Clarifying the population of HMXBs in the Small Magellanic Cloud

The Astrophysical Journal, 2009

We present the most likely optical counterparts of 113 X-ray sources detected in our Chandra survey of the central region of the Small Magellanic Cloud (SMC) based on the OGLE-II and MCPS catalogs. We estimate that the foreground contamination and chance coincidence probability are minimal for the bright optical counterparts (corresponding to OB type stars; 35 in total). We propose here for the first time 13 High-Mass X-ray Binaries (HMXBs), of which 4 are Be X-ray binaries (Be-XRBs), and we confirm the previous classification of 18 Be-XRBs. We estimate that the new candidate Be-XRBs have an age of ∼ 15 − 85 Myr, consistent with the age of Be stars. We also examine the "overabundance" of Be-XRBs in the SMC fields covered by Chandra, in comparison with the Galaxy. In luminosities down to ∼ 10 34 erg s −1 , we find that SMC Be-XRBs are ∼ 1.5 times more common when compared to the Milky Way even after taking into account the difference in the formation rates of OB stars. This residual excess can be attributed to the lower metallicity of the SMC. Finally, we find that the mixing of Be-XRBs with other than their natal stellar population is not an issue in our comparisons of Be-XRBs and stellar populations in the SMC. Instead, we find indication for variation of the SMC XRB populations on kiloparsec scales, related to local variations of the formation rate of OB stars and slight variation of their age, which results in different relative numbers of Be stars and therefore XRBs.

2009

We present far-infrared spectra, λ=52-93 µm, obtained with the Spitzer Space Telescope in the Spectral Energy Distribution mode of its MIPS instrument, of a representative sample of the most luminous compact far-infrared sources in the Large Magellanic Cloud. These include carbon stars, OH/IR Asymptotic Giant Branch (AGB) stars, post-AGB objects and Planetary Nebulae, the R CrBtype star HV 2671, the OH/IR red supergiants WOH G064 and IRAS 05280−6910, the three B[e] stars IRAS 04530−6916, R 66 and R 126, the Wolf-Rayet star Brey 3a, the Luminous Blue Variable (LBV) R 71, the supernova remnant N 49, a large number of young stellar objects (YSOs), compact H ii regions and molecular cores, and a background galaxy at a redshift z ≃ 0.175. We use the spectra to constrain the presence and temperature of cold dust and the excitation conditions and shocks within the neutral and ionized gas, in the circumstellar environments and interfaces with the surrounding interstellar medium (ISM). First, we introduce a spectral classification scheme. Then, we measure line strengths, dust temperatures, and IR luminosities. Objects associated with star formation are readily distinguished from evolved stars by their cold dust and/or fine-structure lines. Evolved stars, including the LBV R 71, lack cold dust except in some cases where we argue that this is swept-up ISM. This leads to an estimate of the duration of the prolific dust-producing phase ("superwind") of several thousand years for both RSGs and massive AGB stars, with a similar fractional mass loss experienced despite the different masses. We tentatively detect line emission from neutral oxygen in the extreme RSG WOH G064, which suggests a large dust-free cavity with implications for the wind driving. In N 49, the shock between the supernova ejecta and ISM is revealed in spectacular fashion by its strong [O i] λ63-µm emission and possibly water vapour; we estimate that 0.2 M ⊙ of ISM dust was swept up. On the other hand, some of the compact H ii regions display pronounced [O iii] λ88µm emission. The efficiency of photo-electric heating in the interfaces of ionized gas and molecular clouds is estimated at 0.1-0.3%. We confirm earlier indications of a low nitrogen content in the LMC. Evidence for solid state emission features is found in both young and evolved objects, but the carriers of these features remain elusive; some of the YSOs are found to contain crystalline water ice. The spectra constitute a valuable resource for the planning and interpretation of observations with the Herschel Space Observatory and the Stratospheric Observatory For Infrared Astronomy (SOFIA).

The Astronomical Journal, 2008

To ascertain the nature of the brightest compact mid-infrared sources in the Large Magellanic Cloud (LMC), we have applied an updated version of the Buchanan et al. (2006) 2MASS-MSX color classification system, which is based on the results of Spitzer Space Telescope spectroscopy, to a mid-infrared (8 µm) flux-limited sample of 250 LMC objects for which 2MASS and MSX photometry is available. The resulting 2MASS-MSX ("JHK8") color-based classifications of these sources, which constitute the most mid-IR-luminous objects in the LMC, were augmented, cross-checked, and corrected where necessary via a variety of independent means, such that only 47 sources retain tentative classifications and only 10 sources cannot be classified at all. The sample is found to consist primarily of carbon-rich asymptotic giant branch (AGB) stars (∼ 35%), red supergiants (∼18%), and compact H ii regions (∼ 30%), with additional, small populations of oxygen-rich AGB stars (∼ 4%), dusty, early-type emission-line stars (∼ 3%), and foreground, O-rich AGB stars in the Milky Way (∼ 3%). The very large ratio of C-rich to O-rich objects among the luminous and heavily dust-enshrouded AGB stars in our LMC IR source sample is consistent with the hypothesis that

2012

We present new observations of 34 young stellar object (YSO) candidates in the Small Magellanic Cloud (SMC). The photometric selection required sources to be bright at 24 and 70 μm (to exclude evolved stars and galaxies). The anchor of the analysis is a set of Spitzer Infrared Spectrograph (IRS) spectra, supplemented by ground-based 3-5 μm spectra, Spitzer Infrared Array Camera and Multiband Imaging Photometer for Spitzer photometry, near-infrared (IR) imaging and photometry, optical spectroscopy and radio data. The sources' spectral energy distributions and spectral indices are consistent with embedded YSOs; prominent silicate absorption is observed in the spectra of at least 10 sources, silicate emission is observed towards four sources. Polycyclic aromatic hydrocarbon (PAH) emission is detected towards all but two sources. Based on band ratios (in particular the strength of the 11.3-μm and the weakness of the 8.6-μm bands) PAH emission towards SMC YSOs is dominated by predominantly small neutral grains. Ice absorption is observed towards 14 sources in the SMC. The comparison of H 2 O and CO 2 ice column densities for SMC, Large Magellanic Cloud and Galactic samples suggests that there is a significant H 2 O column density threshold for the detection of CO 2 ice. This supports the scenario proposed by Oliveira et al., where the reduced shielding in metal-poor environments depletes the H 2 O column density in the outer regions of the YSO envelopes. No CO ice is detected towards the SMC sources. Emission due to pure rotational 0-0 transitions of molecular hydrogen is detected towards the majority of SMC sources, allowing us to estimate rotational temperatures and H 2 column densities. All but one source are spectroscopically confirmed as SMC YSOs. Based on the presence of ice absorption, silicate emission or absorption and PAH emission, the sources are classified and placed in an evolutionary sequence. Of the 33 YSOs identified in the SMC, 30 sources populate different stages of massive stellar evolution. The presence of ice-and/or silicate-absorption features indicates sources in the early embedded stages; as a source evolves, a compact H II region starts to emerge, and at the later stages the source's IR spectrum is completely dominated by PAH and fine-structure emission. The remaining three sources are classified as intermediate-mass

The Astrophysical Journal, 2007

We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S 3 MC), which imaged the star-forming body of the Small Magellanic Cloud (SMC) in all seven MIPS and IRAC wavebands. We find that the F 8 /F 24 ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 µm brightness and F 24 /F 70 ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S 3 MC images to compile a photometric catalog of ∼ 400, 000 mid-and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: 1) bright 5.8 µm sources with very faint optical counterparts and very red mid-infrared colors ([5.8] − [8.0] > 1.2), which we identify as YSOs. 2) Bright mid-infrared sources with mildly red colors (0.16 [5.8] − [8.0] < 0.6), identified as carbon stars. 3) Bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars. And, 4) unreddened early B stars (B3 to O9) with a large 24 µm excess. This excess is reminiscent of debris disks, and is detected in only a small fraction of these stars ( 5%). The majority of the brightest infrared point sources in the SMC fall into groups one to three. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (∼ 7%).

The Astronomical Journal, 2008

To ascertain the nature of the brightest compact mid-infrared sources in the Large Magellanic Cloud (LMC), we have applied an updated version of the 2MASS-MSX color classification system, which is based on the results of Spitzer Space Telescope spectroscopy, to a mid-infrared (8 µm) flux-limited sample of 250 LMC objects for which 2MASS and MSX photometry is available. The resulting 2MASS-MSX ("JHK8") color-based classifications of these sources, which constitute the most mid-IR-luminous objects in the LMC, were augmented, cross-checked, and corrected where necessary via a variety of independent means, such that only 47 sources retain tentative classifications and only 10 sources cannot be classified at all. The sample is found to consist primarily of carbon-rich asymptotic giant branch (AGB) stars (∼ 35%), red supergiants (∼18%), and compact H ii regions (∼ 30%), with additional, small populations of oxygen-rich AGB stars (∼ 4%), dusty, early-type emission-line stars (∼ 3%), and foreground, O-rich AGB stars in the Milky Way (∼ 3%). The very large ratio of C-rich to O-rich objects among the luminous and heavily dust-enshrouded AGB stars in our LMC IR source sample is consistent with the hypothesis that -2carbon stars form easily in lower metallicity environments. We demonstrate that very luminous C-rich and O-rich AGB stars and red supergiants, identified here primarily on the basis of their JHK8 colors, also appear as distinct clusters in Spitzer IRAC/MIPS color-color diagrams. Thus, in principle, the IRS-based IR photometric classification techniques applied here to the LMC can be applied to any external galaxy whose most luminous IR point sources are detectable and resolvable by 2MASS and Spitzer.

Monthly Notices of the Royal Astronomical Society

The X-ray binary population of the Small Magellanic Cloud (SMC) contains a large number of massive X-ray binaries, and the recent survey of the SMC by XMM-Newton has resulted in almost 50 more tentative high-mass X-ray binary (HMXB) candidates. Using probability parameters from Haberl and Sturm together with the optical spectra and timing in this work, we confirm six new massive X-ray binaries in the SMC. We also report two very probable binary periods of 36.4 d in XMM 1859 and of 72.2 d in XMM 2300. These Be X-ray binaries are likely part of the general SMC population, which rarely undergoes an X-ray outburst.

Astronomy &amp; Astrophysics, 2005

We present a catalogue of high-mass X-ray binaries in the Magellanic Clouds. The catalogue lists source name(s), coordinates, apparent magnitudes, orbital parameters, and X-ray luminosity of 128 high-mass X-ray binaries, together with stellar parameters of the components, other characteristic properties and a comprehensive selection of the relevant literature. The aim of this catalogue is to provide easy access to the basic information on the X-ray sources and their counterparts in other wavelength ranges (UV, optical, IR, radio). Most of the sources are identified to be Be/X-ray binaries. Some sources, however, are only tentatively identified as high-mass X-ray binaries on the basis of a transient character and/or a hard X-ray spectrum. Further identification in other wavelength bands is needed to finally determine the nature of these sources. In cases where there is some doubt about the high-mass nature of the X-ray binary this is mentioned. Literature published before 1 May 2005 has, as far as possible, been taken into account.

Astronomy and Astrophysics, 2001

This paper presents new imagery and spectrophotometric results for the N26 HII region in the Small Magellanic Cloud. The observations using monochromatic images and low-resolution spectra (3700-10 000Å) reveal a compact and complex nebula composed of two cores A and B where A in the region of Hβ is brighter than B by a factor ∼5 and distance of 2. The core A of FWHM ∼ 2. 1 or 0.6 pc presents a high excitation [O III] λλ5007 + 4959/Hβ up to ∼8 and a high reddening E(B − V) ≤ 0.6, while the core B is less excited but has a higher reddening ≥0.8. Each core contains one exciting source; the brighter one should be responsible for the high excitation of A. The apparent spectral type of the two cores ranges from O7 to O9 V and the gas electron density and temperature were derived from the absorption and emission-line intensities. The total mass of the ionized gas is evaluated at 13 M. The chemical abundances of He, O, N, Ne, S, and Ar were computed. These abundances seem consistent with average abundances for SMC HII regions, except N that appears slightly overabundant. N26A-B is comparable to the objects previously observed in the LMC and SMC that we have called "blobs".

Monthly Notices of the Royal Astronomical Society, 1996

We present N-band photometry for a sample of 21 dust-enshrouded AGB stars in the Large Magellanic Cloud, and three additional sources in the Small Magellanic Cloud. Together with near-infrared photometry, this is used to give a tentative classification into carbon and oxygen-rich atmospheres. Bolometric luminosities are also estimated for these stars. In addition, we present the results of a survey for OH masers in the LMC, which resulted in the discovery of OH maser emission from IRAS04407-7000. Spectra between 600 and 1000 nm have been obtained for two heavily obscured AGB stars in the LMC, confirming them to be highly reddened very late M-type giants. Because the dust-enshrouded stars are clearly undergoing heavy mass loss they are assumed to be very near the termination of their respective Asymptotic Giant Branch phases. The fraction of mass-losing carbon stars decreases with increasing luminosity, as expected from Hot Bottom Burning. The best candidate carbon star, with M bol ∼ −6.8 mag, is the most luminous mass-losing carbon star in the Magellanic Clouds, and amongst the most luminous AGB stars. At lower luminosities (M bol ∼ −5 mag) both oxygen and carbon stars are found. This may be explained by a range in metallicity of the individual mass-losing AGB stars.

The Astronomical Journal, 2010

We present far-infrared spectra, λ=52-93 µm, obtained with the Spitzer Space Telescope in the Spectral Energy Distribution mode of its MIPS instrument, of a selection of luminous compact farinfrared sources in the Small Magellanic Cloud. These comprise nine Young Stellar Objects (YSOs), the compact H ii region N 81 and a similar object within N 84, and two red supergiants (RSGs). We use the spectra to constrain the presence and temperature of cool dust and the excitation conditions within the neutral and ionized gas, in the circumstellar environments and interfaces with the surrounding interstellar medium. We compare these results with those obtained in the LMC. The spectra of the sources in N 81 (of which we also show the ISO-LWS spectrum between 50-170 µm) and N 84 both display strong [O i] λ63-µm and [O iii] λ88-µm fine-structure line emission. We attribute these lines to strong shocks and photo-ionized gas, respectively, in a "champagne flow" scenario. The nitrogen content of these two H ii regions is very low, definitely N (N)/N (O) < 0.04 but possibly as low as N (N)/N (O) < 0.01. Overall, the oxygen lines and dust continuum are weaker in star-forming objects in the SMC than in the LMC. We attribute this to the lower metallicity of the SMC compared to that of the LMC. Whilst the dust mass differs in proportion to metallicity, the oxygen mass differs less; both observations can be reconciled with higher densities inside star-forming cloud cores in the SMC than in the LMC. The dust in the YSOs in the SMC is warmer (37-51 K) than in comparable objects in the LMC (32-44 K). We attribute this to the reduced shielding and reduced cooling at the low metallicity of the SMC. On the other hand, the efficiency of the photo-electric effect to heat the gas is found to be indistinguishable to that measured in the same manner in the LMC, ≈ 0.1-0.3%. This may result from higher cloud-core densities, or smaller grains, in the SMC. The dust associated with the two RSGs in our SMC sample is cool, and we argue that it is swept-up interstellar dust, or formed (or grew) within the bow-shock, rather than dust produced in these metal-poor RSGs themselves. Strong emission from crystalline water ice is detected in at least one YSO. The spectra constitute a valuable resource for the planning and interpretation of observations with the Herschel Space Observatory and the Stratospheric Observatory For Infrared Astronomy (SOFIA).

The Astrophysical …, 2009

We present a large sample (20 in total) of optical spectra of Small Magellanic Cloud (SMC) High-Mass X-ray Binaries obtained with the 2dF spectrograph at the Anglo–Australian Telescope. All of these sources are found to be Be/X-ray binaries (Be–XRBs), while for five sources we present original classifications. Several statistical tests on this expanded sample support previous findings for similar spectral-type distributions of Be–XRBs and Be field stars in the SMC, and of Be–XRBs in the Large Magellanic Cloud and the Milky Way, although this could be the result of small samples. On the other hand, we find that Be–XRBs follow a different distribution than Be stars in the Galaxy, also in agreement with previous studies. In addition, we find similar Be spectral-type distributions between the Magellanic Clouds samples. These results reinforce the relation between the orbital period and the equivalent width of the Hα line that holds for Be–XRBs. SMC Be stars have larger Hα equivalent widths when compared to Be–XRBs, supporting the notion of circumstellar disk truncation by the compact object.

The Astrophysical Journal, 2010

The H ii complex N 159 in the Large Magellanic Cloud is used to study massive star formation in different environments, as it contains three giant molecular clouds (GMCs) that have similar sizes and masses but exhibit different intensities of star formation. We identify candidate massive young stellar objects (YSOs) using infrared photometry, and model their spectral energy distributions to constrain mass and evolutionary state. Good fits are obtained for less evolved Type I, I/II, and II sources. Our analysis suggests that there are massive embedded YSOs in N 159B, a maser source, and several ultracompact H ii regions. Massive O-type YSOs are found in GMCs N 159-E and N 159-W, which are associated with ionized gas, i.e., where massive stars formed a few Myr ago. The third GMC, N 159-S, has neither O-type YSOs nor evidence of previous massive star formation. This correlation between current and antecedent formation of massive stars suggests that energy feedback is relevant. We present evidence that N 159-W is forming YSOs spontaneously, while collapse in N 159-E may be triggered. Finally, we compare star formation rates determined from YSO counts with those from integrated Hα and 24 μm luminosities and expected from gas surface densities. Detailed dissection of extragalactic GMCs like the one presented here is key to revealing the physics underlying commonly used star formation scaling laws.

Astronomy & Astrophysics, 2012

Aims. The H ii regions LMC N191 and SMC N77 are among the outermost massive star-forming regions in the Magellanic Clouds. So far, few works have dealt with these objects despite their interesting characteristics. We aim at studying various physical properties of these objects regarding their morphology (in the optical and Spitzer IRAC wavelengths), ionized gas emission, nebular chemical abundances, exciting sources, stellar content, age, presence or absence of young stellar objects, etc. Methods. This study is based mainly on optical ESO NTT observations, both imaging and spectroscopy, coupled with other archive data, notably Spitzer images (IRAC 3.6, 4.5, 5.8, and 8.0 µm) and 2MASS observations. Results. We show the presence of two compact H ii regions, a low-excitation blob (LEB) named LMC N191A and a high-excitation blob (HEB) named SMC N77A, and study their properties and those of their exciting massive stars as far as spectral type and mass are concerned. We also analyze the environmental stellar populations and determine their evolutionary stages. Based on Spitzer IRAC data, we characterize the YSO candidates detected in the direction of these regions. Massive star formation is going on in these young regions with protostars of mass ∼ 10 and 20 M ⊙ in the process of formation.

Monthly Notices of the Royal Astronomical Society, 2001

We report observations which confirm the identities of the optical counterpart to the transient sources RX J0544.1-7100 and RX J0520.5-6932. The counterparts are suggested to be a B-type stars. Optical data from the observations carried out at ESO and SAAO, together with results from the OGLE data base, are presented. In addition, X-ray data from the RXTE all-sky monitor are investigated for long term periodicities. A strong suggestion for a binary period of 24.4d is seen in RX J0520.5-6932 from the OGLE data.