Stars and Galaxies 1, 1–7 (2018)

Radial Velocity Survey of Nearby OB Stars
Yuki Moritani
The University of Tokyo, Kavli Institute for the Physics and Mathematics of the Universe (WPI),The University of Tokyo Institutes for Advanced Study,
5-1-5, Kashiwa, Chiba 277-8583, Japan
E-mail: yuki.moritani@ipmu.jp

Takuma Suda, Toshikazu Shigeyama
The University of Tokyo, Research Center for the Early Universe,
Hongo 7-3-1, Bunkyo, Tokyo 113-0033, Japan

Takayuki R. Saitoh
Tokyo Institute of Technology, Earth-Life Science Institute,2-12-1, Ookayama, Meguro, Tokyo, 152-8551, Japan,Kobe University, Department of Planetology,1-1, Rokkodai-cho, Nada-ku, Kobe, Hyogo, 657-8501, Japan(Received 2018 October 31; Accepted 2018 December 4)

Abstract
We report on the current status of the radial velocity monitoring of nearby OB stars to lookfor binaries with small mass ratios. The combined data of radial velocities using the domestic
1-2 m-class telescopes seems to confirm the variations of radial velocities in a few weeks for fourout of ten target single-lined spectroscopic binaries. More data are needed to estimate the exact
periods and mass distributions.

Key words: OBstars — Binary — Radial Velocity
1.

Introduction

Binary stars are important probes for the understanding of stellar evolution. Extensive surveys on thecharacteristics of binary stars have revealed the period distribution and the mass ratio distribution of binaries[Duquennoy et al. (1991), Latham et al. (2002), Raghavan et al. (2010)], which provides useful information
on star formation and chemical evolution. Thanks to these large-scale surveys, the number of samples haveincreased to more than 1,000, while the survey sample is, in most cases, restricted to binaries whose primariesare solar-type stars or low-mass stars at the main sequence phase. For massive stars, on the other hand,the number of stars is limited, but it is suggested that the census of massive star binaries are quite different
from low-mass stars. Most of massive binaries are thought to undergo interactions between two stars via
roche-lobe overflow, envelope stripping, or merging during their evolution [Sana et al. (2012)].
The identification of massive star binaries have some limitations in observations. Surveys of binaries
using radial velocity monitorings, proper motion, or other methods rely on existing bright or nearby stars.
It is difficult to determine the nature of binary systems for faint or distant stars that are located in theGalactic halo and should have low metallicity. The mulitplicity fraction at lower metallicity suffers large
uncertainties with the current surveys [Rastegaev (2010), Moe et al. (2018)]. Moreover, it is impossible todetermine directly the properties of massive star binaries that have already ended their lives. Therefore, the–1–

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survey of binary systems for massive (O and early B type) stars can be made only for metallicity comparable
to the solar value and for stars in the solar neighborhood.
In this study, we report on the survey of OB star binaries based on the monitoring of their radialvelocities. In particular, we focus on the binaries with small mass ratios, q = M2 /M1 where M1 and M2
is the mass of primary and secondary, respectively, to compensate for the scarcity of the data. One of theexamples of the confirmed OB star binaries with small mass ratios is HD 164438 [Mayer et al. (2017)] whosemass ratio and period is 0.1 – 0.22 and 10.25 days, respectively. The details of the observations are givenin § 2.. The current results are reported in § 3., followed by brief summary in § 4..

2.

Observations

Our target stars should have binary companions with the mass of ∼ 1 M⊙
• since the primary stars areexpected to have ∼ 10 M⊙• . If the orbital periods of our target stars are in the range from a few days toa few years, the amplitude of the radial velocities should be 1 – 100 km s−1 . This periodic variation isdetectable by the monitoring of the radial velocities using the observational setups as described below.

2–1.

Target Selection

The program stars have been selected from 64112 OB stars listed in the Catalogue of Stellar SpectralClassifications [Skiff (2014)]. Among them, we have excluded extra Galactic sources and binaries alreadyidentified to be double-lined spectroscopic binaries (SB2) [e.g., the 9th Catalogue of Spectroscopic Binary
Orbits [Pourbaix et al. (2004)]], ecllipse binaries [Malkov et al. (2006), for instance], astrometry binaries[e.g., the Washington Double star Catalogue [Mason et al. (2001)], or visual binaries (the Sixth Catalog ofOrbits of Visual Binary Stars1 ), and High Mass X-ray Binaries [Liu et al. (2006)]. We have checked morethan 30 papers and catalogues to search for identified binaries. Besides, we have also explored literatures
to update binary parameters as much as posible.
Among the rest of 56370 stars in the catalogues that potentially include single-lined spectroscopic
binaries (SB1), we have selected 59 SB1 candidates as target stars. This is because the optical spectra
should be dominated by the OB stars that are much brighter than the late-type companion stars. As afirst step of our survey, we have selected ten stars identified as SB1 by [Chini et al. (2012)], which can bemonitored feasibily using the domestic 1–2 m-class telescopes. These ten stars are sufficiently bright (V ≤8) and are located in the northern sky (δ > −25◦). Our tentative target stars are given in Table 1.
It is to be noted that HD 164438 was initially included in our sample, but during the course of ourobservations, the orbital information on this star was determined [Mayer et al. (2017)]. We found that theirresults are qualitatively consistent with those of our tentative analysis, and decided to drop the star fromthe target list.

2–2.

Instruments and Data Reduction

We have conducted a monitoring of the ten program stars since 2016 using the domestic 1–2m-calss telescopes. We used a medium-resolution spectrograph to detect shorter-period (Porb of day–week) binaries, andhigh-resolution spectrogrphs to detect longer-period (Porb of years) binarirs with the estimated variationsof the radial velocities of 1–100 km s−1 .
Medium-resolution spectra were obtailed with Medium And Low-dispersion Long-slit Spectrograph(MALLS) [Ozaki & Tokimasa (2005)] equipped to the Nayuta 2.0 m telescope at the Nishi Harima Astro◦
nomical Observatory (NHAO). To cover 4400–5000 A, where many metal lines exist in addition to strong◦◦
HI and HeI lines, we have set two center wavelengths of the 1800 mm−1 grating; 4400 A and 4800 A. The
spectral resolution is R ∼ 5000. Most of the spectra produce S/N higher than 200.
1 https://www.usno.navy.mil/USNO/astrometry/optical-IR-prod/wds/orb6–2–Stars and Galaxies style file for LATEX

Table 1. Target Stars
Target Name
HD 18326
HD 35439
HD 52266
HD 93521
HD 98664
HD 184279
HD 190864
HD 194739
HD 195592
HD 204827

RA02 59 23.17
05 24 44.83
07 00 21.08
10 48 23.51
11 21 08.19
19 33 36.92
20 05 39.80
20 27 02.14
20 30 34.97
21 28 57.76

Dec+60 33 59.5
+01 50 47.2
-05 49 36.0
+37 34 13.1
+06 01 45.6
+03 45 40.8
+35 36 28.0
+09 05 31.7
+44 18 54.9
+58 44 23.2

V Mag.
8.0
4.9
7.2
7.1
4.1
7.0
7.8
7.5
7.1
8.0

Sp. Type
B0
B3e
OB
B3a
B9B2eO6
B2.5V
B1e
O9.5IV

Note: (a) [Rauw et al. (2008)] reported the spectral type is O9.5V.
High-resolution spectra were obtained using two spectrographs; High Dispersion Echelle Spectrograph(HIDES) [Izumiura (1999), Kambe et al. (2013)] equipped to the 188 cm telescope at the Okayama Astrophysical Observatory (OAO), and Gunma Astronomical Observatory Echelle Spectrograph (GAOES)[Hashimoto et al. (2006)] equipped to the 1.5 m telescope at the Gunma Astronomical Observatory (GAO).
◦Both spectra cover wavelength range of 4000–5000 A with a single grating configuration. The spectral resolution of HIDES and GAOES is R ∼ 60000 and R ∼ 30000, respectively. Most of the spectra produce S/Nhigher than 200.
All the spectra were reduced using the IRAF2 in the standard way — bias subtraction, flat fielding,
extraction to 1-d spectrum, wavelength calibration, intensity normalization, and heliocentric correction.
Radial velocity variations have been estimated by the cross-correlation method. We have used theIRAF rv package to measure the shift of the radial velocity from the reference spectrum. Here, the reference
spectum was chosen from one of the high-resolution spectra by OAO/HIDES and GAO/GAOES. The FXCOR
method of rv package calculates the cross-correlation value of the two spectra by shifting one spectra inpixels. The peak of pixel shift corresponds to our derived relative velocity. A Gaussian fit was made to thecenter of the peak in individual spectrum. The error is estimated by the width of half the power of thefitted peak, i.e., the FWHM of the fitting.

3.

Results

Figures 1 – 4 show the variations of radial velocities during the period of the observations for HD 93521, HD
98664, HD 184279, and HD 190864, respectively. These selected targets apparently show some variationsin the radial velocities. This is almost consistent with the expected rate of binaries using the Monte Carlosimulations of binary population [Suda et al. (2013)]. For the simulations of binary population synthesis,
the initial mass function is assumed to be a Salpeter’s power law function. The period distribution isbased on the compilation of the literature on OB star binaries as described in the previous section. Binary
population synthesis predicts that one out of six SB1 stars have mass ratios of the order of 0.1, whichcorresponds to the binary period of a few days to a few weeks.
HD 93521 (Figure 1) is known to be a peculiar star among our target stars, and needs some attention
in the current survey. The star is an O9.5V type star, located at high-latitude in the Galaxy. A very rapid
rotation (∼ 390 km s−1 ) was reported for HD 93521 [Rauw et al. (2008)]. It is confirmed that there are
1.75 hr and 2.89 hr periods caused by non-radial pulsations, by searching for periodicity using Hα and He
I lines. In this study, the MALLS obserations imply that the radial velocity increases monotonously by ∼2 http://iraf.noao.edu/–3–Y. Moritani et al.

50 km s−1 in 5 days. If this variation is caused by the biniariy, the orbital period is expected to be ∼ 1month, assuming that the amplitude of the variability is ±60 km s−1 as shown in Fig. 1.
No observational constraints on biniary parameters have been reported for the other three candidates
shown in the figures. To confirm the binarities and determine the binary periods, we need radial velocity
data in appropriate intervals.

Relative Radial Velocity [km/s]

600-10-20-30-40-50
425

40200430435

440-20-40-60
050

100

150

200

250

300

350

400

450

HJD - 2457789
Fig. 1. Monitoring history of the radial velocity of HD 93521. The zero point of the day is adjusted to thesecond observation of our program because the first observational datum has a large uncertainties caused
by low signal to noise ratio. The radial velocity on the fiducial observation is also set at zero without
error bars because it is chosen as the reference value in the cross-correlation method. The data with highresolution spectroscopy using OAO or GAOES are shown in red circles, while those with medium-resolution
spectroscopy using MALLS are shown in blue crosses. The box in the top-right corner is the magnification
for the data obtained in short duration.

3–1.

Uncertainties

The errors of individual measurement in Figures 1 – 4 are estimated by FXCOR method as shown in § 2–2..
The uncertainties associated with the fitting procedure are much smaller than the variations in the measured
radial velocity for HD 93521 as long as we use the same spectrograph. For HD 98664 and HD 184279, theresults with MALLS clearly show the large variation well beyond the uncertainty of the estimated radialvelocities.
Systematic differences among different facilities are also investigated by the comparisons of the line
profiles of the same star in the same day. The line profiles obtained by different spectrographs showthat the wavelength calibration is consistent with each other within the wavelength resolution of individual instruments. The systematic offsets in radial velocity using different instruments should be lessthan several km s−1 . Therefore, we can safely conclude that the variations of the radial velocities are realeven for HD 190864. It is to be noted that this kind of comparisons is also made between HIDES andGAOES, for instance, by the monitoring of the line profile variability in the Balmer lines of V725 Tau[Moritani et al. (2013)]. The same monitoring has been performed for the same target using the MALLS
data (Okazaki et al., in prep.).
–4–Stars and Galaxies style file for LATEX

20
Relative Radial Velocity [km/s]

1510
50-5-10-15-20-25-30
050

100

150

200

250

300

350

400

450

HJD - 2457789
Fig. 2. The same as in Fig. 1, but for HD 98664. The zero point of the day is adjusted to the initialobservation of our program.

Relative Radial Velocity [km/s]

7060504030
20
100-10020406080100120

HJD - 2457903
Fig. 3. The same as in Fig. 2, but for HD 184279.

–5–140Y. Moritani et al.

Relative Radial Velocity [km/s]

20-2-4-6-8-10-12-14-16020

406080100120140

HJD - 2457858
Fig. 4. The same as in Fig. 2, but for HD 190864.

4.

SummaryThe paper reports on the preliminary results for the monitoring of radial velocities for ten OB stars thatare known to be single-lined spectroscopic binaries. The purpose of this study is to identify the OB starbinaries with small mass ratios and to determine their periods and mass ratios. We have obtained spectrafrom Medium And Low-dispersion Long-slit Spectrograph (MALLS) on the Nayuta 2.0 m telescope at
the Nishi Harima Astronomical Observatory, High Dispersion Echelle Spectrograph (HIDES) on the 188
cm telescope at the Okayama Astrophysical Observatory, and Gunma Astronomical Observatory Echelle
Spectrograph (GAOES) on the 1.5 m telescope at the Gunma Astronomical Observatory (GAO). The crosscorrelation method to confirm the variations of the radial velocities allow us to conclude that at least four of
our target stars show evidence of short period binaries with a few days to a few weeks. Further monitoringof the target stars will constrain the binary parameters.
This work has been supported by a Grant-in-Aid for Scientific Research (JP16K05287), from the Japan
Society of the Promotion of Science.

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