arXiv subjects

N. I. Morrell

Publications and source records attributed to N. I. Morrell.

35 records · Page 2Linked to original sources

arXiv2011

Supernovae without host galaxies? The low surface brightness host of SN 2009Z

A remarkable fraction of supernovae (SNe) have no obvious host galaxy. Two possible explanations are that (i) the host galaxy is simply not detected within the sensitivity of the available data or that (ii) the progenitor is a hypervelocity star that has escaped its parent galaxy. We use the Type IIb SN 2009Z as a prototype of case (i), an example of how a very faint (here Low Surface Brightness; LSB) galaxy can be discovered via the observation of a seemingly host-less SN. By identifying and studying LSB galaxies that host SNe related to the death of massive stars, we can place constraints on the stellar population and environment of LSB galaxies, which at present are poorly understood. From an HI spectrum, a redshift of z = 0.02513+-0.00001 and an HI mass of (2.96+-0.12) 10^9 M_sun are computed. This redshift is consistent with that obtained from optical emission lines of SN 2009Z. Furthermore, a gas mass fraction of f_g = 0.87+-0.04 is obtained, one of the highest fractions ever measured. The host galaxy shows signs of recently enhanced star formation activity with a far-UV derived extinction-corrected Star Formation Rate (SFR) of 0.44+-0.34 M_sun/yr. Based on the B-band luminosity we estimate an extinction-corrected metallicity following the calibration by Pilyugin (2001) of 12 + log(O/H) = 8.24+-0.70. The presence of a Type IIb SN in an LSB galaxy suggests, contrary to popular belief, that massive stars can be formed in this type of galaxies. Furthermore, our results imply that LSB galaxies undergo phases of small, local burst activity intermittent with longer phases of inactivity.

astro-ph.CO↗

arXiv2011

The Galactic O-Star Spectroscopic (GOSSS) and Northern Massive Dim Stars (NoMaDS) Surveys, the Galactic O-Star Catalog (GOSC), and Marxist Ghost Buster (MGB)

There are several ongoing massive-star (mostly of spectral type O) surveys that are significantly increasing the quality and quantity of the spectroscopic information about these objects. Here we discuss and present results for two of them, GOSSS and NoMaDS. We also discuss recent and future developments on the Galactic O-Star Catalog and announce the upcoming availability of Marxist Ghost Buster, an IDL code that attacks spectral classification (hence the name) by using an interactive comparison with spectral libraries.

astro-ph.SR↗

arXiv2011

Two O2 If*/WN6 stars possibly ejected from the massive young Galactic cluster Westerlund 2

In this paper we report the identification of two new Galactic O2 If*/WN6 stars (WR20aa and WR20c), in the outskirt of the massive young stellar cluster Westerlund 2. The morphological similarity between the near-infrared spectra of the new stars with that of WR20a and WR21a (two of the most massive binaries known to date) is remarkable, indicating that probably they are also very massive stars. New optical spectroscopic observations of WR20aa suggest an intermediate O2 If*/WN6 spectral type. Based on a mosaic made from the 3.6 microns Spitzer IRAC images of the region including part of the RCW49 complex, we studied the spatial location of the new emission line stars, finding that WR20aa and WR20c are well displaced from the centre of Westerlund 2, being placed at ~ 36 pc (15.7 arcmin) and ~ 58 pc (25.0 arcmin) respectively, for an assumed heliocentric distance of 8 kpc. Also very remarkably, a radius vector connecting both stars would intercept the Westerlund 2 cluster exactly at the place where its stellar density reaches a maximum. We consequently postulate a scenario in which WR20aa and WR20c had a common origin somewhere in the cluster core, being ejected from their birthplace by dynamical interacion with some other very massive objects, perhaps during some earlier stage of the cluster evolution.

astro-ph.GA↗

arXiv2011

An accurate position for the black hole candidate XTE J1752-223: re-interpretation of the VLBI data

Using high-precision astrometric optical observations from the Walter Baade Magellan Telescope in conjunction with high-resolution very long baseline interferometric (VLBI) radio imaging with the Very Long Baseline Array (VLBA), we have located the core of the X-ray binary system XTE J1752-223. Compact radio emission from the core was detected following the state transition from the soft to the hard X-ray state. Its position to the south-east of all previously-detected jet components mandated a re-analysis of the existing VLBI data. Our analysis suggests that the outburst comprised at least two ejection events prior to 2010 February 26. No radio-emitting components were detected to the south-east of the core at any epoch, suggesting that the receding jets were Doppler-deboosted below our sensitivity limit. From the ratio of the brightness of the detected components to the measured upper limits for the receding ejecta, we constrain the jet speed to be greater than 0.66c and the inclination angle to the line of sight to be less than 49 degrees. Assuming that the initial ejection event occurred at the transition from the hard intermediate state to the soft intermediate state, an initial period of ballistic motion followed by a Sedov phase (i.e. self-similar adiabatic expansion) appears to fit the motion of the ejecta better than a uniform deceleration model. The accurate core location can provide a long time baseline for a future proper motion determination should the system show a second outburst, providing insights into the formation mechanism of the compact object.

astro-ph.HE↗

arXiv2011

The Galactic O-Star Spectroscopic Survey. I. Classification System and Bright Northern Stars in the Blue-Violet at R~2500

We present the first installment of a massive spectroscopic survey of Galactic O stars, based on new, high signal-to-noise ratio, R~2500 digital observations from both hemispheres selected from the Galactic O-Star Catalog of Maíz Apellániz et al. (2004) and Sota et al. (2008). The spectral classification system is rediscussed and a new atlas is presented, which supersedes previous versions. Extensive sequences of exceptional objects are given, including types Ofc, ON/OC, Onfp, Of?p, Oe, and double-lined spectroscopic binaries. The remaining normal spectra bring this first sample to 184 stars, which is close to complete to B=8 and north of delta = -20 and includes all of the northern objects in Maíz Apellániz et al. (2004) that are still classified as O stars. The systematic and random accuracies of these classifications are substantially higher than previously attainable, because of the quality, quantity, and homogeneity of the data and analysis procedures. These results will enhance subsequent investigations in Galactic astronomy and stellar astrophysics. In the future we will publish the rest of the survey, beginning with a second paper that will include most of the southern stars in Maíz Apellániz et al. (2004).

astro-ph.GA↗

arXiv2010

The Galactic O-Star Spectroscopic Survey (GOSSS)

We present a massive spectroscopic survey of Galactic O stars, GOSSS, based on new, high signal-to-noise ratio, R~2500 blue-violet digital observations from both hemispheres. The sample size and selection criteria; the relationship between GOSSS, the Galactic O-Star Catalog (GOSC), and three sister surveys (OWN, IACOB, and Lucky Imaging); the current status; and our plans for the future are discussed. We also show some of our first results, which include the new Ofc category, two new examples of Of?p stars, a new atlas for O stars, and the introduction of the O9.7 type for luminosity classes III to V. Finally, our scientific objectives are discussed.

astro-ph.SR↗

arXiv2009

A first orbital solution for the very massive 30 Dor main-sequence WN6h+O binary R145

We report the results of a spectroscopic and polarimetric study of the massive, hydrogen-rich WN6h stars R144 (HD 38282 = BAT99-118 = Brey 89) and R145 (HDE 269928 = BAT99-119 = Brey 90) in the LMC. Both stars have been suspected to be binaries by previous studies (R144: Schnurr et al. 2008b; R145: Moffat 1989). We have combined radial-velocity (RV) data from these two studies with previously unpublished polarimetric data. For R145, we were able to establish, for the first time, an orbital period of 158.8 days, along with the full set of orbital parameters, including the inclination angle i, which was found to be i = 38 \pm 9 deg. By applying a modified version of the shift-and-add method developed by Demers et al. (2002), we were able to isolate the spectral signature of the very faint-line companion star. With the RV amplitudes of both components in R145, we were thus able to estimate their absolute masses. We find minimum masses M_WR sin^{3}i = (116 \pm 33) M_sol and M_O sin^{3}i = (48 \pm 20)$ M_sol for the WR and the O component, respectively. Thus, if the low inclination angle were correct, resulting absolute masses of the components would be at least 300 and 125 M_sol, respectively. However, such high masses are not supported by brightness considerations when R145 is compared to systems with known, very high masses such as NGC3603-A1 or WR20a. An inclination angle close to 90 degrees would remedy the situation, but is excluded by the currently available data. More and better data are thus required to firmly establish the nature of this puzzling, yet potentially very massive and important system. As to R144, however, the combined data sets are not sufficient to find any periodicity.

astro-ph.SR↗

arXiv2008

A Spectroscopic Survey of WNL Stars in the LMC: General Properties and Binary Status

We report the results of an intense, spectroscopic survey of all 41 late-type, nitrogen-rich Wolf-Rayet (WR) stars in the Large Magellanic Cloud (LMC) observable with ground-based telescopes. This survey concludes the decade-long effort of the Montréal Massive Star Group to monitor every known WR star in the Magellanic Clouds except for the 6 crowded WNL stars in R136, which will be discussed elsewhere. The focus of our survey was to monitor the so-called WNL stars for radial-velocity (RV) variability in order to identify the short- to intermediate-period ($P \la 200$ days) binaries among them. Our results are in line with results of previous studies of other WR subtypes, and show that the binary frequency among LMC WNL stars is statistically consistent with that of WNL stars in the Milky Way. We have identified four previously unknown binaries, bringing the total number of known WNL binaries in the LMC to nine. Since it is very likely that none but one of the binaries are classical, helium-burning WNL stars, but rather superluminous, hence extremely massive, hydrogen-burning objects, our study has dramatically increased the number of known binaries harbouring such objects, and thus paved the way to determine their masses through model-independent, Keplerian orbits. It is expected that some of the stars in our binaries will be among the most massive known. With the binary status of each WR star now known, we also studied the photometric and X-ray properties of our program stars using archival MACHO photometry as well as Chandra and ROSAT data. We find that one of our presumably single WNL stars is among the X-ray brightest WR sources known. We also identify a binary candidate from its RV variability and X-ray luminosity which harbours the most luminous WR star known in the Local Group.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

arXiv2004

SB9: The Ninth Catalogue of Spectroscopic Binary Orbits

The Ninth Catalogue of Spectroscopic Binary Orbits (http://sb9.astro.ulb.ac.be) continues the series of compilations of spectroscopic orbits carried out over the past 35 years by Batten and collaborators. As of 2004 May 1st, the new Catalogue holds orbits for 2,386 systems. Some essential differences between this catalogue and its predecessors are outlined and three straightforward applications are presented: (1) Completeness assessment: period distribution of SB1s and SB2s; (2) Shortest periods across the H-R diagram; (3) Period-eccentricity relation.

astro-ph↗

arXiv2002

The massive double-lined O-type binary HD 165052

We present a new optical spectroscopic study of the O-type binary HD 165052 based on high- and intermediate-resolution CCD observations. We re-investigated the spectral classification of the binary components, obtaining spectral types of O6.5 V and O7.5 V for the primary and secondary, respectively, finding that both stars display weak C {\sc iii} $\lambda5696$ emission in their spectra. We also determined a radial-velocity orbit for HD 165052 with a period of 2.95510 $\pm$ 0.00001 d, and semiamplitudes of 94.8 and 104.7 $\pm$ 0.5 km s$^{-1}$, resulting in a mass ratio $Q = 0.9$. From a comparison with previous radial-velocity determinations, we found evidence of apsidal motion in the system. Several signatures of wind-wind collision, such as phase-locked variability of the X-ray flux and the Struve-Sahade effect, are also considered. It was also found that the reddening in the region should be normal, in contrast with previous determinations.

astro-ph↗

arXiv2001

Calculation of the Masses of the Binary Star HD 93205 by Application of the Theory of the Apsidal Motion

We present a method to calculate masses for components of both eclipsing and non-eclipsing binary systems as long as their apsidal motion rates are available. The method is based on the fact that the equation that gives the rate of apsidal motion is a supplementary equation that allows the computation of the masses of the components, if the radii and the internal structure constants of them can be obtained from theoretical models. For this reason the use of this equation makes the method presented here model dependent. We apply this method to calculate the mass of the components of the non-eclipsing massive binary system HD 93205 (O3V+O8V), which is suspected to be a very young system. To this end, we computed a grid of evolutionary models covering the mass range of interest, and taking the mass of the primary (M_1) as the only independent variable, we solve the equation of apsidal motion for M_1 as a function of the age of the system. The mass of the primary we find ranges from M_1= 60+-19 msun for ZAMS models, which sets an upper limit for M_1, down to M_1= 40+-9 msun for an age of 2 Myr. Accordingly, the upper limit derived for the mass of the secondary (M_2= Q M_1) is M_2= 25 msun is in very good agreement with the masses derived for other O8V stars occurring in eclipsing binaries.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

arXiv2000

Determination of Masses and Radii of the Massive Eclipsing Binary System HV 2543 in the Large Magellanic Cloud

We present a V light curve of the eclipsing binary system HV 2543 (Sk-67 117) in the Large Magellanic Cloud based on CCD images acquired between the years 1995 and 1998. We have analysed this light curve and published radial velocity data, finding that this system is semidetached, with the secondary (less massive and less luminous component) filling its Roche lobe. From our analysis with the Wilson-Devinney code, we estimated the following masses and radii for the components of HV 2543: M1=25.63 +- 0.7, R1=15.54 +- 0.4, M2=15.63 +- 1.0 and R2=13.99 +- 0.4, in solar units. On the basis of B and V photometry of the field stars, we found that HV 2543 is member of an OB association, perhaps related to which the massive binary system Sk-67 105 belongs.

astro-ph↗