S. Fabrika

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.

Subaru and Gemini Observations of SS 433: New Constraint on the Mass of the Compact Object

We present results of optical spectroscopic observations of the mass donor star in SS 433 with Subaru and Gemini, with an aim to best constrain the mass of the compact object. Subaru/Faint Object Camera and Spectrograph observations were performed on four nights of 2007 October 6-8 and 10, covering the orbital phase of = 0.96 – 0.26. We first calculate the cross-correlation function (CCF) of these spectra with that of the reference star HD 9233 in the wavelength range of 4740-4840 A. This region is selected to avoid strong absorption lines accompanied with contaminating emission components, which most probably originate from the surroundings of the donor star, such as the wind and gas stream. The same analysis is applied to archive data of Gemini/GMOS taken at = 0.84 – 0.30 by Hillwig & Gies. From the Subaru and Gemini CCF results, the amplitude of the radial velocity curve of the donor star is determined to be 58.3 ± 3.8 km s–1 with a systemic velocity of 59.2 ± 2.5 km s–1. Together with the radial velocity curve of the compact object, we derive the mass of the donor star and compact object to be M O = 12.4 ± 1.9 M ☉ and M X = 4.3 ± 0.6 M ☉, respectively. We conclude, however, that these values should be taken as upper limits. From the analysis of the averaged absorption line profiles of strong lines (mostly ions) and weak lines (mostly neutrals) observed with Subaru, we find evidence for heating effects from the compact object. Using a simple model, we find that the true radial velocity amplitude of the donor star could be as low as 40 ± 5 km s–1 in order to produce the observed absorption-line profiles. Taking into account the heating of the donor star may lower the derived masses to M O = 10.4+2.3 –1.9 M ☉ and M X = 2.5+0.7 –0.6 M ☉. Our final constraint, 1.9 M ☉ ≤M X≤ 4.9 M ☉, indicates that the compact object in SS 433 is most likely a low mass black hole, although the possibility of a massive neutron star cannot be firmly excluded.

A new luminous variable in M33

We present a new luminous star in M33 located in the nuclear region. The star shows strong Fe II and [Fe II] forest, hydrogen emissions in the spectrum, as well as nebular lines. Ti II and Si II lines were detected in absorption, their radial velocity shifted by ≈− 30 km s −1 relative to emission lines. The star is variable over 7 yr with 0.5 mag variations over a year. We studied its spectral energy distribution together with five confirmed luminous blue variables (LBVs) and Var A in M33 using homogeneous data and methods. We found the star’s bolometric luminosity to be log L/L � ≈ 6.27, a surface temperature of T ∼ 16 000 K and blackbody temperatures of two dust components of T ∼ 900 and 420 K. The new star has properties intermediate between the LBVs and Var A (probable cool hypergiant). At the same time, it has a hot photosphere, LBV-like luminosity and an extensive circumstellar material (strong [Ca II] lines). In these seven luminous variables in M33, we find the total range of the hot component luminosities is 1.0 dex, but that of the dust components is 2.0 dex. We conclude that the dust phenomenon in the luminous variables is temporary and variable, and that the dust activity may follow strong eruptions.

Evidence of supercritical disc funnel radiation in X-ray spectra of SS 433

We analysed the XMM-Newton spectra of SS 433 using a standard model of adiabatically and radiatively cooling X-ray jets. The multitemperature thermal jet model reproduces the strongest observed emission line fluxes well. Fitting the He- and H-like iron line fluxes, we find that the visible blue jet base temperature is ~ 17 keV, the jet kinetic luminosity L k ~ 2 × 10 39 er g s -1 and the absorbing column density N H ~ 1.5 × 10 22 cm -2 . All these parameters are in line with previous studies. The thermal model alone cannot reproduce the continuum radiation in the XMM spectral range, nor the fluorescent iron line and some of the broad spectral features. Using the thermal jet-plus-reflection model, we find a notable contribution of ionized reflection to the spectrum in the energy range from ~3 to 12 keV. The reflecting surface is highly ionized (ξ ~ 300), and the illuminating radiation photon index changes from Γ ≈ 2 (a flat spectrum) in the range 7-12 keV to Γ ≈ 1.6 in the range 4-7 keV, and to Γ ≲ 1 in the range 2-4 keV. We conclude that the reflected spectrum is evidence of the supercritical disc funnel, whereby the illuminating radiation comes from deeper funnel regions, to be further reflected in the outer visible funnel walls (r ≳ 2 x 10 11 cm). In the multiple scatterings in the funnel, the harder radiation >7 keV may survive absorption, but softer radiation is absorbed, making the illuminating spectrum curved. We have not found any evidence of reflection in the soft 0.8-2 keV energy range; instead, a soft excess is detected, which does not depend on the details of the thermal jet model. However, the soft component spectrum is basically unknown. This soft component might prove to be the direct radiation of the visible funnel wall. It is represented here either as blackbody radiation with a temperature of θ bb ≈ 0.1 keV and a luminosity of L bb ~ 3 x 10 37 erg s -1 , or with a multicolour funnel model. The soft spectral component has parameters roughly the same as those found in ultraluminous X-ray sources.

Design and modeling of a moderate-resolution astronomic spectrograph with volume-phase holographic gratings

We present an optical design of astronomic spectrograph based on a cascade of volume-phase holographic gratings. The cascade consists of three gratings. Each of them provides moderately high spectral resolution in a narrow range of 83nm. Thus the spectrum image represents three lines covering region 430-680nm. Two versions of the scheme are described: a full-scale one with estimated resolving power of 5300-7900 and a small-sized one intended for creation of a lab prototype, which provides the resolving power of 1500-3000. Diffraction efficiency modeling confirms that the system throughput can reach 75%, while stray light caused by the gratings crosstalk is negligible. We also propose a design of image slicer and focal reducer allowing to couple the instrument with a 6m-telescope. Finally, we present concept of the instrument’s optomechanical design.

Optical counterparts of two ULXs in NGC 5474 and NGC 3627 (M 66)

We identified two optical counterparts of brightest ultraluminous X-ray sources (ULXs) in galaxies NGC 5474 and NGC 3627 (M66). The counterparts in Hubble Space Telescope images are very faint, their V magnitudes are 24.7 (MV ≈ −4.5) and 25.9 (MV ≈ −4.2), respectively. NGC 5474 X-1 changes the X-ray flux more than two orders of magnitude, in its bright state it has LX ≈ 1.6 × 1040 erg s−1, the spectrum is best fitted by an absorbed power law model with a photon index ≈ 0.94. M66 X-1 varies in X-rays with a factor of ∼2.5, its maximal luminosity being 2.0 × 1040 erg s−1 with ≈ 1.7. Optical spectroscopy of the NGC 5474 X-1 has shown a blue spectrum, which however was contaminated by a nearby star of 23 mag, but the counterpart has a redder spectrum. Among other objects captured by the slit are a background emission-line galaxy (z = 0.359) and a new young cluster of NGC 5474. We find that these two ULXs have largest X-ray-to-optical ratios of LX/Lopt ∼ 7000 for NGC 5474 X-1 (in its bright state) and 8000 for M66 X-1 both with the faintest optical counterparts ever measured. Probably their optical emission originates from the donor star. If they have super-Eddington accretion discs with stellar-mass black holes, they may also have the lowest mass accretion rates among ULXs such as in M81 X-6 and NGC 1313 X-1.

Spectral multiplexed VPHG based on photopolymers: the first application on a spectrograph

Many of the current spectrographs available at state-of-the-art telescopes facilities, possess specifications that are strongly limited by the dispersing elements that are used. Therefore, a refurbishment of these devices would potentially increase the performances if innovative designs are considered. We propose a solution for designing stacked VPHG that is able to secure efficiently different spectra in a single shot. This could be possible considering parameters that are specific for a particular class of holographic material, the photopolymers, that are well known for bringing reliability and precise throughput. We demonstrate the applicability of our solution, through the example of the new spectrograph designed for the 1m telescope at SAO RAS. The spectrograph will cover a spectral range 444-706 nm with the spectral resolving power of R=4273-5176 and the throughput maximum of 64%. The working ranges of the gratings are selected to provide more diffraction efficiency around the main important lines used in astrophysics.

Spectral analysis of LBV stars in M31: AF And and Var 15

We study spectra of two bona fide LBV stars in M31: AF And and Var 15. The spectra were obtained with the 6-m telescope (Russia) from 2005 to 2012. The model spectra were calculated with the CMFGEN code. We have not found strong changes in the spectra of the LBV stars in that time interval, however a certain variability has been detected. We estimate the star and wind parameters, such as luminosity, temperature, raduis, mass loss rate, escape velocity, hydrogen content, and reddening. We study the stars on the Hertzsprung-Russell diagram and find their initial masses using evolutionary tracks by Meynet et al . ( 1994 ).