Multiplicity of chemically peculiar stars

Abstract

Recently, with the goal to study multiplicity of chemically peculiar stars, we carried out a survey of 40 stars using diffraction limited near infrared (IR) imaging with NAOS-CONICA (NACO) at the VLT. Here, we announce the detection of 27 near IR companion candidates around 25 late B-type chemically peculiar stars exhibiting strong overabundances of the chemical elements Hg and Mn in their atmospheres. A key point for the understanding of the abundance patterns in these stars may be connected with binarity and multiplicity. It is intriguing that more than half of the sample of HgMn stars studied previously by speckle interferometry and recently using the adaptive optics system NACO belong to multiple systems.

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arXiv:astro-ph/0510302v1 11 Oct 2005
Multiplicity of chemically peculiar stars
Swetlana Hubrig1, Nancy Ageorges1, and Markus Sch¨ oller1
European Southern Observatory (ESO), Casilla 19001, Santiago, Chile
Summary. Recently, with the goal to study multiplicity of chemically peculiar
stars, we carried out a survey of 40 stars using diffraction limited near infrared
(IR) imaging with NAOS-CONICA (NACO) at the VLT. Here, we announce the
detection of 27 near IR companion candidates around 25 late B-type chemically pe-
culiar stars exhibiting strong overabundances of the chemical elements Hg and Mn
in their atmospheres. A key point for the understanding of the abundance patterns
in these stars may be connected with binarity and multiplicity. It is intriguing that
more than half of the sample of HgMn stars studied previously by speckle inter-
ferometry and recently using the adaptive optics system NACO belong to multiple
systems.
1 Introduction
Chemically peculiar (CP) stars are main sequence A and B type stars in
the spectra of which lines of some elements are abnormally strong or weak.
The class of CP stars is represented by roughly three subclasses including
magnetic Ap and Bp stars, metallic-line Am stars and HgMn stars which are
late B-type stars showing extreme overabundances of Hg (up to 6dex) and/or
Mn (up to 3dex). Among the magnetic Ap stars the rate of binaries is 43% [1].
The main result of this most recent study of multiplicity of magnetic stars is
that, statistically, the orbital parameters of Ap stars do not differ from those
of normal stars, except for an almost complete lack of orbital periods shorter
than 3days. The studies of the evolutionary state of magnetic Ap stars in
binaries indicate that all of them are rather old main sequence stars and are
well evolved from the zero-age main sequence (e.g. Wade et al. (1996) [2]),
fully in agreement with results of the study of single magnetic Ap stars by
Hubrig et al. (2000) [3].
The number of double-lined spectroscopic binary (SB2) systems among
magnetic Ap stars is abnormally low (only 3 SB2 systems are known to date)
and no eclipsing binary comprising a magnetic Ap star has ever been identi-
fied. The rate of binaries is much smaller among magnetic Bp stars, ∼20%,
and only two double-lined eclipsing binaries have recently been discovered:
HD123335 was discussed by Hensbergen et al. (2004) [4] and AOVel by
Gonz´ alez et al. (2005) [5]. A total of six magnetic Bp/Ap stars are known to

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2Swetlana Hubrig, Nancy Ageorges, and Markus Sch¨ oller
be multiple and are listed in the Multiple Star Catalogue of Tokovinin (1997)
[6].
The metallic-line Am stars show an overabundance of heavy elements and
an underabundance of Ca and Sc. A very high fraction of these stars, at least
90%, are SB systems with orbital periods between 2.5 and 100 days.
HgMn stars are rather young objects and many of them are found in young
associations like Sco-Cen, Orion OB1 or Auriga OB1. HgMn stars do not have
strong large-scale organized magnetic fields and exhibit marked abundance
anomalies of numerous elements. In contrast to Bp and Ap stars with large-
scale organized magnetic fields, they generally do not show overabundances
of rare earths, but exhibit strong overabundances of heavy elements like W,
R, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi. Another important distinctive feature of
these stars is their slow rotation (?v sini? ≈ 29kms−1, Abt et al. (1972)
[7]). The number of HgMn stars decreases sharply with increasing rotational
velocity[8]. Evidence that stellar rotation does affect abundance anomalies
in HgMn stars is provided by the rather sharp cutoff in such anomalies at a
projected rotational velocity of 70–80 kms−1[9].
The mechanisms responsible for the development of the chemical anoma-
lies of HgMn stars are not fully understood yet. A key point for the un-
derstanding of the abundance patterns may be connected with binarity and
multiplicity. More than 2/3 of the HgMn stars are known to belong to spectro-
scopic binaries[10]. Quite a number of HgMn stars belong to triple or even
quadruple systems[11, 12]. Out of 30 SB HgMn stars observed in speckle
interferometry, 15 appear to have more than two components. Indirect evi-
dence for the presence of a third component was found in four other HgMn
SBs (HD11905, HD34364, HD78316 and HD141556) on the basis of spectro-
scopic and photometric arguments. Further evidence that other HgMn stars
frequently are members of multiple systems is inferred from the results of the
ROSAT all-sky survey. X-ray emission was detected through this survey in
12 HgMn stars (7 SB1s, 3 SB2s and 2 for which no radial velocity data are
available). Previous X-ray observations with the Einstein Observatory and
theoretical estimates had suggested that stars in the spectral range B2–A7
are devoid of any significant X-ray emission. In most cases when emission had
been detected in such stars, it was found to originate from a cool companion.
This suggests that the X-ray emission found in HgMn SBs does not origi-
nate from the HgMn primary. From observations investigating late-B X-ray
sources using the ESO 3.6-m with ADONIS[13] we found faint companions
for 4 HgMn stars that were part of the X-ray selected late-B stars observed,
strengthening this interpretation.
In the catalogue of multiple stars by Tokovinin (1997) [6], which com-
piles data on 612 stellar systems of different spectral types, we found four
additional multiple systems containing HgMn stars. It is especially intriguing
that if the relative frequency of HgMn stars in multiple systems is studied,
every third system with a primary in the spectral range between B6 and

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Multiplicity of CP stars3
B9 involves an HgMn star. These observational results clearly show that the
study of multiple systems with an HgMn component is of prime interest for
star formation. In the following we report our results of the recent study of
multiplicity of this amazing class of objects using NACO K-band imaging.
2 Observations
Observations of 40 HgMn stars have been carried out with NACO in service
mode from October 2004 to March 2005. We used the S13 camera, to be able
to discover practically all components down to K=14 with a signal-to-noise
ratio of the order of 12.
Here, we announce the detection of 27 near IR companion candidates in
eight binaries, 16 triple and one quadruple system. The detected companion
candidates have K magnitudes between 5.m5 and 13.m5 and angular separa-
tions ranging from 0.′′1 to 7.′′3 (7-1500AU).
In Figs.1–3 we show adaptive optics K-band images obtained with NACO.
The field of view displayed was selected according to the angular distances of
the companions. The intensity scale was adjusted to visualize the respective
companions.
One of our NACO targets, the HgMn star HD75333, has already been
observed in May 2001 with the adaptive optics system at KeckII. The Keck
images of this system have been presented in Hubrig et al. (2005) [14]. These
observations revealed that this star has two low-mass companions in a binary
system at a separation of 1.′′34. This system is not displayed in Figs.1–3, since
the separation between the two low-mass companions in the binary system
is only 0.′′06. The diffraction limit for NACO installed at the 8m telescope is
lower than that for KeckII, hence these companions do not appear resolved
in our NACO images. In Fig.4 we show the distribution of the projected
separations for the studied multiple systems with HgMn primaries. For most
of the systems the separations are smaller than 100AU.
If all detected IR objects around the HgMn stars are true companions, the
resulting multiplicity rate is 68%. In Table1 we present the list of the observed
HgMn stars. Their visual magnitudes and spectral types were retrieved from
the SIMBAD data base. In the last column we give some remarks about their
multiplicity.
3 Discussion
The results of our study clearly confirm that HgMn stars are frequently found
in multiple systems. It is especially intriguing that out of the 40 HgMn stars
in the sample studied only two stars, HD65950 and HD70235, are not known
to belong to a binary or multiple system. However, companionship can not be
established based on K photometry alone, and confirming the nature with a

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4 Swetlana Hubrig, Nancy Ageorges, and Markus Sch¨ oller
Fig. 1. NACO images of the wide systems in our sample. Upper row from left to
right: HD32964, HD34880, HD36881 and HD53929. Lower row from left to right:
HD120709, HD129174, HD165493 and HD178065. The field of view in each frame
is 7.95′′×7.95′′.
Fig. 2. NACO images of the intermediate systems in our sample. Upper row from
left to right: HD33904, HD35548, HD53244, HD59067 and HD73340. Lower row
from left to right: HD78316, HD101189, HD110073, HD158704 and HD221507.
The field of view in each frame is 1.33′′×1.33′′.
near infrared spectrograph is essential for establishing their true companion-
ship. Our program to carry out NACO K-band spectroscopy of the discovered
IR-candidate companions has been scheduled at the VLT in service mode for
the period October 2005 to March 2006. Using these observations we will
be able to determine the mass of the IR companions much more accurately,
and explore the physics in their atmospheres by comparison of observed and
synthetic spectra. Since the HgMn type primaries have all Hipparcos par-
allaxes (σ(π)/π < 0.2), their age is known, and assuming coevality we will

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Multiplicity of CP stars5
Fig. 3. NACO images of the close systems in our sample. Upper row from left to
right: HD21933, HD28217 and HD29589. Lower row from left to right: HD31373,
HD33647 and HD216494. The field of view in each frame is 0.66′′×0.66′′.
0 200400600 8001000 12001400
Projected separation [AU]
0
2
4
6
8
10
12
14
N
Fig. 4. Distribution of the projected separations for the studied systems with HgMn
primaries.