arXiv subjects

M. Heydari-Malayeri

Publications and source records attributed to M. Heydari-Malayeri.

28 records · Page 2Linked to original sources

arXiv2001

STIS spectroscopy of newborn massive stars in SMC N81

Using Hubble Space Telescope observations with STIS, we study the main exciting stars of N81, a high excitation compact Hii region in the Small Magellanic Cloud (SMC). These far UV observations are the first spectroscopic measurements of stars in such a region and reveal features characteristic of an O6-O8 stellar type. The astonishing weakness of their wind profiles and their sub-luminosity (up to ~ 2 mag fainter in Mv than the corresponding dwarfs) make these stars a unique stellar population in the Magellanic Clouds. Our analysis suggests that they are probably in the Hertzsprung-Russell diagram locus of a particularly young class of massive stars, the so-called Vz luminosity class, as they are arriving on the zero age main sequence.

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arXiv2001

The youngest massive star clusters in the Magellanic Clouds

High resolution observations with HST have recently allowed us to resolve and study several very tight clusters of newly born massive stars in the Magellanic Clouds. Situated in an extremely rare category of HII regions, being only 5 to 10 arcsecs across and of high excitation and extinction, these stars are just hatching from their natal molecular clouds. Since the SMC is the most metal-poor galaxy observable with very high angular resolution, this work may provide valuable templates for addressing issues of star formation in the very distant metal-poor galaxies of the early Universe.

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arXiv2001

2 um Narrow-band Adaptive Optics Imaging in the Arches Cluster

Canada-France-Hawaii-Telescope adaptive optics bonnette images through narrow-band filters in the $K-$band are presented for the Arches cluster. Continuum fluxes, line fluxes, and equivalent widths are derived from high angular resolution images, some near diffraction limited, for the well known massive stars in the Arches cluster. Images were obtained in the lines of \ion{He}{1} 2.06 \mic, \ion{H}{1} Br$γ$ (2.17 \mic), and \ion{He}{2} 2.19 \mic as well as continuum positions at 2.03 \mic, 2.14 \mic, and 2.26 \mic. In addition, fluxes are presented for \ion{H}{1} P$α$ (1.87 \mic) and a nearby continuum position (1.90 \mic) from Hubble Space Telescope archival data. The 2 \mic and P$α$ data reveal two new emission-line stars and three fainter candidate emission-line objects. Indications for a spectral change of one object between earlier observations in 1992/1993 and our data from 1999 are found. The ratio of \ion{He}{2} 2.19 \mic to Br$γ$ emission exhibits a narrow distribution among the stars, suggesting a narrow evolutionary spread centered predominantly on spectral types O4 If or Wolf-Rayet stars of the WN7 sub-type. From the approximate spectral types of the identified emission-line stars and comparisons with evolutionary models we infer a cluster age between $\sim$ 2 and 4.5 Myr.

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arXiv2001

HST observations of the LMC compact HII region N11A

We present a study of the LMC compact HII region N11A using Hubble Space Telescope imaging observations which resolve N11A and reveal its unknown nebular and stellar features. The presence of a sharp ionization front extending over more than 4'' (1 pc) and fine structure filaments as well as larger loops indicate an environment typical of massive star formation regions, in agreement with high [OIII]/Hb line ratios. N11A is a young region, as deduced from its morphology, reddening, and especially high local concentration of dust, as indicated by the Balmer decrement map. Our observations also reveal a cluster of stars lying towards the central part of N11A. Five of the stars are packed in an area less than 2'' (0.5 pc), with the most luminous one being a mid O type star. N11A appears to be the most evolved compact HII region in the Magellanic Clouds so far studied.

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arXiv2001

HST study of the LMC compact star forming region N83B

High resolution imaging with the HST uncovers the so far hidden stellar content and the nebular features of the high excitation compact HII region N83B in the Large Magellanic Cloud (LMC). We discover that the HII region is powered by the most recent massive starburst in the OB association LH5 and the burst has created about 20 blue stars spread over ~30" on the sky (7.5 pc). Globally N83B displays a turbulent environment typical of newborn massive star formation sites. It contains an impressive ridge, likely created by a shock and a cavity with an estimated age of only ~30,000 yr, sculpted in the ionized gas by the powerful winds of massive stars. The observations bring to light two compact HII blobs, N83B-1 and N83B-2, and a small arc-nebula, N83B-3, lying inside the larger HII region. N83B-1, only ~2.8" (0.7 pc) across, is the brightest and most excited part of N83B. It harbors the presumably hottest star of the burst and is also strongly affected by dust with an extinction of Av=2.5 mag. The second blob, N83B-2, is even more compact, with a size of only ~1" (0.3 pc). All three features are formed in the border zone between the molecular cloud and the ionized gas possibly in a sequential process triggered by the ionization front of an older HII region. Our HST imaging presents an interesting and rare opportunity to observe details in the morphology of the star formation in very small spatial scales in the LMC which are in agreement with the concept of the fractal structure of molecular star forming clouds. A scenario which supports hierarchical massive star formation in the LMC OB association LH5 is presented.

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arXiv2000

Compact star clusters of the LMC HII region N11C

Based on imaging and spectroscopy obtained at the ESO NTT telescope and using an efficient image analysis algorithm, we study the core of the LMC OB association LH13, particularly the two compact stellar clusters Sk-6641 and HNT in the HII, region N11C. We resolve Sk-6641 into 15 components and for the first time the HNT cluster into 70 stars, and derive photometry for the members. Moreover, from medium resolution spectroscopy we determine the spectral types for sixteen stars in N11C. We compare the color-magnitude diagrams of the clusters with that of the field stars and discuss the cluster ages. With an age of ~100 Myr, the HNT cluster appears significantly older than the very young (< 5 Myr) Sk-6641 starburst. We suggest that most of the `field' O-stars in the core of N11C have actually been ejected from Sk-6641 through dynamical interactions in the compact cluster. The properties of the Sk-6641 and HNT clusters suggest that we are viewing different star formation regions lying at different distances along the same line of sight.

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arXiv1999

The `Papillon' nebula: a compact HII blob in the LMC resolved by HST

We present high spatial resolution HST imaging of the LMC compact HII region N159-5. This high excitation blob is revealed to be a `papillon' or butterfly-shaped ionized nebula with the `wings' separated by ~2.3" (0.6 pc). Two subarcsecond features resembling a `smoke ring' and a `globule' are detected in the wings, the origin of which is briefly discussed. N159-5 may represent a new type of HII region in the Magellanic Clouds overlooked so far because of insufficient spatial resolution. Our images also show a strikingly turbulent medium around the Papillon in the giant HII region N159, which manifests itself by a large number of subarcsecond filaments, arcs, ridges, and fronts carved in the ionized gas by the stellar winds from massive stars in the N159 complex.

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arXiv1999

HST observations of the very young SMC ``blob'' N88A

High-resolution Hubble Space Telescope images have allowed us for the first time to resolve the compact SMC ionized ``blob'' N88A (diameter 3.5 arcsec or 1 pc). This very young HII region, which is hatching from its natal molecular cloud, is heavily affected by absorbing dust associated with the cloud. The interstellar reddening towards N88A is on average Av~1.5 mag and strikingly rises to more than 3.5 mag in a narrow dust band crossing the core of the HII region. Such a high extinction is unprecedented for an HII region in the metal-poor SMC. We present the photometry of some 70 stars lying towards the OB association at the center of which lies N88A. The exciting star(s) of N88A is not detected, due to the heavy extinction. The chronology of star formation is discussed for the whole region.

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arXiv1999

Very young massive stars in the Small Magellanic Cloud, revealed by HST

High spatial resolution imaging with the Hubble Space Telescope allowed us to resolve the compact HII, region N81 lying in the Small Magellanic Cloud (SMC). We show the presence of a tight cluster of newborn massive stars embedded in this nebular ``blob'' of about 10 arcsecs, across. This is the first time the stellar content and internal morphology of such an object is uncovered. These are among the youngest massive stars in this galaxy accessible to direct observations at ultraviolet and optical wavelengths. Six of them are grouped in the core region of about 2 arcsecs diameter, with a pair of the main exciting stars in the very center separated by only 0.27 arcsecs or 0.08 pc. The images display violent phenomena such as stellar winds, shocks, ionization fronts, typical of turbulent starburst regions. Since the SMC is the most metal-poor galaxy observable with very high angular resolution, these observations provide important templates for studying star formation in the very distant metal-poor galaxies which populate the early Universe.

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arXiv1997

The LMC transition star R84 and the core of the LH 39 OB association

On the basis of sub-arcsecond optical imaging we resolve and study the core components of the LMC OB association LH 39. The central star of the association, the rare transition object R84, is also investigated using CASPEC echelle spectroscopy. A new, powerful image restoration code that conserves the fluxes allows us to obtain the magnitudes and colors of the components. We bring out some 30 stars in a ~16"x16", area centered on R84. At a resolution of 0.19", the closest components to R84 are shown to lie at 1.1" NW and 1.7" NW respectively of the transition star. One of these stars is the reddest star of the field after R84 but turns out to be too faint to correspond to the red M2 supergiant previously reported to contaminate the spectrum of R84. If the late-type spectrum is due to a line-of-sight supergiant with a luminosity comparable to R84, it should lie closer than 0.12" to R84. The transition star shows spectral variability between 1982 and 1991. We also note some slight radial velocity variations of the Of emission lines over timescales of several years. Furthermore, we discuss the apparent absence of O type stars in this association.

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