Wolf-Rayet Candidates Research Articles

Nature of the diffuse emission sources in the H i supershell in the galaxy IC 1613

ABSTRACT We present a study of the nearby low-metallicity dwarf galaxy IC 1613, focusing on the search for massive stars and related feedback processes, as well as for faint supernova remnants (SNR) in late stages of evolution. We obtained the deepest images of IC 1613 in the narrow-band H α, He ii and [S ii] emission lines and new long-slit spectroscopy observations using several facilities (6-m BTA, 2.5m SAI MSU, and 150RTT telescopes), in combination with the multiwavelength archival data from MUSE/VLT, VLA, XMM–Newton, and Swift/XRT. Our deep narrow-band photometry identifies several faint shells in the galaxy, and we further investigate their physical characteristics with the new long-slit spectroscopy observations and the archival multiwavelength data. Based on energy balance calculations and assumptions about their possible nature, we propose that one of the shells is a possible remnant of a supernova explosion. We study five out of eight Wolf–Rayet (WR) star candidates previously published for this galaxy using the He ii emission line mapping, MUSE/VLT archival spectra, and new long-slit spectra. Our analysis discards the considered WR candidates and finds no new ones. We found P Cyg profiles in H α line in two stars, which we classify as Luminous Blue Variable (LBV) star candidates. Overall, the galaxy IC 1613 may have a lower rate of WR star formation than previously suggested.

The detectability of Wolf–Rayet stars in M33-like spirals up to 30 Mpc

ABSTRACT We analyse the impact that spatial resolution has on the inferred numbers and types of Wolf–Rayet (WR) and other massive stars in external galaxies. Continuum and line images of the nearby galaxy M33 are increasingly blurred to mimic effects of different distances from 8.4 to 30 Mpc, for a constant level of seeing. We use differences in magnitudes between continuum and He ii line images, plus visual inspection of images, to identify WR candidates via their ionized helium excess. The result is a surprisingly large decrease in the numbers of WR detections, with only 15 per cent of the known WR stars predicted to be detected at 30 Mpc. The mixture of WR subtypes is also shown to vary significantly with increasing distance (poorer resolution), with cooler WN stars more easily detectable than other subtypes. We discuss how spatial clustering of different subtypes and line dilution could cause these differences and the implications for their ages, this will be useful for calibrating numbers of massive stars detected in current surveys. We investigate the ability of ELT/HARMONI to undertake WR surveys and show that by using adaptive optics at visible wavelengths even the faintest (MV = –3 mag) WR stars will be detectable out to 30 Mpc.

Applications of machine-learning algorithms for infrared colour selection of Galactic Wolf–Rayet stars

We have investigated and applied machine-learning algorithms for Infrared Colour Selection of Galactic Wolf-Rayet (WR) candidates. Objects taken from the GLIMPSE catalogue of the infrared objects in the Galactic plane can be classified into different stellar populations based on the colours inferred from their broadband photometric magnitudes ($J$, $H$ and $K_s$ from 2MASS, and the four \textit{Spitzer}/IRAC bands). The algorithms tested in this pilot study are variants of the $k$-Nearest Neighbours ($k$-NN) approach, which is ideal for exploratory studies of classification problems where interrelations between variables and classes are complicated. The aims of this study are (1) to provide an automated tool to select reliable WR candidates and potentially other classes of objects, (2) to measure the efficiency of infrared colour selection at performing these tasks and, (3) to lay the groundwork for statistically inferring the total number of WR stars in our Galaxy. We report the performance results obtained over a set of known objects and selected candidates for which we have carried out follow-up spectroscopic observations, and confirm the discovery of 4 new WR stars.

A deep near-infrared spectroscopic survey of the Scutum-Crux arm for Wolf-Rayet stars★

We present an NTT/SOFI spectroscopic survey of infrared selected Wolf-Rayet candidates in the Scutum-Crux spiral arm (298 < l < 340, |b| < 0.5). We obtained near-IR spectra of 127 candidates, revealing 17 Wolf-Rayet stars - a ~13% success rate - of which 16 are newly identified here. The majority of the new Wolf-Rayet stars are classified as narrow-lined WN5-7 stars, with 2 broad-lined WN4-6 stars and 3 WC6-8 stars. The new stars, with distances estimated from previous absolute magnitude calibrations, have no obvious association with the Scutum-Crux arm. Refined near-infrared (YHJK) classification criteria based on over a hundred Galactic and Magellanic Cloud WR stars, providing diagnostics for hydrogen in WN stars, plus the identification of WO stars and intermediate WN/C stars. Finally, we find that only a quarter of WR stars in the survey region are associated with star clusters and/or HII regions, with similar statistics found for Luminous Blue Variables in the Milky Way. The relative isolation of evolved massive stars is discussed, together with the significance of the co-location of LBVs and WR stars in young star clusters.

A Modern Search for Wolf–Rayet Stars in the Magellanic Clouds. III. A Third Year of Discoveries*

Abstract For the past three years we have been conducting a survey for Wolf–Rayet (WR) stars in the Large and Small Magellanic Clouds (LMC, SMC). Our previous work resulted in the discovery of a new type of WR star in the LMC, which we are calling WN3/O3. These stars have the emission-line properties of a WN3 star (strong N v, but no N iv), plus the absorption-line properties of an O3 star (Balmer hydrogen plus Pickering He ii, but no He i). Yet, these stars are 15 times fainter than an O3 V star, ruling out the possibility that WN3/O3s are WN3+O3 binaries. Here we report the discovery of two more members of this class, bringing the total number of these objects to 10, 6.5% of the LMC’s total WR population. The optical spectra of nine of these WN3/O3s are virtually indistinguishable from each other, but one of the newly found stars is significantly different, showing a lower excitation emission and absorption spectrum (WN4/O4-ish). In addition, we have newly classified three unusual Of-type stars, including one with a strong C iii line, and two rapidly rotating “Oef” stars. We also “rediscovered” a low mass X-ray binary, RX J0513.9-6951, and demonstrate its spectral variability. Finally, we discuss the spectra of 10 low priority WR candidates that turned out to not have He ii emission. These include both a Be star and a B[e] star.

Machine-learning approaches to select Wolf-Rayet candidates

AbstractThe WR stellar population can be distinguished, at least partially, from other stellar populations by broad-band IR colour selection. We present the use of a machine learning classifier to quantitatively improve the selection of Galactic Wolf-Rayet (WR) candidates. These methods are used to separate the other stellar populations which have similar IR colours. We show the results of the classifications obtained by using the 2MASS J, H and K photometric bands, and the Spitzer/IRAC bands at 3.6, 4.5, 5.8 and 8.0μm. The k-Nearest Neighbour method has been used to select Galactic WR candidates for observational follow-up. A few candidates have been spectroscopically observed. Preliminary observations suggest that a detection rate of 50% can easily be achieved.

The Wolf-Rayet population of the nearby barred spiral galaxy NGC 5068 uncovered by the Very Large Telescope and Gemini

We present a narrow-band VLT/FORS1 imaging survey of the SAB(rs)cd spiral galaxy NGC 5068, located at a distance of 5.45Mpc, from which 160 candidate Wolf-Rayet sources have been identified, of which 59 cases possess statistically significant 4686 excesses. Follow-up Gemini/GMOS spectroscopy of 64 candidates, representing 40% of the complete photometric catalogue, confirms Wolf-Rayet signatures in 30 instances, corresponding to a 47% success rate. 21 out of 22 statistically significant photometric sources are spectroscopically confirmed. Nebular emission detected in 30% of the Wolf-Rayet candidates spectrally observed, which enable a re-assessment of the metallicity gradient in NGC 5068. A central metallicity of log(O/H)+12 ~ 8.74 is obtained, declining to 8.23 at R_25. We combine our spectroscopy with archival Halpha images of NGC 5068 to estimate a current star formation rate of 0.63(+0.11/-0.13)Msun/yr, and provide a catalogue of the 28 brightest HII regions from our own continuum subtracted Halpha images, of which ~17 qualify as giant HII regions. Spectroscopically, we identify 24 WC and 18 WN-type Wolf-Rayet stars within 30 sources since emission line fluxes indicate multiple Wolf-Rayet stars in several cases. We estimate an additional ~66 Wolf-Rayet stars from the remaining photometric candidates, although sensitivity limits will lead to an incomplete census of visually faint WN stars, from which we estimate a global population of ~170 Wolf-Rayet stars. Based on the Halpha-derived O star population of NGC 5068 and N(WR)/N(O)~0.03, representative of the LMC, we would expect a larger Wolf-Rayet population of 270 stars. Finally, we have compared the spatial distribution of spectroscopically confirmed WN and WC stars with SDSS-derived supernovae, and find both WN and WC stars to be most consistent with the parent population of Type Ib SNe.

RED EYES ON WOLF-RAYET STARS: 60 NEW DISCOVERIES VIA INFRARED COLOR SELECTION

We have spectroscopically identified 60 Galactic Wolf-Rayet (WR) stars, including 38 nitrogen types (WN) and 22 carbon types (WC). Using photometry from the Spitzer/GLIMPSE and 2MASS databases, the WRs were selected via a method we have established that exploits their unique infrared colors, which is mainly the result of excess radiation from free-free scattering within their dense ionized winds. The selection criteria has been refined since our last report, and now yields WRs at a rate of ~20% in spectroscopic follow-up of candidates that comprise a broad color space defined by the color distribution of all known WRs having B>14 mag. However, there are subregions within the broad color space which yield WRs at a rate of >50%. Cross-correlation of WR candidates with archival X-ray point-source catalogs increases the WR detection rate of the broad color space to ~40%; ten new WR X-ray sources have been found, in addition to a previously unrecognized X-ray counterpart to a known WR. The extinction values, distances, and galactocentric radii of all new WRs are calculated using the method of spectroscopic parallax. Although the majority of the new WRs have no obvious association with stellar clusters, two WC8 stars reside in a previously unknown massive-star cluster that lies near the intersection of the Scutum-Centaurus Arm and the Galaxy's bar, in which five OB supergiants were also identified. In addition, two WC and four WN stars were identified in association with the stellar clusters Danks 1 and 2. A WN9 star has also been associated with the cluster [DBS2003] 179. This work brings the total number of known Galactic WRs to 476, or ~7-8% of the total empirically estimated population. An examination of their Galactic distribution reveals a tracing of spiral arms and an enhanced WR surface density toward several massive-star formation sites (abridged).

CIRCUMSTELLAR STRUCTURE AROUND EVOLVED STARS IN THE CYGNUS-X STAR FORMATION REGION

We present observations of newly discovered 24 micron circumstellar structures detected with the Multiband Imaging Photometer for Spitzer (MIPS) around three evolved stars in the Cygnus-X star forming region. One of the objects, BD+43 3710, has a bipolar nebula, possibly due to an outflow or a torus of material. A second, HBHA 4202-22, a Wolf-Rayet candidate, shows a circular shell of 24 micron emission suggestive of either a limb-brightened shell or disk seen face-on. No diffuse emission was detected around either of these two objects in the Spitzer 3.6-8 micron Infrared Array Camera (IRAC) bands. The third object is the luminous blue variable candidate G79.29+0.46. We resolved the previously known inner ring in all four IRAC bands. The 24 micron emission from the inner ring extends ~1.2 arcmin beyond the shorter wavelength emission, well beyond what can be attributed to the difference in resolutions between MIPS and IRAC. Additionally, we have discovered an outer ring of 24 micron emission, possibly due to an earlier episode of mass loss. For the two shell stars, we present the results of radiative transfer models, constraining the stellar and dust shell parameters. The shells are composed of amorphous carbon grains, plus polycyclic aromatic hydrocarbons in the case of G79.29+0.46. Both G79.29+0.46 and HBHA 4202-22 lie behind the main Cygnus-X cloud. Although G79.29+0.46 may simply be on the far side of the cloud, HBHA 4202-22 is unrelated to the Cygnus-X star formation region.

Wolf-Rayet stars in M33 - II. Optical spectroscopy of emission-line stars in giant H ii regions

We present optical spectra of 14 emission-line stars in M33’s giant H II regions NGC 592, 595 and 604: five of them are known Wolf–Rayet (WR) stars, for which we present a better quality spectrogram, eight are WR candidates based on narrow-band imagery and one is a serendipitous discovery. Spectroscopy confirms the power of interference filter imagery to detect emission-line stars down to an equivalent width of about 5 A in crowded fields. We have also used archival Hubble Space Telescope (HST)/WFPC2 (Wide Field and Planetary Camera 2) images to correctly identify emission-line stars in NGC 592 and 588.

A survey of the Wolf–Rayet population of the barred, spiral galaxy NGC 1313

Abstract We present a VLT/FORS1 survey of Wolf–Rayet (WR) stars in the spiral galaxy NGC 1313. In total, 94 WR candidate sources have been identified from narrow-band imaging. Of these, 82 have been spectroscopically observed, for which WR emission features are confirmed in 70 cases, one of which also exhibits strong nebular He iiλ4686 emission. We also detect strong nebular He iiλ4686 emission within two other regions of NGC 1313, one of which is a possible supernova remnant. Nebular properties confirm that NGC 1313 has a metal content log(O/H) + 12 = 8.23 ± 0.06, in good agreement with previous studies. From continuum-subtracted Hα images we infer a global star formation rate of 0.6 M⊙ yr−1. Using template LMC WR stars, spectroscopy reveals that NGC 1313 hosts a minimum of 84 WR stars. Our census comprises 51 WN stars, including a rare WN/C transition star plus 32 WC stars. In addition, we identify one WO star which represents the first such case identified beyond the Local Group. The bright giant H ii region PES 1, comparable in Hα luminosity to NGC 595 in M 33, is found to host a minimum of 17 WR stars. The remaining photometric candidates generally display photometric properties consistent with WN stars, such that we expect a global WR population of ∼115 stars with N(WR)/N(O) ∼ 0.01 and N(WC)/N(WN) ∼ 0.4.

On the optical counterpart of NGC 300 X-1 and the global Wolf-Rayet content of NGC 300

(Conext:) Surveys of Wolf-Rayet (WR) populations in nearby galaxies provide tests of evolutionary models plus Type Ib/c supernova progenitors. This spectroscopic study complements the recent imaging survey of the spiral galaxy NGC 300 by Schild et al. (Aims): Revisions to the known WR content of NGC 300 are presented. We investigate the WR nature of candidate #41 from Schild et al. which is spatially coincident with the bright X-ray point source NGC 300 X-1; (Methods:) VLT/FORS2 multi-object spectroscopy of WR candidates in NGC 300 is obtained; (Results:) We establish an early-type WN nature of #41, i.e. similar to the optical counterpart of IC 10 X-1, which closely resembles NGC 300 X-1. We confirm 9 new WR stars, bringing the current WR census of the inner disk to 31, with N(WC)/N(WN)~0.9. (Conclusions:) If #41 is the optical counterpart for NGC 300 X-1, we estimate a WR mass of 38 Msun based upon ground-based photometry, from which a black hole mass of > 10 Msun results from the 32.8 hr period of the system and WR wind velocity of 1250 km/s. We estimate an 95% completeness among WC stars and 70% among WN stars, such that the total WR content is ~40, with N(WC)/N(WN)~0.7. From the Halpha-derived star formation rate of the inner galaxy, we infer N(WR)/N(O)~0.04

Searching for hidden Wolf-Rayet stars in the Galactic plane - 15 new Wolf-Rayet stars

We report the discovery of 15 previously unknown Wolf–Rayet (WR) stars found as part of an infrared (IR) broad-band study of candidate WR stars in the Galaxy. We have derived an empirically based selection algorithm which has selected ∼5000 WR candidate stars located within the Galactic plane drawn from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (mid-IR) and Two-Micron All-Sky Survey (near-IR) catalogues. Spectroscopic follow-up of 184 of these reveals 11 nitrogen-rich (WN) and four carbon-rich (WC) WR stars. Early WC subtypes are absent from our sample and none shows evidence for circumstellar dust emission. Of the candidates which are not WR stars, ∼120 displayed hydrogen emission-line features in their spectra. Spectral features suggest that the majority of these are in fact B supergiants/hypergiants, ∼40 of these are identified Be/B[e] candidates. Here, we present the optical spectra for six of the newly detected WR stars, and the near-IR spectra for the remaining nine of our sample. With a WR yield rate of ∼7 per cent and a massive star detection rate of ∼65 per cent, initial results suggest that this method is one of the most successful means for locating evolved, massive stars in the Galaxy.

A Wolf-Rayet/black-hole X-ray binary candidate in NGC 300

Context. Wolf Rayet/black hole binaries are believed to exist as a later evolutionary product of high-mass X-ray binaries. Hundreds of such binaries may exist in galaxies, but only a few of them are close enough to be observed as X-ray binaries. Only a couple of candidates have been reported so far. Aims. Based on XMM-Newton observations, we report the positional coincidence of the brightest X-ray source in NGC 300 (NGC 300 X-1) with a Wolf-Rayet candidate. Temporal and spectral analysis of the X-ray source is performed. Methods. We determine an accurate X-ray position of the object, and derive light curves, spectra and flux in four XMM-Newton observations. Results. The positions of the X-ray source and the helium star candidate coincide within 0.11+-0.45. The X-ray light curves show irregular variability. During one XMM-Newton observation, the flux increased by about a factor of ten in 10 hours. The spectrum can be modelled by a power-law with Gamma~2.45 with additional relatively weak line emission, notably around 0.95 kev. The mean observed (absorbed) luminosity in the 0.2-10 kev band is ~2x10^38 erg/s. Conclusions. NGC 300 X-1 is a good candidate for a Wolf-Rayet/black-hole X-ray binary: its position coincides with a Wolf-Rayet candidate and the unabsorbed X-ray luminosity reached L(0.2-10 kev)~1x10^39 erg/s, suggesting the presence of a black hole.

Gemini observations of Wolf-Rayet stars in the Local Group starburst galaxy IC 10

We present Gemini-N GMOS and CFHT MOS spectroscopy of Wolf-Rayet candidates in the Local Group dwarf galaxy IC10 that were previously identified by Massey et al. and Royer et al. From the present spectroscopic survey, the WC/WN ratio for IC10 remains unusually high, given its low metallicity, although none of the WC9 stars suspected from narrow-band imaging are confirmed. Our spectroscopy confirms 9 newly discovered Wolf-Rayet candidates from Royer et al., whilst spectral types of 14 Wolf-Rayet stars previously observed by Massey & Armandroff are refined here. In total, there are 26 spectroscopically confirmed Wolf-Rayet stars in IC~10. All but one of the fourteen WC stars are WC4-6 stars, the exception being # 10 from Massey et al., a broad-lined, apparently single WC7 star. There are a total of eleven WN stars, which are predominantly early WN3-4 stars, but include a rare WN10 star, #8 from Royer et al. #5 from Massey et al. is newly identified as a transition WN/C star. Consequently, the WC/WN ratio for IC10 is 14/11=1.3, unusually high for a metal-poor galaxy. Re-evaluating recent photometric data of Massey & Holmes, we suggest that the true WC/WN ratio may not be as low as 0.3. Finally, we present ground-based finding charts for all confirmed WR stars, plus HST/WFPC2 charts for twelve cases.

A large Wolf-Rayet population in NGC 300 uncovered by VLT-FORS2

We have detected 58 Wolf-Rayet candidates in the central region of the nearby spiral galaxy NGC 300, based on deep VLT-FORS2 narrow-band imaging. Our survey is close to complete except for heavily reddened WR stars. Of the objects in our list, 16 stars were already spectroscopically confirmed as WR stars by Schild & Testor and Breysacher et al., to which 4 stars are added using low resolution FORS2 datasets. The WR population of NGC 300 now totals 60, a threefold increase over previous surveys, with WC/WN > 1/3, in reasonable agreement with Local Group galaxies for a moderately sub-solar metallicity. We also discuss the WR surface density in the central region of NGC 300. Finally, analyses are presented for two apparently single WC stars - #29 (alias WR3, WC5) and #48 (alias WR13, WC4) located close to the nucleus, and at a deprojected radius of 2.5 kpc, respectively. These are among the first models of WR stars in galaxies beyond the Local Group, and are compared with early WC stars in our Galaxy and LMC.

An IRAS-based search for new dusty WCL Wolf-Rayet stars

I have examined all InfraRed Astronomy Satellite (IRAS) data relevant to the 173 galactic Wolf-Rayet (WR) stars in an updated catalogue provided by van der Hucht (priv. comm.), including the 13 stars newly discovered by Shara et al. (1991). Using the exact coordinates given in these lists, I have examined the IRAS Point Source Catalog (PSC), the Faint Source Catalog, Faint Source Reject Catalog, and generated 1-dimensional spatial profiles (“ADDSCAN”s), and 2-dimensional full-resolution images (“FRESCO”s). The goal was to assemble the best set of observed IRAS colour indices for different WR types, in particular for known dusty WCL objects. These colour indices define zones in the IRAS colour-colour plane. By searching the PSC for otherwise unassociated sources that satisfy these colours, I have identified potential new WR candidates, too faint to have been recognized in previous optical searches. I have extracted these candidates' IRAS Low Resolution Spectrometer (LRS) data and compared the spectra with the highly characteristic LRS shape for known dusty WCL stars. Any surviving candidates must now be examined by optical spectroscopy. This work represents a much more rigorous and exhaustive version of the LRS study that identified IRAS 17380-3031 (WR98a) as the first new WR (WC9) star discovered by IRAS (Cohen et al. 1991).

IC 10 - A 'poor cousin' rich in Wolf-Rayet stars

We have obtained narrow-band CCD imaging of the Local Group galaxy IC 10 in order to identify and characterize the Wolf-Rayet star population. We have found sixteen « high significance » Wolf-Rayet candidates and obtained verification spectra of four of them. We infer a WC/WN ratio of ≃0.5 and a surface density of Wolf-Rayet stars in IC 10 considerably larger than in NGC 6822, a galaxy well matched in morphology and composition; the inferred formation rate for massive stars in IC 10 is as high as any other galaxy in the Local Group.