# Binarity among objects with the Be and B[e] phenomena

A. S. Miroshnichenko, S. V. Zharikov, D. Korčaková, N. Manset, R.

Mennickent, S. A. Khokhlov, S. Danford, A. Raj, O. V. Zakhozhay

► To cite this version:

A. S. Miroshnichenko, S. V. Zharikov, D. Korčaková, N. Manset, R. Mennickent, et al., Binarity among objects with the Be and B[e] phenomena. Contributions of the Astronomical Observatory Skalnaté Pleso, 2020, 50, pp.513-517. 10.31577/caosp.2020.50.2.513. insu-04863694

HAL Id: insu-04863694

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# Binarity among objects with the Be and B[e] phenomena

A.S. Miroshnichenko<fnref n="1" />,<fnref n="2" />,<fnref n="3" />, S.V. Zharikov<fnref n="4" />, D. Korčaková<fnref n="5" />,
N. Manset<fnref n="6" />, R. Mennickent<fnref n="7" />, S.A. Khokhlov<fnref n="3" />,<fnref n="8" />, S. Danford<fnref n="1" />,
A. Raj<fnref n="9" /> and O.V. Zakhozhay<fnref n="10" />

Received: October 31, 2019; Accepted: February 6, 2020

**Abstract.** Many B-type stars exhibit two phenomena that are symbolized Be and B[e]. The former refers to presence of a circumstellar gaseous disk without significant circumstellar dust, while the latter have dust. Although neither phenomenon is fully understood, growing evidence suggests that binarity plays an important role. Recent results on Be and B[e] binaries, along with methods of their discovery, are reviewed.

**Key words:** Stars: emission-line, Be, B[e] – Stars: binaries: spectroscopic

## 1. Introduction

Two emission line phenomena, found almost exclusively in stars of spectral type B, are due to large amounts of circumstellar material. These Be and B[e] stars

<footnote n="1">¹ Department of Physics and Astronomy, University of North Carolina at Greensboro, P.O. Box 26170, Greensboro, NC 27402-6170, USA, (E-mail: a_mirosh@uncg.edu)</footnote>

<footnote n="2">² Pulkovo Astronomical Observatory of the Russian Academy of Sciences, Pulkovskoe shosse 65-1, St. Petersburg, 196140, Russia</footnote>

<footnote n="3">³ Fesenkov Astrophysical Institute, Observatory, 23, Almaty, 050020, Kazakhstan</footnote>

<footnote n="4">⁴ Instituto de Astronomia, Universidad Nacional Autónoma de México, Apdo. Postal 877, Ensenada, 22800, Baja California, México</footnote>

<footnote n="5">⁵ Astronomical Institute, Charles University, V Holešovičkách 2, CZ-180 00 Praha 8, Czech Republic</footnote>

<footnote n="6">⁶ CFHT Corporation, Kamuela, HI 96743, USA</footnote>

<footnote n="7">⁷ Departamento di Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile</footnote>

<footnote n="8">⁸ Physical and Technical Faculty, Al-Farabi Kazakh National University, Al-Farabi Ave., 71, 050040, Almaty, Kazakhstan</footnote>

<footnote n="9">⁹ Indian Institute of Astrophysics, Block II, Koramangala, Bangalore 560034, India</footnote>

<footnote n="10">¹⁰ Main Astronomical Observatory, National Academy of Sciences of Ukraine, Kyiv 03680, Ukraine</footnote>
were not expected to be mostly in binary systems, but their estimated binary
fractions have steadily grown with time.

The Be phenomenon was discovered at the dawn of astronomical spec-
troscopy in the middle of the 19th century by visual observing of hydrogen
emission lines in the spectrum of γ Cassiopeae (Secchi, 1866). The B[e] phe-
nomenon was discovered 110 years later in the course of an early near-infrared
(IR) photometric survey combined with optical spectroscopy. It was defined as
the presence of both permitted and forbidden line emission along with a large
IR excess due to radiation of circumstellar (CS) dust (Allen & Swings, 1976).
Major features of both phenomena are summarized in Table 1.

## 2. Properties of objects with both phenomena and connections to binary systems

2.1. The Be phenomenon

One of the main features of the Be phenomenon is rapid rotation that may be
due to initial fast rotation or to later mass transfer in a binary system. Many
B-type stars with broad absorption lines (known as Bn stars) have not been
found to exhibit emission lines, thus suggesting that fast rotation alone may
not be the only mechanism for the phenomenon creation, especially in strong-
lined Be stars. A binary hypothesis as a reason for the phenomenon was first
proposed by Kriz & Harmanec (1975) and initially attributed to ongoing trans-
fer of mass and angular momentum to the B-type component. However, this
hypothesis was taken with caution because only ~40 binaries were recognized
among over 1000 known Be stars by the end of the 20th century (e.g., Gies,
2000; Harmanec, 2001). The advent of high-resolution spectroscopy, including a
large contribution from the amateur community collected in the BeSS database
(Neiner et al., 2011), allowed to observe Be stars more frequently and sharply
increased the number of confirmed Be binaries. Currently 14 out of the bright-
est 24 Be stars (V ≤ 4 mag) and 75 out of 237 Be stars with V ≤ 7.5 mag are
known binary systems (Miroshnichenko, 2016). Apparent decrease of the binary
fraction toward fainter objects is due to observational selection effect, as fainter
stars are not observed as frequently as brighter ones.

The line emission in Be stars is variable at different levels. Some objects
show very little variation for decades (e.g., β CMi), while others exhibit periods
of strong line emission followed by “normal” star phases, when the emission
lines and IR excess disappear completely. Some examples of the latter include
π Aqr (Bjorkman et al., 2002), 66 Oph, and φ And (see Fig. 1, left panel). Be
disks may also reappear unexpectedly in previously known Be stars (e.g., π Aqr,
Zharikov et al., 2013) or form around B-type stars for the first time (e.g., δ Sco,
Miroshnichenko et al., 2001). It has been suggested that disk formation can be
triggered by non-radial pulsations (see Rivinius et al., 2013, for a recent review)
or by periastron passages in highly eccentric binaries (δ Sco). The disks are
<table><caption>**Table 1.** Main features of the Be and B[e] phenomena</caption><thead><tr><th>Feature</th><th>Be</th><th>B[e]</th></tr></thead><tbody><tr><td>Emission lines</td><td>Permitted transitions:<br>H I, Fe II except the coolest<br><br>no forbidden lines</td><td>Permitted and forbidden transitions:<br>same + Na I<br><br>[O I], [Fe II], [N II], [S II]<br>[O III] - B0-B2</td></tr><tr><td>IR-excess</td><td>free-free + free-bound CS gas</td><td>CS gas + dust thermal radiation</td></tr><tr><td>Rotation</td><td>close to break-up velocity</td><td>slower than that of Be stars</td></tr></tbody></table>

typically Keplerian, but the CS gas may both fall back onto the star or move away due to viscosity.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Left panel: Parts of the spectra of two Be stars that lost their disks. The spectra were taken with a 0.81 m telescope of the Three College Observatory in North Carolina, USA, with a spectral resolving power of *R* ~ 12,000. The dates of observation along with the objects names and spectral lines shown are indicated. The intensity is normalized to the local continuum, the wavelength scale is given in Angströms. Right panel: Brightness distribution of Galactic Be stars and FS CMa objects with the B[e] phenomenon.</figcaption></figure>

## 2.2. The B[e] phenomenon

The B[e] phenomenon is present in objects of 4 groups with known nature (pre-main-sequence Herbig Ae/Be stars, a small group of supergiants, symbiotic binaries, and some proto-planetary and planetary nebulae, Lamers et al., 1998). However, half of the originally found 65 objects were not classified, because they either had no reliable luminosity estimates or showed properties of more than one of the groups. Recently Miroshnichenko (2007) suggested that most unclassified objects with the B[e] phenomenon may actually be binary systems that
have undergone non-conservative mass transfer, which was responsible for for-
mation of the CS material, including dust. This group was named FS CMa type
objects after the prototype object with the B[e] phenomenon (Swings, 2006).
The group's SEDs show a lack of cold dust compared to young stars and plane-
tary nebulae and typically very strong emission-line spectra. The group has been
expanded several times (Miroshnichenko et al., 2007, 2017) and nearly 20 binary
systems have been found among its members by various methods, including five
with measured orbital periods. It is harder to reveal their binarity than in Be
stars due to a more complicated structure and variability of the CS medium, as
well as their lower brightness (see Fig. 1, right panel). However a growing num-
ber of their observations, especially at big telescopes (including SALT, CFHT,
the 2.1 m telescope of the Observatorio Astronómico Nacional San Pedro Mar-
tir, and 2 m Himalayan Chandra Telescope) will eventually result in verifying
the binary hypothesis.

3. Conclusions

There is growing evidence that many stars with the Be and B[e] phenomena
are binary systems, most of which have undergone mass transfer between the
components due to Roche lobe overflow. The number of confirmed Be binaries
is steadily growing. The situation with B[e] objects is similar: symbiotic systems
are binaries by definition, while most B[e] supergiants and a number of FS CMa
objects have been found to be binaries as well. Binarity can be revealed with
several methods, which include observing regular radial velocity variations or
orbital phase locked intensity variations of double-peaked emission-line profiles
(in weak-lined Be stars Zharikov et al., 2013), eclipses, and resolving individual
components with interferometry and spectro-astrometry. Spectroscopic methods
are still the most successful but require long-term observing campaigns and
collaboration between professional and amateur astronomers.

**Acknowledgements.** A.M. thanks the American Astronomical Society for support-
ing his trip to the conference site with an International Travel Grant. A.M. and S.Zh.
acknowledge support from DGAPA/PAPIIT Project IN 100617. The work was carried
out within the framework of Project No. BR05236322 "Studies of physical processes
in extragalactic and galactic objects and their subsystems" financed by the Ministry
of Education and Science of the Republic of Kazakhstan.

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