Pulsation in cool main-sequence Am stars is a rare phenomenon observed in a few marginal Am stars and only one classical Am star. The chemically peculiar star HD 13079 is shown to be pulsating with a 78-min period and a peak-to-peak B amplitude of 0.02 mag. The Hipparcos parallax, uvbyβ photometry and pulsations together suggest that HD 13079 is an Am star near the zero-age main sequence, and that it is a fundamental-mode pulsator on the red edge of the instability strip

Ashoka, B. N.

Catanzaro, G.

Chaubey, U. S.

Gupta, S. K.

Kasturirangan, K.

Kurtz, D. W.

Leone, F.

Martinez, Peter

Raj, E.

Sagar, R.

Seetha, S.

English

Publications of the IAS Fellows

Mon. Not. R. Astron. Soc. 309, 871±874 (1999)
Discovery of pulsations in the Am star HD 13079
Peter Martinez,1w D. W. Kurtz,2 B. N. Ashoka,3 U. S. Chaubey,4 S. K. Gupta,4 F. Leone,5
G. Catanzaro,5 R. Sagar,4 E. Raj,3 S. Seetha3 and K. Kasturirangan6
1South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
2Department of Astronomy, University of Cape Town, Rondebosch 7700, South Africa
3Technical Physics Division, ISRO Satellite Applications Centre, Airport Road, Vimanapura PO, Bangalore 560 017, India
4Uttar Pradesh State Observatory, Manora Peak, Naini Tal 263 129, India
5Osservatorio Astrofisico di Catania, Citta Universitaria, I-95125 Catania, Italy
6Indian Space Research Organisation Headquarters, Antariksh Bhavan, New BEL Road, Bangalore 560 094, India
Accepted 1999 June 16. Received 1999 June 11; in original form 1998 November 10
A B S T R A C T
Pulsation in cool main-sequence Am stars is a rare phenomenon observed in a few marginal
Am stars and only one classical Am star. The chemically peculiar star HD 13079 is shown to
be pulsating with a 78-min period and a peak-to-peak B amplitude of 0.02 mag. The
Hipparcos parallax, uvbyb photometry and pulsations together suggest that HD 13079 is an
Am star near the zero-age main sequence, and that it is a fundamental-mode pulsator on the
red edge of the instability strip.
Key words: stars: chemically peculiar ± stars: individual: HD 13079 ± stars: oscillations.
1 I N T R O D U C T I O N
The classical instability strip crosses the main sequence among the
A and early F-type stars. About 30 per cent of the stars in this part
of the Hertzsprung±Russell (HR) diagram are d Scuti stars which
pulsate with amplitudes ranging from a few mmag to almost a
magnitude. Although most d Scuti stars are chemically normal,
there are also some chemically peculiar stars which exhibit d
Scuti pulsation. The pulsations are driven by the k mechanism
operating in the He ii ionization zone. The chemical peculiarities
are confined to the surface and are produced by diffusion:
elements with many lines near flux maximum are pushed upward
radiatively while elements with few lines near flux maximum, or
with saturated lines, sink.
Among the chemically peculiar stars which exhibit d Scuti
pulsation are several luminous, cool evolved Am stars, a few
marginal Am stars and one classical Am star, HD 1097 (Kurtz
1989). The classical Am stars are main-sequence stars of H-line
spectral types A4±F1 in which the metallic-line and Ca ii K line
types differ by five or more spectral subclasses. The marginal Am
stars (designated Am:) are those stars of similar spectral
appearance to the classical Am stars, but in which the metallic
and K line spectral types differ by less than the required 5
subclasses. As a group, Am stars are slowly rotating, in most cases
as a result of their being members of close binaries.
The coexistence of pulsation and metallicism in Am stars is not
fully understood. The partial He ii ionization zone is the driving
agent for d Scuti-type pulsation. In the Am stars, sedimentation of
elements owing to suppressed rotational velocities depletes He
from the superficial layers. Earlier work by Cox, King & Hodson
(1979) suggested that classical Am stars should not pulsate
because their He ii ionization regions should be fully depleted of
He. The existence of subsequently discovered pulsating Am stars
was attributed to residual He in the He ii ionization zone being
barely capable of exciting low-amplitude pulsation. However,
more recent work by Gautschy, Saio & Harzenmoser (1998),
incorporating an atmospheric temperature inversion in inhomo-
geneous envelope models, produces contradictory results. In these
models, only the H/He ionization zone is deficient of He and the
He ii ionization region is unmodified. The hydrogen partial
ionization zone has a damping influence which overcompensates
the driving action of the He ii ionization region, thus suppressing
the low-overtone modes. In this scenario, the presence of
pulsations in certain Am stars indicates that superficial He
drainage is not sufficiently advanced in those stars to suppress
pulsation. See Kurtz (1989, 1998) for additional discussions of the
problems of pulsation and metallicism.
There is one other group of chemically peculiar pulsating stars,
the rapidly oscillating Ap stars, which pulsate with periods in the
range 6±16 min and amplitudes of a few mmag. The short periods
imply pulsation in very high overtones: n . 10±40. See Kurtz
(1990), Matthews (1991) and Martinez & Kurtz (1995) for reviews
of these stars. In the context of this paper, the point we wish to
note is that there is a well-documented exclusion of low-overtone
(i.e. d Scuti-like) pulsation in the Ap stars (Kreidl 1986). This is
discussed in theoretical terms by Gautschy et al. (1998).
2 T H E S TA R H D 1 3 0 7 9
HD 13079 (HIC 10023, CCDM 0209013936) is a double star.
q 1999 RAS
w E-mail: peter@saao.ac.za (PM)
872 P. Martinez et al.
The Hipparcos Hp magnitudes for the two components are
8:989 ^ 0:007 and 11:311 ^ 0:057. The separation and position
angle are r  6:173 ^ 0:017 arcsec and u  2548: 4, respectively.
The StroÈmgren photometric indices for the combined light of the
two stars are: b 2 y  0:203; m1  0:211; c1  0:672; dm1 
20:023; dc1  20:028 and b  2:759. The dereddened indices are
dm1  20:060 and dc1  20:069. These indices are typical of
the strongly line-blanketed spectra of cool Ap and Am stars.
In this paper we report the discovery of low-overtone pulsation
in HD 13079. Since there is a well-established exclusion of low-
overtone pulsation in Ap stars, this argues that HD 13079 is
probably an Am or Am: star.
The metal abundance may be determined from the photometry
using the tight linear correlation determined by Smalley &
Dworetsky (1993) M=H  210:56dm0 1 0:081 which is valid
for metal-rich stars, including classical Am stars. For HD 13079
we obtain M=H  0:33, a value typical of the Am stars.
The Hipparcos parallax of p  5:52 ^ 1:97 mas indicates
MV  2:461:662:96. At b 2 y  0:203, this is about 0.5 mag above
the main sequence, which places the star on the red edge of the
instability strip (Breger 1979). The Hipparcos MV is consistent
with MV  2:46 ^ 0:19 derived from King's (1991) period±
luminosity (P±L) relation for d Scuti stars pulsating in the
fundamental mode.
The B 2 V±b 2 y calibration of Golay (1972) yields
B 2 V  0:29, in good agreement with the Tycho B 2 V
transformed to the Johnson system (ESA 1997);
B 2 V  0:28 ^ 0:05. These values are consistent with the
hydrogen-line type of F0 given in the Henry Draper Catalogue.
Application of the uvbyb photometry to the Teff±log g grids of
Moon & Dworetsky (1985) yields Teff  7350 K and log g  4:3.
Assuming a bolometric correction . 20:09, and using the
relation log M=M(  0:46 2 0:10Mbol, the mass of the star is
derived to be 1.7 M(, well within the range of masses of the d
Scuti stars.
For chemically normal A/F stars, dc1  c1 2 c1zero ÿ
age main sequence; b is a luminosity indicator. In the heavily
blanketed metallic-line stars dc1 is an unreliable luminosity
indicator because c1 is suppressed relative to the chemically
normal stars. For Am stars, Gray & Garrison (1989) recommend
the use of a dc1 index based on the B 2 V colour instead. Using
the b 2 y±B 2 V plot of Crawford, Barnes & Golson (1970)
and Crawford's (1979) A-star calibration, we derived dc1(B2V) 
0.01. Reference to fig. 5 of Gray & Garrison (1989) suggests that
this is a luminosity class V star.
3 P H OT O M E T R I C O B S E RVAT I O N S A N D
A N A LY S I S
In 1997 November we used the Indian Space Research Organisa-
tion (ISRO) high-speed photometer (Venkat Rao et al. 1990)
attached to the 1-m Sampurnanand telescope of the Uttar Pradesh
State Observatory (UPSO) at Naini Tal to search for high-overtone
oscillations in Ap stars. The data were acquired as continuous
single-channel 10-s integrations through a Johnson B filter.
Photometric apertures of 30 arcsec or larger were used to
minimize light losses arising from seeing effects and tracking
drifts. The observations were interrupted for occasional sky
background measurements to take account of changes in sky
brightness during the night. Because the high-overtone pulsations
in Ap stars have periods in the range 6±16 min, the candidates
were monitored continuously and no comparison stars were
observed. The data were corrected for coincidence counting
losses, sky background and atmospheric extinction, respectively.
Low-overtone oscillations in HD 13079 with a period of
, 80 min were discovered on the night of 1997 November 12/13
and subsequently confirmed with better signal-to-noise ratio data
on the night of 1997 November 15/16. Additional multi-channel
photometric observations of HD 13079 were subsequently
acquired on 1999 January 18±20 using the same instrument
attached to the 1-m telescope of the Vainu Bappu Observatory in
Kavalur. The photometric observations are all listed in Table 1. The
table lists the ut civil date, the heliocentric Julian date (HJD) of the
first observation on each night, the duration in hours, the number of
observations and the filling factor or `duty cycle.' Fig. 1 shows two
sample light curves of HD 13079.
Amplitude spectra of the light curves (Fig. 2) were computed
using the Deeming (1975) discrete Fourier transform (DFT)
algorithm as expressed by Kurtz (1985). These are `raw'
transforms in the sense that no low-frequency filtering has been
applied to the data, other than a correction for mean extinction.
Inspection of the nightly amplitude spectra shows that all the
signal power is contained in the range 0±1 mHz and also suggests
that the amplitude of the oscillations is modulated. This is
especially evident in the three lower panels of Fig. 2, which refer
to multi-channel data and should thus be relatively immune to sky
transparency variations.
To refine our frequency determination for the oscillations in HD
13079 and to seek additional component frequencies, we
computed the DFT of the multi-channel data combined from the
three consecutive nights 1999 January 18±20. The top panel of
q 1999 RAS, MNRAS 309, 871±874
Figure 1. Johnson B light curves of HD 13079.
Table 1. Journal of time-series observations of HD
13079.
ut date HJD start T N % fill
2 400 0001 hr
12 Nov 1997 50765.15417 2.03 712 0.97
15 Nov 1997 50768.06686 2.52 866 0.96
18 Jan 1999 51197.09759 3.11 1119 1.00
19 Jan 1999 51198.07968 2.22 799 1.00
20 Jan 1999 51199.08145 2.58 929 1.00
S 12.45 4425
Oscillations of HD 13079 873
Fig. 3 shows the DFT of the combined data for the range 0.0±
0.5 mHz. This Fourier amplitude spectrum is dominated by a
prominent window pattern of amplitude A1  9:4 mmag, centred
at frequency n1  0:2129 mHz 18:39 d21 with daily alias
ambiguities. The lower panel in the figure shows the residuals
after prewhitening a sinusoid A1 sin2pn1t 1 f from the data.
The window patterns in these residuals are not sufficiently
pronounced to warrant further frequency identifications and serve
instead to define an upper limit of 3 mmag on the amplitude of
further frequency components.
4 N E W S P E C T R O S C O P I C O B S E RVAT I O N S
O F H D 1 3 0 7 9
Spectroscopy of HD 13079 in the 585±665 nm region was
performed on 1998 November 24±25 with the 91-cm telescope at
the Catania Astrophysical Observatory equipped with the REOSC
spectrograph (Frasca & Catalano 1994). The data reduction
performed with the iraf package shows that the linear dispersion
is 7.5 AÊ mm21, the instrumental profile is represented by a full
width at half-maximum Gaussian of 0.42 AÊ and a signal-to-noise
ratio of , 95.
The observed spectra were analysed using (a) atlas9 (Kurucz
1993) to compute the stellar atmosphere models and (b) synthe
(Kurucz & Avrett 1981) to identify spectral lines and match the
observations. The iterative procedure described by Leone &
ManfreÁ (1997) was applied to determine the stellar parameters of
HD 13079. We find that the Ha line profile can be matched
assuming an effective temperature Teff  7200 K, the solar metal
opacity scale and ve sin i  45 km s21. Because of the lack of a
large number of unblended lines of a given chemical species
necessary to measure the microturbulent velocity, a value of
2 km s21 was assumed. Given that the Balmer lines are very
weakly dependent on gravity, log g was fixed at 4.0.
As for metal abundances, the iron peak element abundances are
not larger than solar values. Calcium, from the blended Ca i
639.4602, 643.9075, 644.9808, 646.2567, 649.9650 nm lines,
appears to be underabundant. A quantitative evaluation has not
been attempted because of the large number of free parameters
involved and which cannot be separated at our spectral resolution.
We also searched for lines of those other elements which
characterize Am stars, such as scandium and strontium, but
could not find them in our spectra.
We conclude that the solar abundances of the iron peak
elements exclude HD 13079 being an Ap star and that the calcium
underabundance suggests that it is an Am star.
5 D I S C U S S I O N
We have presented evidence for the existence of chemical
peculiarities in HD 13079 and the presence of low-overtone
pulsations in this star. However, the exact peculiarity class of HD
13079 is unclear. Given the pulsation period, the calcium
underabundance and the peculiar colours, the star is most likely
an Am, Am:, or r Pup (evolved Am) star. Indeed, the b index
indicates a somewhat higher effective temperature than (b2y), as
is generally observed in Am stars. The Am and Am: stars are
main-sequence objects, whereas the r Pup stars are cooler and
considerably more luminous (Gray & Garrison 1989). We will
argue that the balance of evidence indicates a main-sequence
luminosity for HD 13079, and hence an Am character.
The standard pulsation relation
Q  P

r= r(
p
:
may be expressed in terms of observable quantities as
log Q  26:454 1 log P 1 0:5 log g 1 0:1Mbol 1 log Teff ;
where the pulsation period P and pulsation constant Q are in units
of days, and g is in cgs units. The value of Q is not too sensitive to
q 1999 RAS, MNRAS 309, 871±874
Figure 2. Discrete Fourier transforms of the light curves listed in Table 1.
Figure 3. Discrete Fourier transform of combined light curves for JD
2451197±99.
874 P. Martinez et al.
Mbol or Teff, but it is sensitive to log g, which is our most uncertain
quantity. Table 2 shows the Q values computed for a variety of
possible log g values. The Q values are almost all identical to three
decimals for Teffb 2 y  7200 K and Teffb  7350 K. For
standard late-A and early-F star models we expect Q  0:033 for
fundamental-mode pulsation and Q  0:025 for first-overtone
pulsation (Stellingwerf 1979). Breger & Bregman (1975) demon-
strated that for Teff , 7800 K, the highest amplitude mode tends
to be the fundamental mode, whereas for hotter temperatures the
first overtone is prevalent. This is consistent with Stellingwerf's
(1979) computed blue edge for fundamental-mode pulsation near
the zero-age main sequence (ZAMS). If one admits this as a
probabilistic argument for fundamental-mode pulsation in HD
13079, then Table 2 is consistent with HD 13079 being near the
ZAMS, as indicated by the Hipparcos parallax and the photo-
metry.
It is instructive to compare this result with the luminosity
calculated from Breger's (1979) empirical d Scuti P±L±C relation
MV  23:052 log P 1 8:456b 2 y2 3:121;
which yields MV  2:46 ^ 0:31, in good agreement with the value
derived from the Hipparcos parallax. The above relation does not
take account of the pulsation mode. King (1991) derived P±L
relations for the fundamental, first overtone, second and third
overtone pulsators. Reference to fig. 2 of King (1991) suggests
that HD 13079 is a fundamental-mode pulsator. This is consistent
with the observed tendency for variables near the red edge of the
instability strip to show a fundamental-mode Q value. This in
itself does not constitute a definitive mode identification of the
pulsations in HD 13079. If additional pulsation frequencies can be
measured in this star, the period ratios will allow further
comparisons with period ratios expected for radial modes.
The coexistence of pulsation and metallicism in Am stars is
important for our understanding of abundance anomalies in the A-
type stars and of diffusion processes in general. HD 13079 appears
to be an Am or Am: fundamental-mode pulsator on the red edge of
the instability strip. It would be instructive to perform a more
detailed spectroscopic analysis of this star. It would also be useful
to obtain more extensive pulsation data to search for further
pulsation modes. The star is situated at declination 1308 and
would thus be a good target for a short multi-site campaign with
0.5- to 1-m telescopes. The fainter star in this double is 6 arcsec
from HD 13079. This separation is too small to exclude that star in
conventional aperture photometry. The companion is only 2.3 mag
fainter, and could be used as the on-chip comparison for
differential CCD photometry.
AC K N OW L E D G M E N T S
The authors thank UPSO for the allotment of observing time and
UPSO staff for their assistance. BNA gratefully acknowledges
travel and logistics support from the Indian Space Research
Organisation.
R E F E R E N C E S
Breger M., 1979, PASP, 91, 5
Breger M., Bregman J. N., 1975, ApJ, 200, 343
Cox A. N., King D. S., Hodson S. W., 1979, ApJ, 231, 798
Crawford D. L., 1979, AJ, 84
Crawford D. L., Barnes J. V., Golson J. C., 1970, AJ, 75, 624
Deeming T. J., 1975, Ap&SS, 36, 137
ESA 1997, The Hipparcos and Tycho Catalogues, European Space
Agency, SP1200. ESA Publications Division, ESTEC, Noordwijk,
The Netherlands
Frasca A., Catalano S., 1994, A&A, 284, 883
Gautschy A., Saio H., Harzenmoser H., 1998, MNRAS, 301, 31
Golay M., 1972, Vistas Astron., 14, 13
Gray R. O., Garrison R. F., 1989, ApJS, 69, 301
King J. R., 1991, Inf. Bull. Variable Stars, 3562
Kreidl T. J., 1986, Lect. Notes Phys., 274, 134
Kurtz D. W., 1985, MNRAS, 213, 773
Kurtz D. W., 1989, MNRAS, 238, 1077
Kurtz D. W., 1990, ARA&A, 28, 607
Kurtz D. W., 1998, in Bradley P. A., Guzik J. A., eds, ASP Conf. Ser. Vol.
135, A Half Century of Stellar Pulsation Interpretations: A Tribute to
Arthur N. Cox. Astron. Soc. Pac., San Francisco, p. 420
Kurucz R. L., 1993, in Dworetsky M. M., Castelli F., Faraggiana R., eds,
Asp Conf. Ser. Vol. 44, IAU Colloq. 138, Peculiar versus normal
phenomena in A-type and related stars. Astron. Soc. Pac., San
Francisco, p. 87
Kurucz R. L., Avrett E. H., 1981, Smithson. Astrophys. Obs. Spec. Rep.,
391
Leone F., ManfreÁ M., 1997, A&A, 320, 257
Martinez P., Kurtz D. W., 1995, ASP Conf. Ser. Vol. 83. Astron. Soc. Pac.,
San Francisco, p. 58
Matthews J. M., 1991, PASP, 103, 5
Moon T. T., Dworetsky M. M., 1985, MNRAS, 217, 305
Smalley B., Dworetsky M. M., 1993, ASP Conf. Ser. Vol. 44. Astron. Soc.
Pac., San Francisco, p. 182
Stellingwerf R. F., 1979, ApJ, 227, 935
Venkat Rao G. et al., 1990, Bull. Astron. Soc. India, 18, 79
This paper has been typeset from a TEX/LATEX file prepared by the author.
q 1999 RAS, MNRAS 309, 871±874
Table 2.
log g Q (days)
4.3 0.034
4.0 0.024
3.5 0.014
3.0 0.008


Discovery of pulsations in the Am star HD 13079

Indian Academy of Sciences

http://repository.ias.ac.in/16596/1/358.pdf

Discovery of pulsations in the Am star HD 13079

Abstract

Similar works

Full text

Available Versions

Publications of the IAS Fellows

Indian Academy of Sciences