arXiv subjects

M. Heydari-Malayeri

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

At least 19 recordsLinked to original sources

arXiv2012

Two compact HII regions at the remote outskirts of the Magellanic Clouds

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. 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 microns) and 2MASS observations. 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 about 10 and 20 M_sun in the process of formation.

astro-ph.SR↗

arXiv2012

A Novel Semantic Software for Astronomical Concepts

We have created a new semantic tool called AstroConcepts, providing definitions of astronomical concepts present on Web pages. This tool is a Google Chrome plug-in that interrogates the Etymological Dictionary of Astronomy and Astrophysics, developed at Paris Observatory. Thanks to this tool, if one selects an astronomical concept on a web page, a pop-up window will display the definition of the available English or French terms. Another expected use of this facility could be its implementation in Virtual Observatory services.

astro-ph.IM↗

arXiv2011

An interesting candidate for isolated massive star formation in the Small Magellanic Cloud

The region of the Small Magellanic Cloud (SMC) with which this paper is concerned contains the highest concentration of IRAS/Spitzer sources, H I emission, and molecular clouds in this neighboring galaxy. However very few studies have been devoted to it, despite these signs of star formation. We present the first detailed study of the compact H II region N33 in the SMC by placing it in a wider context of massive star formation. Moreover, we show that N33 is a particularly interesting candidate for isolated massive star formation. This analysis 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 mic) and 2MASS observations. We derive a number of physical characteristics of the compact H II region N33 for the first time. This gas and dust formation of 7".4 (2.2 pc) in diameter is powered by a massive star of spectral type O6.5-O7 V. The compact H II region belongs to a rare class of H II regions in the Magellanic Clouds, called high-excitation blobs (HEBs). We show that this H II region is not related to any star cluster. Specifically, we do not find any traces of clustering around N33 on scales larger than 10" (~ 3 pc). On smaller scales, there is a marginal stellar concentration, the low density of which, below the 3 sigma level, does not classify it as a real cluster. We also verify that N33 is not a member of any large stellar association. Under these circumstances, N33 is also therefore attractive because it represents a remarkable case of isolated massive-star formation in the SMC. Various aspects of the relevance of N33 to the topic of massive-star formation in isolation are discussed.

astro-ph.SR↗

arXiv2010

A very young component in the pre-eminent starburst region of the Small Magellanic Cloud

We present a study of the compact H II region N66A in the SMC pre-eminent starburst region N66/NGC346. Despite extensive research on various components of the N66/NGC346 complex, few studies have so far focused on N66A, which is a special object in the whole complex and therefore deserves scrutiny. The study of this compact H II region and its fellow objects seems important in the framework of massive star formation in the Magellanic Clouds. This analysis is based mainly on our optical ESO NTT observations, both imaging and spectroscopy, coupled with archive HST ACS data and Spitzer infrared images (IRAC 3.6, 4.5, 5.8, and 8.0 microns). We derive a number of physical characteristics of the compact H II region N66A. Moreover, we present the spectral classification of the main exciting star of N66A for the first time using spectroscopy. Its spectral features indicate a main sequence massive star of type O8. We compare this result with that based on the stellar Lyman continuum flux estimated from the ionized gas H-beta flux. The compact H II region belongs to a rare class of H II regions in the Magellanic Clouds, called High-excitation Blobs (HEBs). N66A most probably represents a very young massive star formation event in the N66 complex, which has a range of ages.

astro-ph.SR↗

arXiv2009

VLT/NACO near-infrared imaging and spectroscopy of N88A in the SMC

We present near-infrared imaging and spectroscopic high spatial resolution observations of the SMC region N88 containing the bright, excited, extincted and compact H II region N88A of size ~ 1 pc. To investigate its stellar content and reddening, N88 was observed using spectroscopy and imagery in the JHKs- and L'-band at a spatial resolution of ~ 0.1-0.3", using the VLT UT4 equipped with the NAOS adaptive optics system. In order to attempt to establish if the origin of the infra-red (IR) excess is due to bright nebulosity, circumstellar material and/or local dust, we used Ks vs J-K colour-magnitude (CM) and JHK colour-colour (CC) diagrams, as well as L' imagery.Our IR-data reveal in the N88 area an IR-excess fraction of geq 30 per cent of the detected stars,as well as an unprecedently detailed morphology of N88A. It consists of an embedded cluster of ~3.5" (~ 1 pc) in diameter, of at least thirteen resolved stars superposed with an unusual bright continuum centered on a very bright star. The four brightest stars in this cluster lie red-ward of H-K geq 0.45 mag, and could be classified as young stellar object (YSO) candidates. Four other probable YSO candidates are also detected in N88 along a south-north bow-shaped thin H2 filament at ~ 7" east of the young central bright star. At 0.2" east of this star, a heavily embedded core is detected that could be a massive class I protostar candidate. The 2.12 mu H2 image of N88A resembles a shell of diameter ~ 3" ~ 0.9 pc) centered on the bright star. The line ratios of H2 2-1 S(1) and 1-0 S(0) relative to 1-0 S(1), as well as the presence of high v lines, are indicative of photodissociation regions, rather than shocks.

astro-ph.CO↗

arXiv2009

A Multilingual on-line Dictionary of Astronomical Concepts

On the occasion of the International Year of Astronomy (IYA2009), we present a new interactive dictionary of astronomy and astrophysics, which contains about 7000 entries. This interdisciplinary and multicultural work is intended for professional and amateur astronomers, university students in astrophysics, as well as terminologists and linguists. A new approach is pursued in the formation of a scientific dictionary, which aims to display additional dimensions of astronomical concepts. Although Virtual Observatories recognize the necessity of efforts to define basic astronomical concepts and establish their reciprocal relations, so far they have mainly been confined to archiving observational data. The present dictionary could be an incipient contribution to cover and inter-relate the whole astronomical lexicon beyond subfields.

astro-ph.IM↗

arXiv2008

Spitzer Mid-infrared Study of Compact HII Regions in the Magellanic Clouds

We present a study of the mid-infrared properties and dust content of a sample of 27 HII ``blobs'', a rare class of compact HII regions in the Magellanic Clouds. A unique feature of this sample is that even though these HII regions are of high and low excitation they have nearly the same physical sizes ~1.5-3 pc. We base our analysis on archival 3-8 microns infrared imagery obtained with the Infrared Array Camera (IRAC) on board the Spitzer Space Telescope. We find that despite their youth, sub-solar metallicity and varied degrees of excitation, the mid-infrared colors of these regions are similar to those of typical HII regions. Higher excitation ``blobs'' (HEBs) display stronger 8 micron emission and redder colors than their low-excitation counterparts (LEBs).

astro-ph↗

arXiv2007

The Persian-Toledan Astronomical Connection and the European Renaissance

This paper aims at presenting a brief overview of astronomical exchanges between the Eastern and Western parts of the Islamic world from the 8th to 14th century. These cultural interactions were in fact vaster involving Persian, Indian, Greek, and Chinese traditions. I will particularly focus on some interesting relations between the Persian astronomical heritage and the Andalusian (Spanish) achievements in that period. After a brief introduction dealing mainly with a couple of terminological remarks, I will present a glimpse of the historical context in which Muslim science developed. In Section 3, the origins of Muslim astronomy will be briefly examined. Section 4 will be concerned with Khwarizmi, the Persian astronomer/mathematician who wrote the first major astronomical work in the Muslim world. His influence on later Andalusian astronomy will be looked into in Section 5. Andalusian astronomy flourished in the 11th century, as will be studied in Section 6. Among its major achievements were the Toledan Tables and the Alfonsine Tables, which will be presented in Section 7. The Tables had a major position in European astronomy until the advent of Copernicus in the 16th century. Since Ptolemy's models were not satisfactory, Muslim astronomers tried to improve them, as we will see in Section 8. This Section also shows how Andalusian astronomers took part in this effort, which was necessary in the path to the Scientific Revolution. Finally, Section 9 presents the Spanish influence on the eve of the Renaissance.

astro-ph↗

arXiv2007

HST's view of the youngest massive stars in the Magellanic Clouds

Accurate physical parameters of newborn massive stars are essential ingredients to shed light on their formation, which is still an unsolved problem. The rare class of compact H II regions in the Magellanic Clouds (MCs), termed ``high-excitation blobs'' (HEBs), presents a unique opportunity to acquire this information. These objects (~ 4" to 10", ~ 1 to 3 pc, in diameter) harbor the youngest massive stars of the OB association/molecular cloud complexes in the MCs accessible through high-resolution near-IR and optical techniques. We present a brief overview of the results obtained with HST mainly on two HEBs, one in the LMC (N159-5) and the other in the SMC (N81).

astro-ph↗

arXiv2007

An Etymological Dictionary of Astronomy and Astrophysics: English-French-Persian

This online dictionary presents the definition of classical as well as advanced concepts of modern astronomy. Moreover, each English entry is accompanied by its French and Persian equivalents. The dictionary is intended to be helpful to professional as well as amateur astronomers. As a notable particularity, it also provides a detailed etymology of English and Persian terms. The etymological material contained in this work may interest linguists, in particular those concerned with the evolution of Indo-European languages, especially with that of their Iranian branch. Apart from educational and outreach objectives in the field of astronomy, one of the main aims of this work is to contribute to the Persian language by creating a comprehensive dictionary of astronomy and astrophysics.

astro-ph↗

arXiv2006

Low-excitation blobs in the Magellanic Clouds

Aims : We study an unknown, or very poorly known, interstellar HII component in the Magellanic Clouds. This is the first study ever devoted to this class of objects, which we call Low-excitation blobs (LEBs). Methods : We used low-dispersion spectroscopy carried out at ESO to obtain emission line intensities of Ha, Hb, and [OIII] (4959+5007) for 15 objects in the Large Magellanic Cloud and 14 objects in the Small Magellanic Cloud. Results are displayed in excitation ([oiii]/Hb ratio) versus Hb luminosity diagrams. Results : We show the presence of an LEB component in the Magellanic Clouds and study its relationship with the already known class of high-excitation blobs (HEBs). The newly found LEBs are lower excitation counterparts of HEBs and are powered by less massive exciting stars. Further study of LEBs is expected to provide new pieces of information for a better understanding the low mass end of the upper initial mass function in the Magellanic Clouds.

astro-ph↗

arXiv2004

A concise review of the Iranian calendar

We present a general description of the Iranian calendar and Nowruz, which starts at the precise instant when the Sun coincides with the vernal equinox. Although the paper aims at the basic astronomical elements of the calendar, historical aspects are not ignored. Using the ephemeris provided by the French Institut de Mecanique Celeste et de Calcul des Ephemerides (IMCCE) for the period A.D. -4000 to +2500, we discuss the various definitions of the year and show that the Iranian calendar is based upon the vernal-equinox year and not on the ``tropical year'' of 365.2422 days formulated by researchers in the field of celestial mechanics. Neglecting the short-term fluctuations, the mean length of the Iranian year at the present epoch is 365.2424 solar days, corresponding to the mean interval between two successive passages of the Sun through the vernal equinox. There is now a widely spread confusion between the "tropical" and vernal-equinox years, leading to the erroneous value of 365.2422 days often given for the mean interval between two successive passages of the Sun through the vernal equinox. We also analyze the intercalation system of the Iranian calendar, which is based on a 33-year cycle, and point out the astronomical foundation of this cycle created by the famous Iranian mathematician, astronomer, and poet Omar Khayyam.

astro-ph↗

arXiv2004

Stellar populations associated with the LMC Papillon Nebula

We study the Large Magellanic Cloud Papillon Nebula (N159-5), a conspicuous High Excitation Blob (HEB) lying in the star forming complex N159. Using JHK near-infrared photometry obtained at the ESO VLT with the ISAAC camera, we examine the stellar populations associated with the Papillon, tracing their history using stellar evolution models. Two populations are revealed: one composed of young, massive stars with an age ~ 3 Myr, and a second consisting of older lower mass stars of age spreading between 1 and 10 Gyr. We analyze the properties of those populations and discuss their significance in the context of N159. We also estimate that if the star at the center of the Papillon is single its initial mass is ~ 50 solar masses and it is affected by an extinction Av ~ 7 mag.

astro-ph↗

arXiv2004

Puzzling wind properties of young massive stars in SMC-N81

We present a quantitative study of the stellar and wind properties of massive stars in the compact star forming region SMC-N81. Non-LTE spherically extended atmosphere models including line-blanketing have been computed with the code CMFGEN (Hillier & Miller 1998) and the analysis of UV STIS spectra lead to the following results: 1) The SMC-N81 components are young (~ 0-4 Myrs) O stars with Teff compatible with medium to late subtypes and with luminosities lower than average Galactic O dwarfs, rendering them possible ZAMS candidates; 2) The winds are extremely weak: the mass loss rates (~1e-8/1e-9 Msol/yr) and the modified wind momenta are 1 to 2 orders of magnitude lower than observed so far for Galactic stars, and are lower than the predictions of the most recent hydrodynamical models. The reasons for such weak winds are investigated with special emphasis on the modified wind momenta: 1) There may be a break-down of the wind momentum - luminosity relation (WLR) for dwarf stars at low luminosity (log L/Lsol < 5.5). However, reasons for such a breakdown remain unknown; 2) The slope of the WLR may be steeper at low metallicity. This is predicted by the radiation driven wind theory, but the current hydrodynamical simulations do not show any change of the slope at SMC metallicity. Moreover, there are indications that some Galactic objects have wind momenta similar to those of the SMC stars; 3) Decoupling in the atmosphere may lead to multicomponent winds, but various tests indicate that this is not likely to be the case for the SMC-N81 stars. Hence, the origin of the weakness of the wind observed in these objects remains unknown. We suggest that this weakness may be linked with the youth of these stars and represents possibly the onset of stellar winds in recently formed massive stars.

astro-ph↗

arXiv2003

The stellar environment of SMC N81

We present near infrared JHK imaging of the Small Magellanic Cloud compact H II region N81 using the ISAAC camera at the ESO Very Large Telescope (Antu). Our analysis of the stellar environment of this young massive star region reveals the presence of three new stellar populations in the surrounding field which are mainly composed of low mass stars. The main population is best fitted by evolutionary models for about 2 solar mass stars with an age of 1 Gyr. We argue that these populations are not physically associated with the H II region N81. Instead they are the result of a number of low mass star forming events through the depth of the SMC south of its Shapley's wing. The populations can rather easily be probed due to the low interstellar extinction in that direction.

astro-ph↗

arXiv2003

Spectroscopic analysis of newborn massive stars in SMC N81

We present the first results of a spectroscopic study of young massive stars in the SMC High Excitation Blob N81. These stars have Mv values which are ~ 2 mag smaller than those of nor mal dwarf stars. Their UV STIS spectra reveal features typical of O stars, but surprisingly weak wind components. The preliminary modelling of these spectr a with the code CMFGEN (Hillier & Miller 1998) indicates a mass loss rate of the order 10e-9 Mdot/yr. If confirmed, such a weak wind may indicate either a breakdown of the wind-momentum luminosity relation at low luminosity, or a steeper slope of this relation at low metallicity.

astro-ph↗

arXiv2003

Tight LMC massive star clusters R 127 and R 128

We study the Large Magellanic Cloud (LMC) star clusters R 127 and R 128 using imaging and spectroscopy obtained at the ESO NTT telescope. An advanced image restoration technique allows us to resolve these two clusters into at least 14 and 33 stars respectively and obtain their photometry. In particular, we show that the core of R 127 is composed of at least four stars and identify the Luminous Blue Variable (LBV) component. The closest neighbor of the LBV (star #8) is 1".5 away. Moreover, from medium dispersion spectroscopy we determine the spectral types for 19 stars in and near both clusters, and in particular present the first spatially resolved observation of the second brightest component of the R 127 cluster (star #3) situated 3".3 from the LBV. By comparing with evolutionary models we also look into the stellar ages. The oldest stars of the cluster are ~ 6-8 Myr old, whereas the most massive star of the region (#7), formed ~ 3 Myr ago as an 80 solar mass star, has turned into an LBV, the ``R 127'' star.

astro-ph↗

arXiv2001

Resolving the compact HII regions in N160A with HST

Using high-resolution imaging with the Hubble Space Telescope, we study the Large Magellanic Cloud HII region N160A and uncover several striking features of this complex massive star-forming site. The two compact high excitation HII blobs (HEBs) A1 and A2 are for the first time resolved and their stellar content and morphology is revealed. A1, being of higher excitation, is powered by a single massive star whose strong wind has created a surrounding bubble. A2 harbors several exciting stars enshrouded inside large quantities of dust. The whole N160A nebula is energized by three star clusters for which we obtain photometry and study their color-magnitude diagram. The HII region is particularly dusty, with extinction values reaching an A_v~2.5 mag in the visible, and it is separated from the molecular cloud by an outstanding ionization front. A previously detected infrared young stellar object is also accurately located with respect to the HII region.

astro-ph↗