arXiv subjects

V. S. Niemela

Publications and source records attributed to V. S. Niemela.

12 recordsLinked to original sources

arXiv2008

The very massive X-ray bright binary system Wack 2134 (= WR 21a)

From the radial velocities of the N IV 4058 and He II 4686 emission lines, and the N V 4604-20 absorption lines, determined in digital spectra, we report the discovery that the X-ray bright emission line star Wack 2134 (= WR 21a) is a spectroscopic binary system with an orbital period of $31.673\pm0.002$ days. With this period, the N IV and He II emission and N V absorption lines, which originate in the atmosphere of the primary component, define a rather eccentric binary orbit (e=0.64$\pm$0.03). The radial velocity variations of the N V absorptions have a lower amplitude than those of the He II emission. Such a behaviour of the emission line radial velocities could be due to distortions produced by a superimposed absorption component from the companion. High resolution echelle spectra observed during the quadrature phases of the binary show H and He II absorptions of both components with a radial velocity difference of about 541 km/s. From this difference, we infer quite high values of the minimum masses, of about 87Mo and 53Mo for the primary and secondary components, respectively, if the radial velocity variations of the He II emission represent the true orbit of the primary. No He I absorption lines are observed in our spectra. Thus, the secondary component in the Wack2134 binary system appears to be an early O type star. From the presence of H, He II and N V absorptions, and N IV and C IV emissions, in the spectrum of the primary component, it most clearly resembles those of Of/WNLha type stars.

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arXiv2008

Discovery of a [WO] central star in the planetary nebula Th 2-A

% context About 2500 planetary nebulae are known in our Galaxy but only 224 have central stars with reported spectral types in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae (Acker et al. 1992; Acker et al. 1996) % aims We have started an observational program aiming to increase the number of PN central stars with spectral classification. % methods By means of spectroscopy and high resolution imaging, we identify the position and true nature of the central star. We carried out low resolution spectroscopic observations at CASLEO telescope, complemented with medium resolution spectroscopy performed at Gemini South and Magellan telescopes. % results As a first outcome of this survey, we present for the first time the spectra of the central star of the PN Th 2-A. These spectra show emission lines of ionized C and O, typical in Wolf-Rayet stars. % conclusions We identify the position of that central star, which is not the brightest one of the visual central pair. We classify it as of type [WO 3]pec, which is consistent with the high excitation and dynamical age of the nebula.

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arXiv2007

The environs of the HII region Gum31

We analyze the distribution of the interstellar matter in the environs of the HII region Gum31, excited by the open cluster NGC3324, located in the complex Carina region, with the aim of investigating the action of the massive stars on the surrounding neutral material. We use neutral hydrogen 21-cm line data, radio continuum images at 0.843, 2.4 and 4.9 GHz, 12CO(1-0) observations, and IRAS and MSX infrared data. Adopting a distance of 3 kpc for the HII region and the ionizing cluster, we derived an electron density of 33+/-3 cm^-3 and an ionized mass of (3.3+/-1.1)x10^3 Mo based on the radio continuum data at 4.9 GHz. The HI 21-cm line images revealed an HI shell surrounding the HII region. The HI structure is 10.0+/-1.7 pc in radius, has a neutral mass of 1500+/-500 Mo, and is expanding at 11 km/s. The associated molecular gas amounts to 1.1+/-0.5)x10^5 Mo, being its volume density of about 350 cm^3. This molecular shell could represent the remains of the cloud where the young open cluster NGC3324 was born or could have originated by the shock front associated with the HII region. The difference between the ambient density and the electron density of the HII region suggests that the HII region is expanding. The distributions of the ionized and molecular material, along with that of the emission in the MSX band A, suggest that a photodissociation region has developed at the interface between the ionized and molecular gas. The characteristics of a relatively large number of the IRAS, MSX, and 2MASS point sources projected onto the molecular envelope are compatible with protostellar candidates, showing the presence of active star forming regions. Very probably, the expansion of the HII region has triggered stellar formation in the molecular shell.

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arXiv2007

The environs of the HII region Gum31

We analyze the distribution of the interstellar matter in the environs of the \hii region Gum 31, excited by the open cluster NGC 3324, located in the complex Carina region, with the aim of investigating the action of the massive stars on the surrounding neutral material. We use 21cm-line data, radio continuum images at 0.843, 2.4 and 4.9 GHz, $^{12}${\bf CO(1-0)} observations, and IRAS and MSX infrared data. Adopting a distance of 3 kpc for the \hii\ region and the ionizing cluster, we have derived an electron density of 33$\pm$3 cm$^{-3}$ and an ionized mass of (3.3$\pm$1.1)$\times10^3$ M$_{\odot}$ based on the radio continuum data at 4.9 GHz. The \hi 21-cm line images revealed an \hi shell surrounding the H {\sc ii} region. The \hi structure is 10.0$\pm$1.7 pc in radius, has a neutral mass of 1500$\pm$500 M$_{\odot}$, and is expanding at 11 km s$^{-1}$. The associated molecular gas amounts to (1.5$\pm$0.5)$\times10^5$ M$_{\odot}$, being its volume density of about 500 cm^{-3}. This molecular material probably represents the remains of the cloud where the young open cluster NGC 3324 was born. The distributions of the ionized and molecular material, along with that of the emission in the MSX band A suggest that a photodissociation region has developed at the interface between the ionized and molecular gas. We conclude that either the massive stars in the open cluster have weak stellar winds or the stellar winds have blown during a very short period of time to create an interstellar bubble in an interstellar medium as dense as observed. IRAS, MSX, and 2MASS point sources projected onto the molecular envelope are compatible with protostellar candidates, showing the presence star forming regions. The expansion of the \hii region has triggered stellar formation in the molecular shell.

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arXiv2007

The stellar upper mass limit in the solar neighborhood

We are using HST GO programs 10205, 10602, and 10898 to test the stellar upper mass limit in the solar vicinity by attempting to detect optical close companions, thus lowering the calculated evolutionary masses. We have observed with ACS/HRC all the known (as of early 2005) Galactic O2/3/3.5 stars. We also have observations with HST/FGS and ground-based spectroscopy from LCO and CASLEO. Here we discuss our results for Pismis 24 and HD 93129A.

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arXiv2007

Pismis 24-1: The Stellar Upper Mass Limit Preserved

Is there a stellar upper mass limit? Recent statistical work seems to indicate that there is and that it is in the vicinity of 150 solar masses. In this paper we use HST and ground-based data to investigate the brightest members of the cluster Pismis 24, one of which (Pismis 24-1) was previously inferred to have a mass greater than 200 solar masses, in apparent disagreement with that limit. We determine that Pismis 24-1 is composed of at least three objects, the resolved Pismis 24-1SW and the unresolved spectroscopic binary Pismis 24-1NE. The evolutionary zero-age masses of Pismis 24-1SW, the unresolved system Pismis 24-1NE, and the nearby star Pismis 24-17 are all approximately 100 solar masses, very large but under the stellar upper mass limit.

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arXiv2006

Optical Spectroscopy of X-Mega targets in the Carina Nebula - VI. FO 15: a new O-Type double-lined eclipsing binary

We report the discovery of a new O-type double-lined spectroscopic binary with a short orbital period of 1.4 days. We find the primary component of this binary, FO 15, to have an approximate spectral type O5.5Vz, i.e. a Zero-Age-Main-Sequence star. The secondary appears to be of spectral type O9.5V. We have performed a numerical model fit to the public ASAS photometry, which shows that FO 15 is also an eclipsing binary. We find an orbital inclination of ~ 80 deg. From a simultaneous light-curve and radial velocity solution we find the masses and radii of the two components to be 30 +/- 1 and 16 +/- 1 solar masses and 7.5 +/- 0.5 and 5.3 +/- 0.5 solar radii. These radii, and hence also the luminosities, are smaller than those of normal O-type stars, but similar to recently born ZAMS O-type stars. The absolute magnitudes derived from our analysis locate FO 15 at the same distance as Eta Carinae. From Chandra and XMM X-ray images we also find that there are two close X-ray sources, one coincident with FO 15 and another one without optical counterpart. This latter seems to be a highly variable source, presumably due to a pre-main-sequence stellar neighbour of FO 15.

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arXiv2004

The Nearest Star of spectral type O3: A Component of the Multiple System HD 150136

From radial velocities determined in high signal-to-noise digital spectra, we report the discovery that the brightest component of the binary system HD 150136 is of spectral type O3. We also present the first double-lined orbital solution for this binary. Our radial velocities confirm the previously published spectroscopic orbital period of 2.6 days. He II absorptions appear double at quadratures, but single lines of N V and N IV visible in our spectra define a radial velocity orbit of higher semi-amplitude for the primary component than do the He II lines. From our orbital analysis, we obtain minimum masses for the binary components of 27 and 18$\modot$. The neutral He absorptions apparently do not follow the orbital motion of any of the binary components, thus they most probably arise in a third star in the system.

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arXiv2002

An X-ray investigation of the NGC346 field in the SMC (1) : the LBV HD5980 and the NGC346 cluster

We present results from a Chandra observation of the NGC346 cluster. This cluster contains numerous massive stars and is responsible for the ionization of N66, the most luminous HII region and the largest star formation region in the SMC. In this first paper, we will focus on the characteristics of the main objects of the field. The NGC346 cluster itself shows only relatively faint X-ray emission (with L_X^{unabs} ~ 1.5 times 10^{34} erg s^{-1}), tightly correlated with the core of the cluster. In the field also lies HD5980, a LBV star in a binary (or possibly a triple system) that is detected for the first time at X-ray energies. The star is X-ray bright, with an unabsorbed luminosity of L_X^{unabs} ~ 1.7 times 10^{34} erg s^{-1}, but needs to be monitored further to investigate its X-ray variability over a complete 19d orbital cycle. The high X-ray luminosity may be associated either with colliding winds in the binary system or with the 1994 eruption. HD5980 is surrounded by a region of diffuse X-ray emission, which is a supernova remnant. While it may be only a chance alignment with HD5980, such a spatial coincidence may indicate that the remnant is indeed related to this peculiar massive star.

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arXiv2002

The Massive Wolf-Rayet Binary SMC WR7

We present a study of optical spectra of the Wolf--Rayet star AzV 336a (= SMC WR7) in the Small Magellanic Cloud. Our study is based on data obtained at several Observatories between 1988 and 2001. We find SMC WR7 to be a double lined WN+O6 spectroscopic binary with an orbital period of 19.56 days. The radial velocities of the He absorption lines of the O6 component and the strong He{\sc ii} emission at $λ$4686Åof the WN component describe antiphased orbital motions. However, they show a small phase shift of $\sim$ 1 day. We discuss possible explanations for this phase shift. The amplitude of the radial velocity variations of He {\sc ii} emission is twice that of the absorption lines. The binary components have fairly high minimum masses, $\sim$ 18 $\modot$ and 34 $\modot$ for the WN and O6 components, respectively.

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arXiv2001

Optical spectroscopy of X-MEGA targets I. CPD -59 2635: A New Double-Lined O type Binary in the Carina Nebula

Optical spectroscopy of CPD -59 2635, one of the O-type stars in the open cluster Trumpler 16 in the Carina Nebula, reveals this star to be a double-lined binary system. We have obtained the first radial velocity orbit for this system, consisting of a circular solution with a period of 2.2999 days and semi amplitudes of 208 and 273 km/s. This results in minimum masses of 15 and 11 Msol for the binary components of CPD -59 2635, which we classified as O8V and O9.5V, though spectral type variations of the order of 1 subclass, that we identify as the Struve-Sahade effect, seem to be present in both components. From ROSAT HRI observations of CPD -59 2635 we determine a luminosity ratio log(L_x/L_bol)~ -7, which is similar to that observed for other O-type stars in the Carina Nebula region. No evidence of light variations is present in the available optical or X-rays data sets.

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arXiv2001

Optical spectroscopy of X-Mega targets in the Carina Nebula. II. The massive double-lined O-type binary HD 93205

A new high-quality set of orbital parameters for the O-type spectroscopic binary HD93205 has been obtained combining echelle and coude CCD observations. The radial velocity orbits derived from the HeII 4686 A (primary component) and HeI 4471 A (secondary component) absorption lines yield semiamplitudes of 133 +/- 2 and 314 +/- 2 km/s for each binary component, resulting in minimum masses of 31 and 13 Msol (q = 0.42). We also confirm for the binary components the spectral classification of O3V+O8V previously assigned. Assuming for the O8V component a ``normal'' mass of 22 - 25 Msol we would derive for the primary O3V a mass of ``only'' 52 - 60 Msol and an inclination of about 55 degrees for the orbital plane. We have also determined for the first time a period of apsidal motion for this system, namely 185 +/- 16 years using all available radial velocity data-sets of HD 93205 (from 1975 to 1999). Phase-locked variations of the X-ray emission of HD 93205 consisting of a rise of the observed X-ray flux near periastron passage, are also discussed.

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