The pulsating l Bootis star HD 105759

Peter Martinez,1* C. Koen,1² G. Handler2* and E. Paunzen2*1South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa2Institut fuÈr Astronomie der UniversitaÈt Wien, TuÈrkenschanzstr. 17, A-1180 Wien, Austria

Accepted 1998 August 17. Received 1998 July 17

A B S T R A C T

We present spectroscopic and multisite time series photometric observations of the star

HD 105759 which con®rm that this is a new pulsating l Bootis star. Our spectroscopy indicates

an overall metallicity of �Z� � log Z ÿ log Z( � ÿ1 and that Teff � 8000 6 300 K and

log g � 4:0 6 0:2. The photometric data reveal that this star pulsates with at least ®ve

frequencies: 8.62, 12.69, 20.15, 21.27 and 23.66 dÿ1. These frequencies do not all match those

found in an earlier single-site study, indicating that the pulsation spectrum changes with time.

Key words: stars: chemically peculiar ± stars: individual: HD 105759 ± stars: oscillations ±

stars: variables: other.

1 I N T R O D U C T I O N

The l Bootis stars are a small class of chemically peculiar, non-

magnetic, A to F-type Population I dwarfs characterized by marked

de®ciencies of metals. They contrast the Am and Ap stars, which

have spectra characterized by marked excesses in metals. The l

Bootis stars are de®ned spectroscopically to be those stars with a

hydrogen-line type between A0 and F0, a K-line type of A0 or

slightly later, and a weak Mg II 4481-AÊ line. For the hydrogen-line

type, the metallic spectrum is weak (Gray 1988). StuÈrenburg (1993)

analysed 13 stars extensively and concluded the following

abundance pattern for the metals.

(1) The lighter metals (C, N, O and S) have solar abundance.

(2) The heavier metals (Mg, Al, Ca and Fe) are underabundant by

up to a factor of 100.

The l Bootis stars are also readily identi®ed in the UV

range, where they exhibit certain characteristic features in low-

dispersion IUE spectra. On the basis of optical and UV criteria it

is possible to designate about 25 stars as de®nite members of the

l Bootis class.

It has been suggested (e.g. Gray & Corbally 1993, and references

therein) that rapid rotation is a characteristic of the l Boo phenom-

enon, and that only stars with v sin i > 50 km sÿ1 should be admitted

to the class. Paunzen et al. (1997) compared the distributions of

rotational velocity of the known l Boo stars with normal stars in the

same spectral range, and concluded that the two distributions are

probably the same, and that the lack of l Boo stars with

v sin i < 50 km sÿ1 is probably due to small-number statistics.

They therefore suggested that the v sin i membership criterion be

dropped. More recently, Paunzen et al. (1998a) discovered a slowly

rotating l Boo star, further strengthening the case against a v sin i

membership criterion.

The origin of the l Bootis stars is not clear. It has been suggested

that they are related to the other chemically peculiar (CP) stars of

the upper main-sequence. However, diffusion/mass-loss models

that explain a variety of CP stars fail to produce the l Boo

phenomenon in A stars until nearly the end of their main-sequence

lifetimes (Michaud & Charland 1986). In support of this, a number

of candidate evolved l Boo stars have been identi®ed, although

their membership of the class is contentious.

An alternative explanation for the source of the l Boo phenom-

enon has been advanced by Venn & Lambert (1990), who invoke the

accretion of metal-poor gas from interstellar medium. Theoretically

this mechanism can produce l Bootis peculiarities in very young

stars near the ZAMS (Turcotte & Charbonneau 1993). At least three

l Boo stars appear to be young objects; two are members of young

binary systems, and the other is a member of the Orion OB1

association.

The discovery of pulsation in the l Bootis star HD 142994 by

Weiss et al. (1994) opened a new window on these objects. By

studying the pulsations one may hope to obtain a better under-

standing of the global and atmospheric properties of these stars,

thereby shedding light on the origin of the l Boo phenomenon.

Paunzen et al. (1998b) list the 22 pulsating l Boo stars known as of

that writing. With this paper we add HD 105759 to that list. These l

Boo stars pulsate with periods in the range 0.5±2 h, except for one

star, HD 142994, which has a period close to 4 h. In all well-studied

cases the pulsations are multiperiodic. Remarkably, it seems that at

least 50 per cent of the l Bootis stars inside the classical instability

strip are pulsators (Paunzen et al. 1998b), compared to the one-third

incidence of variability among chemically normal stars inside the

instability strip. Otherwise the pulsational properties of this group

are similar to those of the `normal' d Scuti stars, indicating that the

mechanism driving the pulsations is very similar. This is consistent

Mon. Not. R. Astron. Soc. 301, 1099±1103 (1998)

q 1998 RAS

*E-mail: peter@saao.ac.za (PM); handler@galileo.ast.univie.ac.at (GH);

paunzen@galileo.ast.univie.ac.at (EP)

²Visiting Astronomer: Cerro Tololo Inter-American Observatory, operated

by the Associated Universities for Research in Astronomy, Inc., under

contract with the National Science Foundation.

with both the diffusion/mass-loss and accretion models, as they

both produce abundance patterns con®ned to the surface.

2 H D 1 0 5 7 5 9 ± T H E S T O RY S O FA R

The star HD 105759 (II Vir) was previously announced as variable

by Colomba, De Benedetto & Ielo (1991). The only published

spectroscopic classi®cation of the star which we could ®nd is given

in the Henry Draper catalogue, namely A0. If correct, the classi®-

cation would place the star just outside the accepted d Scuti

instability strip.

The StroÈmgren indices of this star were measured by Koen at al.

(1995) to be: �b ÿ y� � 0:142; m1 � 0:142; c1 � 0:880. We have

determined the Hb index from ®ve observations spread over 1.5 h:

b � 2:777 6 0:003. Comparison of these indices with the Teff ±

log g calibration of Napiwotzki, SchoÈnberner & Wenske (1993)

based on the grids of Moon & Dworetsky (1985) yields

Teff � 7500 K and log g � 3:8, values more suggestive of an

A7IV star than A0. A low-dispersion spectrogram (100 AÊ mmÿ1)

of the star obtained by Koen et al. shows the strength of the Ca II K

line to be much too weak for spectral type A7, and more comparable

with that for an A3-type star. The Mg II 4481-AÊ line is very weak in

their spectrogram; in fact, it is not unambiguously detected. There

are no published observations of this star in the infrared or

ultraviolet, nor have detailed abundance analyses been derived.

The discrepancy between the photometric and spectroscopic clas-

si®cations is probably due to the star being metal-weak. This is

supported by the position of the star in the m1 versus �b ÿ y�

diagram, where HD 105759 is well within the region occupied by

the l Bootis stars (Paunzen et al. 1997). Gray (unpublished) has

assigned a spectral type of kA2hF0m A2V (Lambda Boo) to this

star.

The Hipparcos (ESA 1997) parallax of HD 105759 is

9.0260.92 mas, which yields MV � 1:33 6 0:22. This corresponds

roughly to an A5IV star. For comparison, the photometry yields

MV � 1:5 (with an estimated error of 0.3 mag), based on Craw-

ford's (1979) relation MV � MV �ZAMS� ÿ 9dc0, which accentu-

ates the impact of metallicity effects on the c1 index in CP stars. The

photometric uvbyb calibration of MV does not take into account the

effects of metallicity or rotational velocity. Antonello & Mante-

gazza (1997) have shown that there are signi®cant differences in the

absolute magnitudes of d Scuti stars derived from Hipparcos

parallaxes and uvbyb photometry; these differences are correlated

with rotational velocity and metallicity.

An exploratory study of the pulsations in HD 105759 was

performed by Koen et al. (1995), who presented Johnson B and V

light curves of HD 105759 which revealed that the oscillations are

multiperiodic, with two sets of frequencies clustered around 12 and

20 cycles per day. Koen et al. were unable to identify the frequen-

cies of oscillation unambiguously, because their single-site data

suffered from alias ambiguities introduced by the daily gaps.

In this paper we present new spectroscopic observations which

con®rm the suspected l Bootis nature of HD 105759, and we also

present a frequency analysis of multisite photometric data of the

oscillations in this star.

3 S P E C T R O S C O P I C O B S E RVAT I O N S A N D

A N A LY S I S

Spectroscopic observations of II Vir were performed with the

Sandiford Cassegrain Echelle Spectrograph (McCarthy et al.

1993) attached to the McDonald Observatory 2.1-m telescope on

1998 February 9. Three 20-min exposures were taken, with a

wavelength coverage of 4355 ± 4880 AÊ and an effective resolution

of R < 45 000.

The frames were corrected for bias and ¯at-®elded. The different

orders were then extracted and wavelength-calibrated. After con-

tinuum normalization, the different orders were combined, and the

three resulting spectra were averaged into a 60-min exposure to be

analysed.

All model calculations were carried out with Kurucz's ATLAS9

(Kurucz 1993). In addition to the classical treatment of convection

(mixing-length theory with or without overshooting), we used the

CM-model (Canuto & Mazzitelle 1991). Atomic parameters for the

spectrum synthesis were taken from the Vienna Atomic Line

Database (VALD; Piskunov et al. 1995). The different steps of

the abundance determination ± calculating the model atmosphere,

selecting and synthesizing spectral lines, deriving abundances ±

were performed by the semi-automatic tool AAP (Gelbmann et al.

1997). The projected rotational velocity v sin i was estimated with

the program ROTATE (Piskunov 1992).

Synthetic spectra ranging from Teff � 7000 to 9000 K

and log g � 3:5 to 4.5 were investigated. An overall metallicity of

�Z� � log Z ÿ log Z( � ÿ1 was chosen. The best ®t was obtained

with an (8000 K, 4.0) model. The ®t does not change very much

within 6300 K and 60.2, but gets worse beyond these values. The

®tting errors do not exclude the (7500 K, 3.8) model suggested by

the photometry, but our best ®t is obtained with the (8000 K, 4.0)

model. Fig. 1 shows our spectrum and the ®t obtained for the

(8000 K, 4.0) model. Most of the features are strong blends of

several elements. A v sin i value of 12065 km sÿ1 was determined

from about 25 spectral lines.

The following individual element abundances were determined:

�Fe� � ÿ0:9 6 0:2 (17 lines measured); �Ti� � ÿ0:8 6 0:3 (15);

�Mg� � ÿ1:0 6 0:4 (3); �Cr� � ÿ1:0 6 0:3 (6) and �Mn� �

ÿ1:0 6 0:5 (2). These values are well within the range established

for the l Bootis stars (Paunzen et al. 1998a). As a caveat, we

mention that we were unable to determine the abundances of the

lighter elements (C, N, O and S); these elements are present at solar

abundance in the l Bootis stars.

4 M U LT I S I T E T I M E S E R I E S P H OT O M E T RY

Time series observations of HD 105759 and two comparison stars

(Table 1) were obtained on seven nights, 1996 March 19±26, at the

Cerro Tololo Inter-American Observatory (CTIO) and the South

African Astronomical Observatory (SAAO). The SAAO observa-

tions were acquired using the Modular Photometer attached to the

0.5-m telescope at Sutherland. The CTIO observations were

acquired using the ASCAP photometer attached to the 1-m tele-

scope at Cerro Tololo (Table 2).

The stars were observed in Johnson B and V in the sequence C1,

V, C2, V, C1, V,¼, with occasional measurements of the sky

background as necessary. Photometric apertures of 30 arcsec or

larger were used to minimize light losses due to seeing, scintillation

and telescope tracking errors. An integration time of 120 s was used

throughout for both bandpasses in the SAAO observations, while the

CTIO observations comprised 60-s integrations. Typically, the civil

time between two successive programme star measurements was

6 min at both sites, yielding about 10 points per cycle. With only two

sites active in the campaign and some time lost to weather, we

achieved an overall duty cycle of roughly 35 per cent. The longest

uninterrupted time series achieved was about 12.8 h, signi®cantly

longer than could have been achieved from a single site.

1100 P. Martinez et al.

q 1998 RAS, MNRAS 301, 1099±1103

The data from the two sites were ®rst reduced in two separate

batches. Each batch was corrected for coincidence counting losses,

sky background and extinction, respectively. The time of each

observation was converted to a Heliocentric Julian Date. The

light curves of the programme and comparison stars were inspected,

and any obviously bad points were removed before producing a

®nal set of differential magnitudes (HD 105759ÿC1), (HD 105759

ÿ C2) and (C1 ÿ C2) in each of the ®lters. From the (C1 ÿ C2)

differentials we con®rmed that both comparison stars were constant

at the limit of these observations. This allowed us to combine the

data for the two sets of differentials for HD 105759 to produce a

single light curve with double the sampling frequency. For each

®lter, the differential light curves (HD 105759 ÿ C1) and

(HD 105759 ÿ C2) were shifted on to the same zero-point to

produce one differential light curve whose points alternate between

the two comparison stars.

The differential B light curves are shown in Fig. 2. Data from the

two sites match well where they overlap in time, illustrating the

quality of the observing conditions and subsequent data reduction.

5 F R E Q U E N C Y A N A LY S I S

The data were analysed for their component frequencies using

The pulsating l Bootis star HD 105759 1101

q 1998 RAS, MNRAS 301, 1099±1103

Figure 1. The observed (crosses) and synthetic (line) spectrum of HD 105759. The parameters of the synthetic spectrum are given in Section 3.

Table 1. Programme and comparison stars.

HD B V HD Sp

C1 HD 104887 8.25 8.20 A0

C2 HD 104728 7.25 7.14 A0

V HD 105759 6.74 6.55 A0

Table 2. Journal of photometric observations of HD 105759.

HJD start HJD end Site/Obs T N

2 450 000+ 2 450 000+ hr

162.38722 162.59465 SAAO/PM 4.98 46

163.42694 163.53547 SAAO/PM 2.60 25

164.33937 164.58470 SAAO/PM 5.89 49

164.57512 164.87346 CTIO/CK 7.16 58

165.45494 165.59411 SAAO/PM 3.34 33

165.57584 165.86021 CTIO/CK 6.82 58

166.45837 166.54908 SAAO/PM 2.29 25

166.60895 166.86310 CTIO/CK 6.10 52

167.40075 167.58192 SAAO/PM 4.35 45

167.61247 167.85996 CTIO/CK 5.94 52

168.37096 168.58058 SAAO/PM 5.03 57

168.56456 168.79808 CTIO/CK 5.60 35

S 60.10 535

Deeming's (1975) Discrete Fourier Transform (DFT) algorithm.

This yielded a set of frequencies which were subsequently re®ned in

an iterative non-linear least-squares ®t which simultaneously

adjusts the frequency, amplitude and phase of each component.

Table 3 and Fig. 3 show the ®ve most signi®cant frequencies we

established using this procedure for the B and V data. There are

further frequencies present in the data, but they cannot be identi®ed

in both data sets (not even as 1 dÿ1 aliases of each other) and are thus

ill-determined. Since the B and V time series are completely

independent, the agreement in the results from these two sets in

Table 3 gives us con®dence in our results.

Koen et al. (1995) identi®ed four frequencies in HD 105759 (viz.

11.67, 12.31, 19.10 and 20.13 dÿ1) in their data. Our analysis yields

two of these frequencies: 11.67 dÿ1� n2 ÿ 1 dÿ1 and

20.13 dÿ1� n4. Given the paucity and aliasing in their data, it is

not surprising that the other two frequencies were not con®rmed.

What is remarkable, though, is that there is no hint of n1 � 23:66 dÿ1,

our most signi®cant frequency, in Koen et al.'s data. This suggests that

the modes in HD 105759 are intrinsically variable.

For completeness, we examined the Hipparcos epoch photome-

try of HD 105759 to see if the frequencies listed in Table 3 are

present in the Hipparcos data. 90 transits of HD 105759 were

observed by the satellite, of which 80 were considered to be

photometrically acceptable. The measurements span 3.19 yr in

groups of 1 to 10 measurements per day. In the case of multiple

measurements per day, consecutive observations are mostly spaced

by 20.6 min. As it was not practical to search all of frequency space

at a resolution of dn � 1 ´ 10ÿ9 Hz, we concentrated on frequency

regions 4-mHz wide, centred on the frequencies listed in Table 3.

The results of this exercise were not particularly informative, since

the noise level in the DFT of Hipparcos data is , 7 mmag. Taking

the highest peak to emerge from each of these regions, we managed

to identify n1; n2 and n3 at very marginal signal-to-noise ratios, but

this exercise is otherwise uninstructive.

The Hipparcos data are better suited to searches for long-term

variation on time-scales $ 1 d. We therefore computed the DFT of

these data in the frequency range 0 # n # 1 dÿ1. The resulting

spectrum was ¯at, with a noise level around 7 mmag.

6 D I S C U S S I O N

We have presented photometric and spectroscopic evidence that

HD 105759 is indeed a l Bootis star, as was speculated by Koen et

al. (1995). We have also presented multisite photometric data of the

pulsations of HD 105759 which reveal that this star pulsates with at

least ®ve frequencies. The most signi®cant frequency identi®ed in

our study, n1 � 23:66 dÿ1, was not present in the earlier light curves

of Koen et al. This suggests intrinsic growth/decay of individual

frequencies.

The computed eigenspectra for pulsating l Boo stars vary in

richness depending on the evolutionary state of the star. In the ®rst

part of main-sequence evolution the number of excited modes is

10±30. As the star begins to evolve off the main sequence, the

computed eigenspectra become very rich. In contrast to this, the

number of observed modes in HD 105759 is very small. This is also

the case for two other intensively studied pulsating l Boo stars,

HD 142994 and 111786 (Paunzen et al. 1998c). We have no

explanation for this disparity between the number of observed

and predicted modes as the mode selection mechanism in these

stars is not known.

With the high v sin i of this star, the 6.4-d span of the data should

suf®ce to resolve possible rotationally split frequencies. However,

with the data we have in hand, we are unable to ascribe any of the

frequencies we observe to rotational sidebands. The identi®cation

of such sidebands is not straightforward because, due to second-

order effects, the rotational mode splitting is non-equidistant (see

Paunzen et al. 1998c).

It is possible that the paucity of observed modes compared with

theoretical expectations may be due to the short span of the

observations. The problem is not one of frequency resolution, but

rather one of observing different modes at different times. This sort

1102 P. Martinez et al.

q 1998 RAS, MNRAS 301, 1099±1103

Figure 2. Light curves of the combined SAAO (circles) and CTIO (crosses)

B data listed in Table 2. The number in the top left corner of each panel

denotes the night.

Table 3. Frequencies in HD 105759. The phases are given with respect to

T0 � 245 0165:500 00.

Frequency Frequency Amplitude Phase

(mHz) (dayÿ1) (mmag) (radian)

B

6 0.01 6 0.8

n1 273.85 6 0.07 23.66 11.9 2.00 6 0.07

n2 146.92 6 0.07 12.69 11.4 0.71 6 0.08

n3 099.85 6 0.08 08.62 10.4 1.73 6 0.08

n4 233.16 6 0.10 20.15 8.9 ÿ2.84 6 0.10

n5 246.16 6 0.11 21.27 8.3 1.05 6 0.11

V

6 0.01 6 0.7

n1 273.97 6 0.07 23.67 9.6 1.92 6 0.07

n2 146.85 6 0.08 12.69 8.3 0.74 6 0.09

n3 099.79 6 0.07 08.62 8.7 1.77 6 0.08

n4 233.15 6 0.11 20.14 6.6 ÿ2.81 6 0.12

n5 246.14 6 0.12 21.27 6.4 1.18 6 0.12

of behaviour has been observed in the DA white dwarf pulsator

G29-38 by Kleinman et al. (1998). Since our data contain frequen-

cies that were clearly not present in the earlier data of Koen et al., it

would be worthwhile having another intensive multisite campaign

on this star. This would serve as a check on the frequencies we

identi®ed, as well as to search for different excited modes, thereby

building up a picture of the `total' pulsation spectrum. After all, it is

possible that the paucity of observed modes compared with theore-

tical expectations may be due to the fairly short time-span of the

extant data sets. Spectroscopy of possible line-pro®le variations

associated with the photometric variations could also lead to mode

identi®cations.

ACKNOWLEDGMENTS

CK acknowledges the commendable hospitality and friendly assis-

tance during his observing at CTIO. GH and EP acknowledge

®nancial support from the Austrian Fonds zur FoÈrderung der

Wissenschaftlichen Forschung (projects S7303-AST and S7304-

AST) and from Digital Equipment Corporation's European Exter-

nal Research Programme (project STARPULS).

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797

This paper has been typeset from a TEX=LATEX ®le prepared by the author.

The pulsating l Bootis star HD 105759 1103

q 1998 RAS, MNRAS 301, 1099±1103

Figure 3. Discrete Fourier Transforms (DFTs) of the combined B multisite data listed in Table 2. The top panel is the spectral window. The middle panel is the

DFT of the data with all its component frequencies. The bottom panel shows the residuals after prewhitening n1 ± n5 from Table 3.