V. V. Gvaramadze

Revealing evolved massive stars with Spitzer

Massive evolved stars loss a large fraction of their mass via copious stellar wind or instant outbursts and during certain evolutionary phases they can b e identified via the presence of their circumstellar nebulae. In this paper, we present the r esults of search for compact nebulae (reminiscent of circumstellar nebulae around evolved m assive stars) using archival 24 μm data obtained with the Multiband Imaging Photometer for Spitzer. We discovered 115 nebulae, most of which bear a striking resemblance to the circums tellar nebulae associated with Luminous Blue Variables (LBVs) and late WN-type (WNL) WolfRayet (WR) stars in the Milky Way and the Large Magellanic Cloud (LMC). We interpret this similarity as an indication that the central stars of detected nebulae are either LB Vs or related evolved massive stars. Our interpretation is supported by follow-up spectroscopy f two dozens of these central stars, most of which turns out to be either candidate LBVs (cLBVs), b lue supergiants or WNL stars. We expect that the forthcoming spectroscopy of the remainin g objects from our list, accompanied by the spectrophotometric monitoring of the already discovered cLBVs, will further increase the known population of Galactic LBVs, which in tur n would have profound consequences for better understanding the LBV phenomenon and its role in the transition between hydrogen burning O stars and helium burning WR stars. We also rep rt the detection of an arc-like structure attached to the cLBV HD 326823 and an arc a ssociated with the LBV R99 (HD 269445) in the LMC.

On the origin of high-velocity runaway stars

We explore the hypothesis that some high-velocity runaway stars attain their peculiar velocities in the course of exchange encounters between hard massive binaries and a very massive star (either an ordinary 50-100 M-circle dot star or a more massive one, formed through runaway mergers of ordinary stars in the core of a young massive star cluster). In this process, one of the binary components becomes gravitationally bound to the very massive star, while the second one is ejected, sometimes with a high speed. We performed three-body scattering experiments and found that early B-type stars (the progenitors of the majority of neutron stars) can be ejected with velocities of greater than or similar to 200-400 km s(-1) (typical of pulsars), while 3-4 M-circle dot stars can attain velocities of greater than or similar to 300-400 km s(-1) (typical of the bound population of halo late B-type stars). We also found that the ejected stars can occasionally attain velocities exceeding the Milky Wayss escape velocity.

Very massive runaway stars from three-body encounters

Very massive stars preferentially reside in the cores of their parent clusters and form binary or multiple systems. We study the role of tight very massive binaries in the origin of the field population of very massive stars. We performed numerical simulations of dynamical encounters between single (massive) stars and a very massive binary with parameters similar to those of the most massive known Galactic binaries, WR20a and NGC3603-A1. We found that these three-body encounters could be responsible for the origin of high peculiar velocities (> 70 kms) observed for some very massive (> 60 − 70M⊙) runaway stars in the Milky Way and the Large Magellanic Cloud (e.g., λCep, BD+43 3654, Sk−6722, BI 237, 30Dor 016), which can hardly be explained within the framework of the binary-supernova scenario. The production of high-velocity massive stars via three-body encounters is accompanied by the recoil of the binary in the opposite direction to the ejected star. We show that the relative position of the very massive binary R145 and the runaway early B-type star Sk−69 206 on the sky is consistent with the possibility that both objects were ejected from the central cluster, R136, of the star-forming region 30Doradus via the same dynamical event – a three-body encounter.

Hyperfast pulsars as the remnants of massive stars ejected from young star clusters

Recent proper motion and parallax measurements for the pulsar PSR B1508+55 indicate a transverse velocity of similar to 1100 km s(-1), which exceeds earlier measurements for any neutron star. The spin-down characteristics of PSR B1508+55 are typical for a non-recycled pulsar, which implies that the velocity of the pulsar cannot have originated from the second supernova disruption of a massive binary system. The high velocity of PSR B1508+55 can be accounted for by assuming that it received a kick at birth or that the neutron star was accelerated after its formation in the supernova explosion. We propose an explanation for the origin of hyperfast neutron stars based on the hypothesis that they could be the remnants of a symmetric supernova explosion of a high-velocity massive star which attained its peculiar velocity (similar to that of the pulsar) in the course of a strong dynamical three- or four-body encounter in the core of dense young star cluster. To check this hypothesis, we investigated three dynamical processes involving close encounters between: (i) two hard massive binaries, (ii) a hard binary and an intermediate-mass black hole (IMBH) and (iii) a single stars and a hard binary IMBH. We find that main-sequence O-type stars cannot be ejected from young massive star clusters with peculiar velocities high enough to explain the origin of hyperfast neutron stars, but lower mass main-sequence stars or the stripped helium cores of massive stars could be accelerated to hypervelocities. Our explanation for the origin of hyperfast pulsars requires a very dense stellar environment of the order of 10(6)- 10(7) stars pc(-3). Although such high densities may exist during the core collapse of young massive star clusters, we caution that they have never been observed.

MN112: a new Galactic candidate Luminous Blue Variable ?

We report the discovery of a new Galactic candidate Luminous Blue Variable (cLBV) via detection of an infrared circular nebula and follow-up spectroscopy of its central star. The nebula, MN112, is one of many dozens of circular nebulae detected at 24µm in the Spitzer Space Telescope archival data, whose morphology is similar to that of nebulae associated with known (c)LBVs and related evolved massive stars. Specifically, the core-halo morphology of MN112 bears a striking resemblance to the circumstellar nebula associated with the Galactic cLBV GAL079.29+00.46, which suggests that both nebulae might have a similar origin and that the central star of MN112 is a LBV. The spectroscopy of the central star showed that its spectrum is almost identical to that of the bona fide LBV PCygni, which also supports the LBV classification of the object. To further constrain the nature of MN112, we searched for signatures of possible high-amplitude (& 1 mag) photometric variability of the central star using archival and newly obtained photometric data covering a 45 year period. We found that the B magnitude of the star was constant ( 17.1� 0.3 mag) over this period, while in the I band the star brightened by 0.4 mag during the last 17 years. Although the non-detection of large photometric variability leads us to use the prefix ‘candidate’ in the classification of MN112, we remind that the long-term photometric stability is not unusual for genuine LBVs and that the brightness of PCygni remains relatively stable during the last three centuries.

On the origin of hyperfast neutron stars

We propose an explanation for the origin of hyperfast neutron stars (e.g. PSR B1508+55, PSR B2224+65, RX J0822-4300) based on the hypothesis that they could be the remnants of a symmetric supernova explosion of a high-velocity massive star (or its helium core) which attained its peculiar velocity (similar to that of the neutron star) in the course of a strong three- or four-body dynamical encounter in the core of a young massive star cluster. This hypothesis implies that the dense cores of star clusters (located either in the Galactic disk or near the Galactic centre) could also produce the so-called hypervelocity stars - ordinary stars moving with a speed of ~ 1 000 km s−1.