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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

WFPC2 Observations of Star Clusters in the Magellanic Clouds: The Oldest Star Clusters in the Small Magellanic Cloud - Report 2

We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) observations in F45OW ( approximately B) and F555W (approximately V) of the intermediate-age populous star clusters NGC 121, NGC 339, NGC 361, NGC 416, and Kron 3 in the Small Magellanic Cloud. We use published photometry of two other SMC populous star clusters, Lindsay 1 and Lindsay 113, to investigate the age sequence of these seven populous star clusters in order to improve our understanding of the formation chronology of the SMC. We analyzed the V vs B-V and M(sub V) vs (B-V)(sub 0) color-magnitude diagrams of these populous Small Magellanic Cloud star clusters using a variety of techniques and determined their ages, metallicities, and reddenings. These new data enable us to improve the age-metallicity relation of star clusters in the Small Magellanic Cloud. In particular, we find that a closed-box continuous star-formation model does not reproduce the age-metallicity relation adequately. However, a theoretical model punctuated by bursts of star formation is in better agreement with the observational data presented herein.

Mighell, Kenneth J.↗
NASA NTRS1985

Ultraviolet interstellar absorption toward stars in the Small Magellanic Cloud. III - The structure and kinematics of the Small Magellanic Cloud

The structural and kinematic properties of the Small Magellanic Cloud (SMC) were studied by combining IUE satellite observations of individual bright stars (Sk 159 and HD 5980) with existing optical and radio data. It is shown that the SMC has a complex structure due to gravitational interaction with the Milky Way and the Large Magellanic Cloud (LMC). Stars observed in the southwest regions of the SMC were associated with the 134 km/s H I complex. H II regions in the SMC were distributed according to a rotational velocity of 167 km/s. All IUE high-dispersion targets exhibit ultraviolet absorption near 100 km/s and 200 km/s. It is shown that the absorption arises in the widespread H I complexes at velocities similar to the H II regions. Some possible explanations for the origin of H II and H I regions in the SMC are discussed. The locations of the nine high-dispersion IUE targets in the SMC are given.

Fitzpatrick, E. L.↗
NASA NTRS1971

Cepheid variables and the Magellanic Clouds.

The Magellanic Cloud variables differ from variables in the Galaxy by the number of variables at different periods, by the period-amplitude relation, and by the mean color. Schematic evolutionary tracks are used to demonstrate how a proposed difference between Galactic evolution and Small Magellanic Cloud evolution might be responsible for the observed mean colors and frequencies of occurrence of cepheid variables with different periods. The frequency of short-period cep heids is seen to be a very sensitive indicator of some, not yet fully understood, aspect of abundances. It is demonstrated that the characteristic second-bump feature appearing in the light curves of cepheids with the period range from 7 to 15 days can be used to determine stellar mass and radius on the basis of calculations of interior motions.-

Christy, R. F.↗
NASA NTRS1982

Tidal stability of gas clouds in the Large Magellanic Cloud and M101

The tidal stability of the giant atomic gas clouds in the Large Magellanic Cloud (LMC) and in M101 is examined. The giant atomic clouds in the Large Magellanic Cloud are found to be unstable against tidal disruption by the gravity of the LMC, but the clouds in M101 are in approximate tidal balance. It is unlikely that there is sufficient unobserved molecular gas to stabilize the LMC clouds, although they may have substantial molecular cores which are tidally stable. The time scale for tidal disruption of the LMC clouds is of the order of 20-30 million years. Because most of the clouds have associated H II regions, the onset of star formation in these clouds must occur rapidly. In order to balance the loss due to tidal disruption, the clouds would have to be forming at a rate of approximately 0.09 solar masses/year per sq kpc, a value comparable to that derived for the Milky Way.

Blitz, L.↗
NASA NTRS1993

The distance to the Large Magellanic Cloud - Constraints from Cepheids in Large Magellanic Cloud star clusters

We have used recent observational data for the Cepheids in the rich, young LMC clusters NGC 1866 and NGC 2031 to constrain the cluster distances with the mass-equivalency (ME) method. The basis of this approach is to fix the cluster distances by requiring the Cepheid evolutionary and pulsational masses to be equal. Using evolutionary models incorporating a mild amount of core and envelope overshooting along with recent pulsational models, we derive distance moduli of 18.51 +/- 0.21 and 18.32 +/- 0.20 for NGC 1866 and NGC 2031, respectively. The quoted errors are dominated by the uncertainties in the heavy element abundances of the clusters (assumed to be 0.3 dex for both clusters), with a smaller contribution due to the apparently intrinsic spread in the masses of the Cepheids in each cluster. For the ME method, we find that Delta(m-M)(0)/Delta sub Z(1) = 0.69, where Z(1) = log (Z/0.016). This result implies that the cluster distances can be determined to better than +/- 5 percent if the cluster abundances can be measured to better than about +/- 0.15 dex.

Bertelli, Gianpaolo↗
NASA NTRS1985

Ultraviolet interstellar absorption toward stars in the Small Magellanic Cloud. IV - Highly ionized gas associated with the Small Magellanic Cloud

High-dispersion International Ultraviolet Explorer satellite spectra of seven stars in the Small Magellanic Cloud (SMC) are examined to study the properties of interstellar C IV and Si IV absorption in the SMC. Absorption by C IV or Si IV or both is found near 160 km/s for all the stars. The velocity and the relative C IV and Si IV strengths suggest UV-photoionized nebular gas as the origin of this absorption. In addition, the stars show absorption by C IV and, sometimes, Si IV in the velocity range 100-130 km/s. This velocity is 30-60 km/s more negative than that expected for normal nebular gas, and the relative C IV and Si IV strengths indicate an ionization source other than stellar UV photoionization by normal Population I stars. Possible global origins are considered for this absorption, including a hot phase of the SMC interstellar medium and a circum-SMC distribution of highly ionized gas. The only detection of interstellar N V toward a SMC star is for HD 5980. The line is broad, possibly complex, and spans the velocity range of the nebular absorption and the 100-130 km/s absorption.

Fitzpatrick, E. L.↗
NASA NTRS2019

Complex Organic Molecules in Star-Forming Regions of the Magellanic Clouds

The Large and Small Magellanic Clouds (LMC and SMC), gas-rich dwarf companions of the Milky Way, are the nearest laboratories for detailed studies on the formation and survival of complex organic molecules (COMs) under metal poor conditions. To date, only methanol, methyl formate, and dimethyl ether have been detected in these galaxies – all three toward two hot cores in the N113 star-forming region in the LMC, the only extragalactic sources exhibiting complex hot core chemistry. We describe a small and diverse sample of the LMC and SMC sources associated with COMs or hot core chemistry, and compare the observations to theoretical model predictions. Theoretical models accounting for the physical conditions and metallicity of hot molecular cores in the Magellanic Clouds have been able to broadly account for the existing observations, but fail to reproduce the dimethyl ether abundance by more than an order of magnitude. We discuss future prospects for research in the field of complex chemistry in the low-metallicity environment. The detection of COMs in the Magellanic Clouds has important implications for astrobiology. The metallicity of the Magellanic Clouds is similar to galaxies in the earlier epochs of the Universe, thus the presence of COMs in the LMC and SMC indicates that a similar prebiotic chemistry leading to the emergence of life, as it happened on Earth, is possible in low-metallicity systems in the earlier Universe.

Magellanic Clouds, star formation, astrochemistry,↗
NASA NTRS1992

Interstellar dust from the Milky Way to the Magellanic Clouds

Interstellar dust in the Magellanic Clouds, with a weak or nearly absent 2175 A extinction feature, may be of interest in studies of galaxies in early stages of chemical evolution. To this inquiry, we extend the graphite-silicate grain model, introduced by Mathis, Rumpl, and Nordsieck and developed by Draine and Lee (1984) from the Milky Way to the Magellanic Clouds. We find that the empirical extinction curves in the Large and Small Magellanic Clouds can be reproduced by adjusting only the relative abundances of graphite and silicate grains, while leaving all other model properties fixed to those appropriate for the Galactic extinction curve. Using the graphite-silicate models, we calculate the absorption and scattering optical depths, the mass-density ratio of interstellar dust to neutral hydrogen, and the Kramers-Kronig relation for all three galaxies. We also present a fitting function for the three extinction curves, valid not only over the observed range of wavelengths but also over the full range as predicted by the models. All the quantities we derived here are independent of the dust-to-gas ratios in the Milky Way and the Magellanic Clouds, and can be applied to other galaxies if they contain Galactic or Magellanic-type dust.

Pei, Yichuan C.↗
NASA NTRS2019

Complex Organic Molecules in the Star-Forming Regions of the Magellanic Clouds

The Large and Small Magellanic Clouds (LMC and SMC), gas-rich dwarf companions of the Milky Way, are the nearest laboratories for detailed studies on the formation and survival of complex organic molecules (COMs) under metal-poor conditions. To date, only methanol, methyl formate, and dimethyl ether have been detected in these galaxiesall three toward two hot cores in the N113 star-forming region in the LMC, the only extragalactic sources exhibiting complex hot-core chemistry. We describe a small and diverse sample of the LMC and SMC sources associated with COMs or hot-core chemistry, and compare the observations to theoretical model predictions. Theoretical models accounting for the physical conditions and metallicity of hot molecular cores in the Magellanic Clouds have been able to broadly account for the existing observations, but they fail to reproduce the dimethyl ether abundance by more than an order of magnitude. We discuss future prospects for research in the eld of complex chemistry in the low- metallicity environment. The detection of COMs in the Magellanic Clouds has important implications for astrobiology. The metallicity of the Magellanic Clouds is similar to that of galaxies in the earlier epochs of the universe; thus, the presence of COMs in the LMC and SMC indicates that a similar prebiotic chemistry leading to the emergence of life, as it happened on Earth, is possible in low-metallicity systems in the earlier universe.

Sewilo, Marta M.↗
NASA NTRS1976

Apollo 16 far-ultraviolet imagery and spectra of the Large Magellanic Cloud

The Large Magellanic Cloud was observed by the far ultraviolet camera spectrograph from the lunar surface during the Apollo 16 mission 22 April 1972. Images were obtained with about 3 arc min resolution, in the 1,050 to 1,600 and 1,250 to 1,600 A wavelength ranges, of nearly the entire LMC. Spectra were also obtained in the 1,050 to 1,600 and 900 to 1,600 A ranges along a strip 1/4 deg wide (determined by the instrument's grid collimator) passing across the LMC. The images and spectra have been scanned with a PDS microdensitometer, and isodensity contour plots have been prepared using the Univac 1108 computer.

Page, T.↗
NASA NTRS1977

Apollo 16 far-ultraviolet imagery and spectra of the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) was observed with a far-ultraviolet camera/spectrograph (Experiment S-201) from the lunar surface during the Apollo 16 mission. Images were obtained with about 3-arcmin resolution, in the 1050-1600- and 1250-1600-A wavelengths ranges, of nearly the entire LMC. Spectra were also obtained in the 1050-1600- and 900-1600-A ranges with 30-40-A resolution along a strip 0.25 deg wide passing across the LMC. The images and spectra have been scanned with a microdensitometer, and analyses of the data to date are discussed.

Page, T. L.↗
NASA NTRS1984

Far IR Observations of the Magellanic Clouds

Several Large Magellanic and Small Magellanic Cloud H II regions were observed at 50 and 100 microns. Observations were made on three flights of the Kuiper Airborne Observatory using a six channel detector system. Fully sampled maps were made simultaneously at both wavelengths. The integrated properties of the H II regions are listed and interpretations of the results regarding stellar formation are presented.

Jones, T. J.↗
NASA NTRS2000

Hot Gas in the Large Magellanic Cloud

The Large Magellanic Cloud (LMC) offers an excellent laboratory to study the physical structure of the interstellar medium (ISM) because of its proximity, nearly face-on orientation, and small foreground and internal extinction. Optical and radio surveys of the LMC ISM have revealed interstellar structures of sizes ranging from a few parsecs to over 1000 parsecs. ROSAT X-ray mosaics of the LMC have detected abundant 10 (exp 6) K hot gas, some of which is bounded by large shell structures while the rest, does not appear to be associated with any visible interstellar structure. The X-ray observations have been analyzed to determine the physical conditions of the hot gas. The distribution of the hot gas can be compared to those of the cooler gas and massive stars, in order to determine the production mechanism of the hot gas. UV observations of interstellar absorption lines of high ions, such as C IV, N V, and O VI, can be used to study the interfaces between the 10 (exp 6) K gas and cooler ionized gas, and to provide constraints on the location of 10 (exp 6) K gas with respect to the cooler gas along the line of sight.

Chu, You-Hua↗
NASA NTRS2022

Physical and Chemical Conditions in the N113 Star-Forming Region in the Low-Metallicity Large Magellanic Cloud

The Large Magellanic Cloud (LMC) is the nearest (~50 kpc) star-forming galaxy characterized by a low metallicity (Z~0.3-0.5 Z⨀) similar to galaxies during the early phases of their assembly. As a result, star formation studies in the LMC provide a stepping stone to understanding star formation at earlier epochs of the universe where these processes cannot be directly observed. N113 is one of the most prominent star-forming regions in the LMC hosting one of the most massive giant molecular clouds.N113 is small enough to be imaged in its entirety, but large enough to showcase many important phenomena such as multiple generations of stars, stellar feedback, and different environments. We present our findings from an investigation of the early stages of star formation in the N113 region using the Atacama Large Millimeter/submillimeter Array (ALMA)molecular line data probing a wide density range: 12CO, 13CO, and C18O (2-1), 13COand C18O (1-0), HCN (1-0), HCO+ (1-0), H13CN (1-0) and (3-2), H13CO+ (1-0) and (3-2), CS (2-1) and (5-4), as well as 1.3 mm and 3 mm continuum. We used the Python package quick clump to identify molecular clumps. We utilized the multiline non-LTE fitting tool based on models from RADEX developed by Finn et al. (2021, ApJ, 917, 106)to construct the CO, HCN, HCO+, and CS temperature and column density, and the H2density maps of N113. We constructed a catalog of molecular clumps including their physical properties, chemical abundances, sizes, velocities, and velocity dispersions. To establish the evolutionary status of the clumps, their positions were compared with previously identified young stellar objects (YSOs) from the Spitzer/SAGE and Herschel/HERITAGE surveys, as well as water and OH masers. We compared the properties of the clumps in N113 to those in the Galaxy and other regions in the LMC to assess the impact of the environment (e.g., metallicity, stellar feedback) on the star formation process.

Jonathon Noswitz↗
NASA NTRS1987

Star formation in the Magellanic clouds

Because of their proximity, the Magellanic Clouds provide the opportunity to conduct a detailed study of the history and current state of star formation in dwarf irregular galaxies. There is considerable evidence that star formation in the Clouds was and is proceeding in a manner different from that found in a typical well-ordered spiral galaxy. Star formation in both Clouds appears to have undergone a number of relatively intense bursts. There exist a number of similarities and differences in the current state of star formation in the Magellanic Clouds and the Milky Way. Examination of Infrared Astronomy Satellite (IRAS) sources with ground based telescopes allows identification of highly evolved massive stars with circumstellar shells as well as several types of compact emission line objects.

Frogel, Jay A.↗
NASA NTRS1984

Ages of intermediate-age Magellanic Cloud star clusters

Ages of intermediate-age Large Magellanic Cloud star clusters have been estimated without locating the faint, unevolved portion of cluster main sequences. Six clusters with established color-magnitude diagrams were selected for study: SL 868, NGC 1783, NGC 1868, NGC 2121, NGC 2209, and NGC 2231. Since red giant photometry is more accurate than the necessarily fainter main-sequence photometry, the distributions of red giants on the cluster color-magnitude diagrams were compared to a grid of 33 stellar evolutionary tracks, evolved from the main sequence through core-helium exhaustion, spanning the expected mass and metallicity range for Magellanic Cloud cluster red giants. The time-dependent behavior of the luminosity of the model red giants was used to estimate cluster ages from the observed cluster red giant luminosities. Except for the possibility of SL 868 being an old globular cluster, all clusters studied were found to have ages less than 10 to the 9th yr. It is concluded that there is currently no substantial evidence for a major cluster population of large, populous clusters greater than 10 to the 9th yr old in the Large Magellanic Cloud.

Flower, P. J.↗
NASA NTRS1992

Ionized carbon (CII) in the Magellanic Clouds

We present preliminary results of recently obtained observations of the important cooling line of singly ionized carbon in the Magellanic Clouds. Three large Magellanic Cloud (LMC) and six HII region/CO cloud complexes have been detected and mapped. Comparison shows that the strength and distribution of (CII) regions is well-correlated with those of dust complexes emitting far-infrared radiation, but not with molecular cloud complexes traced by CO emission. The results confirm that photo-dissociation processes are of relatively greater importance in Magellanic Cloud star formation regions than in Galactic regions.

Israel, F. P.↗
NASA NTRS1999

A Reconnaissance of the 900-1200 A Spectra of Early O Stars in the Magellanic Clouds

Far-UV spectrograms of ten very hot stars in the Magellanic Clouds were obtained with the Hopkins Ultraviolet Telescope during the Astro-2 mission in March 1995. Very few normal OB stars were observed during Astro-1, and the only significant prior sample of such objects in this wavelength range was that obtained by Copernicus of stars near the Sun. The Magellanic Cloud sample offers the advantages of low reddening, a range in metallicity, and more extreme temperatures and luminosities than represented in the near solar neighborhood. Several interesting phenomena were found in the HUT MC sample, including very strong O VI wind profiles in O3 spectra; anomalous CNO wind features corresponding to abundances altered by stellar nucleosynthesis; weak wind features due to the metal deficiency of the Small Magellanic Cloud; and interstellar molecular hydrogen features originating in the MCs. The results were presented and discussed as a small atlas, and they were shown at the Pittsburgh AAS meeting in June 1995. A correlative atlas of Copernicus data was also prepared and published under the auspices of this grant. These publications will serve as resources for the characterization of the FUSE observations of related objects to hopefully be obtained soon. I believe that the large differences among the strong wind features in the HUT data, corresponding to CNO and systemic metallicity effects, are amenable to some relative quantitative analysis, and I shall endeavor to promote such in conjunction with HST data at longer wavelengths for the same stars (which has also not yet been completely analyzed) at a future opportunity.

Walborn, Nolan↗