Download SPECTRAL ANALYSIS OF A NEWLY DISCOVERED HgMn STAR

Dig Sites of Stellar Archeology: Giant Stars in the Milky Way
Ege Uni. J. of Faculty of Sci., Special Issue, 2014, 194-199
SPECTRAL ANALYSIS OF A NEWLY DISCOVERED
HgMn STAR: HD 318126
Tolgahan Kılıçoğlu1,*, Richard Monier2,3, Luca Fossati4 , Berahitdin Albayrak1
1
Ankara University, Faculty of Science, Astronomy and Space Sciences Department, Ankara, Tūrkiye
2
3
Laboratoire Lagrange, Université de Nice-Sophia, Nice, France
Argelander-Institude for Astronomy, Bonn University, Bonn, Germany
Abstract: We present a spectroscopic analysis of HD 318126, a member of the M6 (NGC 6405,
age ~100Myr) open cluster, using low and high-resolution (R~7500, R~25000) spectra stretching from
4500-5840 Å. The data were obtained with the multi-fiber spectrograph GIRAFFE with MEDUSA,
mounted at UT2 (Kueyen), the 8 meter class VLT telescope, in Chile. The atmospheric parameters of the
star were determined from Geneva photometry and hydrogen line modeling (Te = 12000 ± 200 K, log g =
4.1 ± üü0.1). The projected rotational velocity of 55 ± 5 km s-1 was derived from several unblended
metallic lines. The abundances of 10 elements were determined by fitting synthetic spectral lines to
observed ones (using a best-fitting method). We derived a strong overabundance of mercury (+5.00 ±
0.25 dex, relative to the Sun) and manganese (+2.3 ± 0.3 dex, relative to the Sun). Ti, Cr, Ni, Zr, Y, and
Ba elements are overabundant as well. These abundance anomalies indicate that HD318126 is a HgMn
type star (CP3).
Keywords: keyword; keyword; keyword; keyword; keyword (at least 3, maximum 7 keywords)
1. INTRODUCTION
The HgMn stars are very interesting objects since they allow us to test the radiative
diffusion hypothesis that used the explain CP stars, as discussed originally by [1].
Moreover, the abundances of the elements can be derived precisely due to the
distinctive lines of some elements such as Hg, Mn, Y, and Ba.
The HgMn stars have weak or non-existent magnetic fields [2]. Their effective
temperatures range between 10500-15000 K and their photospheres exhibit
overabundance of some elements: Sc, Cr,, Mn, Ga, Sr, Y, Zr, Ba, Ce, and Hg [3]. They
may also have extreme overabundances of heavy elements such as W, Re, Os, Ir, Pt,
Au, Hg, Tl, Pb, or Bi [4].
*
Corresponding Author:
Tel: +90 312 2126720 / 1350
Fax: +90 312 2232395
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E-mail: [email protected]
T. KILIÇOĞLU et al. / Ege Uni. J. of Faculty of Sci., Special Issue, 2014, 194-199
The HgMn stars rotate more slowly than normal stars having similar spectral class.
An average rotational velocity of 29 km s-1 was derived by [5]. This velocity is large
enough to affect the abundance anomalies in HgMn stars. Most of the HgMn stars
belong to double or multiple star system [4].
In this paper we present atmospheric parameters and abundances of the elements of a
newly-discovered HgMn star: HD 318126.
2. OBSERVATIONS
The spectrum of HD 318126 was acquired with the multi-fiber spectrograph
GIRAFFE with MEDUSA, mounted at UT2 (Kueyen), the 8 meter class VLT telescope,
in Chile, in May and June, 2007. The spectral regions cover three wavelength intervals:
4500-5100 Å, 5140-5350 Å, and 5590-5840 Å at resolving powers of about 7500,
25900, 24200, respectively.
3. ABUNDANCE ANALYSIS
Model atmospheres were calculated using ATLAS9 [6], assuming a plane parallel
geometry, a gas in hydrostatic and radiative equilibrium and LTE. Prescriptions given
by [7] were used for the mixing-length ratios. Synthetic spectra were computed with the
SYNSPEC48 code [8], assuming solar composition given by [9]. The line list was first
constructed from Kurucz’s gfall.dat, and then updated by using VALD and NIST.
Hyperfine structure was taken into account.
The atmospheric parameters were
estimated from Geneva Photometry. The observed H line was then compared to
β
synthetic profiles to derive more accurate atmospheric parameters (Fig. 1). The
chemical abundance analysis was carried out by searching for the best fit between the
observed spectrum and theoretical one (Fig. 1). The derived physical quantities of HD
318126 are given in Table 1.
The projected rotational velocity (v sin i) was determined by fitting synthetic line
profiles to several unblended metallic lines. The microturbulent velocity was assumed to
be 0.0 ± 1.0 km s-1. We derived chemical abundances for Si, Ti, Cr, Mn, Fe, Ni, Zr, Y,
Ba, Hg. Only the Hg II 5769.823 line was used to derive the abundance of mercury,
while 39 Mn II lines were used for manganese.
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T. KILIÇOĞLU et al. / Ege Uni. J. of Faculty of Sci., Special Issue, 2014, 194-199
Figure 1. The low-resolution (R=7500) spectrum of HD318126 with computed synthetic spectra for
12000 K and 4.10 dex (log g).
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T. KILIÇOĞLU et al. / Ege Uni. J. of Faculty of Sci., Special Issue, 2014, 194-199
Star
No.
99
Other Id.
mB
mV
HD 318126
9.51
9.41
Johnson
Ph.
Te
(K)
10911
Geneva 7color
Ph.
Te
log g
(K)
(dex)
12369
4.33
Spectroscopy
Te
(K)
12000
±200
log g
(dex)
4.1
±0.1
v sin i
(km s-1)
55
±5
Table 1. Physical quantities of HD 318126.
4. RESULTS
We detected extreme overabundance of Mn (2.26 dex), Zr (3.30 dex), Y (3.27 dex),
and Hg (5.00 dex). We derived also overabundances of Ti (1.20 dex), Cr (0.76 dex), and
Ni (0.63 dex). We failed to identify more Hg II lines in the range from 5590 to 5840 Å
due to the telluric lines. Telluric lines should be reduced to obtain more a precise
mercury abundance. New observations at high resolution including 3500 to 4500 Å
region may also help to improve the Hg overabundance value.
A noticeable radial velocity shift (36.4 ± 5 km s-1) was detected among the spectra of
HD 318126 taken on two different days that were separated by 5 days (Fig. 2). This
indicates that the star may have a faint secondary component. Strong changes in the
asymmetry of some lines may also lead inhomogeneous surface abundances
distribution.
The chemical abundance pattern (Fig. 3), rotational velocity (55 ± 5 km s-1),
duplicity/multiplicity character of the object indicate that HD 318126 is a newlydiscovered HgMn type star.
ACKNOWLEDGMENT
We kindly thank Pierre North for making his code CALIB available for Geneva
7color calibrations. This research has used SIMBAD, WEBDA, VALD, and NIST
databases.
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T. KILIÇOĞLU et al. / Ege Uni. J. of Faculty of Sci., Special Issue, 2014, 194-199
Figure 2. The high-resolution (R=25900) spectra of HD 318126, taken on different days, with 5 days time interval.
Figure 3. The derived abundances relative to the Sun for HD318126.
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T. KILIÇOĞLU et al. / Ege Uni. J. of Faculty of Sci., Special Issue, 2014, 194-199
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