analogous,
much more pronounced effects, seen in the RV behavior of H
in the
top-right panel of Fig. 2, can be interpreted in the same way: strong
red-shifted overlying H
absorption present in all the spectra
preceding maximum light greatly distorts the H
emission profiles
and velocities calculated from their extrema.
4 Post-maximum helium emission
Perhaps the most surprising aspect of these observations is the reappearance
of helium emission, which becomes clearly visible a second time near maximum
light (
), then slowly weakens and disappears after about
45 mn. To illustrate this emission, barely recognizable in the bottom
spectrum of Fig. 1, I co-added eighteen spectra of RV Oct obtained in the phase interval
to obtain the spectrum displayed in Fig. 3.
The D3 flux during emission maximum exceeds that of the local continuum
flux by four times the rms pixel-to-pixel flux variation in the nearby
continuum. The measured RVs of individual observations used to
construct this spectrum all lie red-ward of the time-average
metallic-line velocity by
km s-1 (see Fig. 2,
upper left panel). I do not know how to estimate the distortions of
measured velocities that might be produced by flanking violet-displaced
absorptions. It is conceivable to me that the entire 10 km s-1
offset could be produced by such distortion, in which case stationary
helium emission may be present throughout the light-rise, only becoming
recognizable as such after overlying absorption weakens sufficiently.
Analogous weak hydrogen emission, if present, is masked by overpowering
absorption at these phases.
![]() |
Figure 3:
The profile of D3 in the spectrum of RV Oct created by co-adding eighteen spectra obtained in the phase interval
|
Open with DEXTER |
Fokin & Gillet (1997) present an awesome calculation of shocks and compression waves that occur during an RR Lyr pulsation cycle (see their Fig. 4). Two of their shocks (s1 and s2) are present simultaneously at and immediately after maximum light. I can only speculate that the appearances of helium emission at these phases may be a manifestation of these shocks. In a subsequent paper Fokin et al. (1999) conclude that the maximum in FWHM that occurs just after maximum light is due to a velocity gradient in the line-producing layers rather than to unresolved shock line-doubling. The helium emission/double absorption profile that persists after phase 0.00 suggests that shock activity may be a more important contributor to atomic absorption line contours at these phases than had been realized previously.
5 Discussion
The nature of the He I spectrum in RRab stars differs substantially from
that of W Vir (Lèbre & Gillet 1992) with respect to the numbers of lines detected thus far, the phase-duration of emission, particularly the
post-maximum light emission at D3, and the complex time-evolution of the
associated absorption components. In these respects, the RR Lyrae stars
present a rich source of observational material with which to explore
atmospheric structure during the shock-wave phases of their pulsation
cycles. Although the behavior of the He and H lines conform in a general way
to the shock model proposed by Schwarzschild (1952), the He lines add
important detail. Because of the large excitation and ionization potentials
of helium relative to those of hydrogen, the appearance and disappearance of
helium lines on a time scale of 2 h add new constraints on the
rapidly evolving temperature structure of the atmosphere during the ``hump''
near phase 0.93 (Gillet & Crowe 1988), and raise questions about the
persistence of emission long after maximum light. Successful models must
account for emission fluxes that arise in or near the shock as well as
absorption line strengths produced by hydrogen and helium in the infalling
and outflowing layers. The effect of shock waves on H
profiles in
RR Lyrae spectra has been modeled by Fokin (1992), but he made no
predictions about helium in emission or absorption. A theoretical treatment
of He I lines that occur under the conditions of temperature and density
encountered in RRab atmospheres near rising light would be most welcome.
Finally, I call attention to the apparent decrease in RV with wavelength in
Cols. 1 and 3 of Table 2 with some diffidence. Paschen bound-free
absorption is the dominant source of atmospheric opacity at optical
wavelengths in the atmosphere of an RRab star during rising light (
K); it increases by a factor of about 5 between 4000 Å
and 8000 Å. Interpreted as a level effect produced by this opacity
gradient, the velocity differences in Table 2 suggest that during the phase
of strong helium emission the deepest layers of the atmosphere that
contribute to
4471 are stationary in the rest frame of the star,
while superincumbent layers are moving outward at a rate that increases with
distance above the photosphere. Further discussion of this preliminary
result best awaits confirmation by observations with higher S/N, and
superior time resolution that will permit study of the time-evolution of
such velocity structure.
The observations reported here were recorded with lengthy (93 s) readout
times of a
pixel
raster in order to maximize echelle spectral coverage at a telescope of
modest (2.5 m) aperture. Integration times of my du Pont
observations vary with pulsation phase, star brightness, and observing
conditions, and they range from 200 s to 1000 s for the stars
in Table 1. Order-of-magnitude improvement in time-resolution (to isolate the phase of maximum emission strength) and S/N (to reduce the substantial errors in continuum location, hence emission flux) could be achieved by
restricting read-out to small portions of adjacent echelle orders that
contain, for example,
6678 and
7281, and by use of
telescopes of larger (6 to 10 m) aperture.
The author wishes to thank referee Dr. Philippe Mathias for his many helpful comments, questions, and suggestions that greatly improved this Research Note.
References
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Footnotes
- ... SIMBAD
- http://simbad.harvard.edu/simbad/
All Tables
Table 1: RRab stars in whose spectra He I lines appear during rising light.
Table 2: Properties of He I lines measured in du Pont echelle spectra of RV Oct.
All Figures
![]() |
Figure 1:
Profile variations of D3 and the sodium D lines ( left panel) and H |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Radial velocities for RV Oct on JD 2454194.5 calculated from
wavelengths of extrema in the absorption/emission profiles as discussed
in the text: ( upper left) for D3; ( upper right) for H |
Open with DEXTER | |
In the text |
![]() |
Figure 3:
The profile of D3 in the spectrum of RV Oct created by co-adding eighteen spectra obtained in the phase interval
|
Open with DEXTER | |
In the text |
Copyright ESO 2009
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