LMC References
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Reionization of Universe (follow this [1] for detailed information)
The ultraviolet (UV) bump at 2175 Ã… is weaker in the LMC than in the MW. Moreover, the far-ultraviolet rise is steeper in the LMC, indicating that grains responsible for this feature could be more abundant. However, the LMC and MW extinction curves are similar in the visible (VIS) and the near-infrared (NIR).
[edit] Parker et al the astronomical journal,116:180-208,1998
Table 1 of UIT Observations of LMC give a comprehensive list of HII regions(by Davis et al 1976) and OB Associations (by Lucke and Hodge 1970), useful for analysis.
B1 (λeff = 1521A,λpeak = 1443A,δλ = 354A)and B5 λeff = 1615A,λpeak = 1518A,δλ = 225A ).
The transmission curve of the B5 filter is similar to that of the B1 filter on the long wavelength side of the peak but does not go to as short wavelengths as the B1 filter, thereby acting as an airglow suppressing filter. The B5 filter was used when observations were made on the dayside of the orbit, and the B1 filter was used for nightside observations.
Objective is to find the O stars, using UV photometry, which discriminates between the temperatures of the hottest stars better than visible photometry but also potentially provides data on hundreds or thousands of stars in the time it takes to get a classification spectrum of a single star.
UV photometry of 37,333 stars, table 2.
Catalog of Magellanic cloud HII regions compiled by Davis, Elliott, and Meaburn 1976 is called DEM regions.
Kennicutt and Hodge 1986 have published H_alpha fluxes for many DEM regions, and Parker et al them with FUV fluxes measured in UIT images over apertures of equal sizes. (section 4).
in Figure 16,they present the plot of the H_alpha flux to FUV observations and modeled fluxes for DEM regions. Following paras are their discussions based on that figure.
We calculated the ratio of the total stellar flux to the integrated aperture flux for our FUV images. The median values of the flux ratios are 0.72 and 0.78 for the B1 and B5 filters, respectively, i.e., about one-quarter of the total FUV flux in a region typically comes from diffuse FUV light. However, note that quite a few regions have flux ratios very close to unity within the errors, indicating very little contribution from diffuse FUV light in those instances. Sources for the diffuse FUV flux could be either scattered light or the light from a population of stars fainter than our PSF photometry limit.
If the light is from faint stars, then the fact that some of the DEM regions have stellar-to-integrated flux ratios close to unity would imply that some regions do not have significant populations of such fainter (lower mass) stars. However, there is no obvious correlation in Figure 16 between brightness of the region and the flux ratio, which argues against the diffuse flux's being due to a constant spatial distribution of faint stars. It also means that the brighter (presumably more active and populous) regions do not have proportionately greater or fewer faint, unresolved stars, e.g., there is not a low-mass cutoff or increase in the star formation activity within the detection limits of the diffuse flux. Cornett et al. (1997) found ratios for the stellar to integrated flux in the SMC somewhat lower than we found in these LMC regions and argued that dust-scattered radiation is a major contributor to the diffuse flux.
[edit] Bernard et al., The Astronomical Journal, 136:919-945, 2008
The article is based on SAGE observations with SPITZER. Relative abundance of dust has been calculated based on comparisons in different observed bands. The authors estimate the dust abundance in LMC to be about 1/3rd of Milky Way and the metallicity to be lower by a factor of about 2. LMC shows excess emission in the 70 micron band suggesting an abundant large grain population.
[edit] Blair et al., The Astrophysical Journal, 538:L61-L64, 2000
The article reports spectroscopic observations of SN Rem N49 in LMC. Strong lines are reported apart from new detections of faint ones. The prominent strong line is the O VI doublet at 1031.9 and 1039.2 AA. Some of the faint lines detected are Ne V (1137, 1146), S VI (933, 944) and Ne VI (999, 1006). The detection of weak lines attests the sensitivity of FUSE for observing faint and diffuse sources.
[edit] Pineda et al., The Astrophysical Journal, 703:736-751, 2009
The authors present CO map of the 30 Dor region of the LMC. The main aim is to estimate the FUV field and study the dependence of CO clouds on this field. They find that that the CO molecular clumps are insensitive to variations in UV field. The authors also find that the average CO to H2 conversion factor in the molecular ridge is approximately twice the value determined from the outer galaxy clouds.
[edit] Brosch et al., THE ASTRONOMICAL JOURNAL, 117:206-224, 1999
HIgh angular resolution HST observations of 198 objests in "general field" of LMC is presented in this paper. The UV emission from all the detected stars comes to be 2.49 x E-7 ergs s-1 cm-1 AA-1 str-1 at 1500 AA. Authors show that the large part of the UV radiation in LMC (at least for >1500 AA) is produced bu field stars and not by objects in clusters or associations. This UV emission accounts for 2/3rd of the UV emission from the field. When correlated with UIT observations, the authors confirm the claim of Parker et al (1998) that LMC field has been forming stars over 1 Gyr.
[edit] Matsuura et al., Mon. Not. R. Astron. Soc. 396, 918-934 (2009)
The authors have analysed the gas and dust budget in the ISM of LMC using SPITZER spectra of AGB stars and SNe. The SFR exceeds the gas feedback from AGB stars and SNe in the LMC, and the current star formation depends on gas already present in the ISM. This suggests that as the gas in the ISM is exhausted, the SFR will eventually decline in the LMC. In the LMC, there is ‘a missing dust source’ problem present. The largest estimate of dust accumulation from AGB stars and SNe cannot account for the full dust mass in the ISM. This is a similar problem as found for high-z galaxies. Additional dust sources are required, possibly located in star-forming regions.
[edit] Bell et al., THE ASTROPHYSICAL JOURNAL, 565:994-1010, 2002
Authors investigate the effects of dust on Large Magellanic Cloud (LMC) H II region spectral energy distributions using arcminute-resolution far-ultraviolet (FUV), Ha, far-infrared (FIR), and radio images. The strong difference between the distributions of the young stars (as probed by the Ha, 1900 AA, or FIR emission) and older stars (as probed by the R band) show that the older stellar populations are strongly concentrated in the bar, with an underlying disklike envelope. In contrast, the younger stars have a clumpy distribution and are predominantly concentrated in the northern half of the LMC. The authors find that the FIR and Ha morphologies are similar and that they differ from the morphology in the UV. This has two interesting implications : First, the assumption used in linking UV to FIR luminosities has broken down. The similarity between the Ha and FIR morphologies implies that there is a good correspondence between the distribution of very young (ionizing) stars an d dust. However, as stars age (so that they are now no longer strongly ionizing stars but still have relatively large UV Ñuxes), the distribution of dust must decouple from that of the stars. Second, the dissimilarity between the Ha and UV morphologies of the LMC stresses a fundamental limitation of this work in its application to the global situation in galaxies. The dichotomy between the sites of Ha and UV emission also leads us to caution against attempts to link the effects of dust on galactic Ha and UV emission too closely : it may be impossible to meaningfully link the Ha and FUV extinction properties of star-forming galaxies in any more than a statistical sense.
[edit] Paradis et al., The Astronomical Journal, 138:196-209, 2009
Using SAGE data, the authors have studied the variations of dust composoition and abundance across the LMC. They show that PAH relative abundance in the LMC is maximum in an extended region corresponding to the old-population stellar bar, a region with no strong current stellar formation activity. This is the first time such an effect is evidenced at Galactic scale. They interpret this finding as an indication that PAHs may have formed efficiently in this region in the past from carbon-rich stars' winds and have remained overabundant in the surrounding ISM. The also show the presence of a region with a significantly increased VSG abundance in the same region, although with a lesser spatial extent. This is likely to reveal that VSGs also formed efficiently from the same process. In agreement with several studies, the VSG abundance distribution appears to follow the star formation activity while that of PAHs could follow more quiescent environments. The region around 30-Doradus shows increased VSG abundance, which could result from the destruction of larger BGs in shocks. Similarly, regions close to the surface of molecular clouds with an increase PAH relative abundance could result from the destruction of VSGs into PAHs by shocks impacting the cloud’s surfaces.
[edit] Cole et al., THE ASTRONOMICAL JOURNAL, 118:2292-2305, 1999
Motivated by new sounding-rocket wide-Ãeld polarimetric images (WISP) of the Large Magellanic Cloud (reported simultaneously by Cole et al.), authors have used a three-dimensional Monte Carlo radiation transfer code to investigate the escape of near-ultraviolet photons from young stellar associations embedded within a disk of dusty material (i.e., a galaxy). As photons propagate through the disk, they may be scattered or absorbed by dust. Scattered photons are polarized and tracked until they escape the dust layer, allowing them to be observed; absorbed photons heat the dust, which radiates isotropically in the far-infrared where the galaxy is optically thin.
Dust-scattered starlight gives rise to linear polarizations ; the magnitude and position angle of the polarization vectors allows to derive information regarding the scattering geometry of stars + dust within the LMC and the optical properties of Magellanic Cloud dust. The authors consider three disk inclinations between 28 deg and 45 deg and five values for the phase function asymmetry parameter g between 0.64 and 0.90.
The conclusions are:
The inclination of the disk of the LMC to the plane of the sky is 36 deg. The most likely value for g of 0.70 ; the uncertainty in the observations does not permit to rule out any value for g. The model predicts that scattered light from 30 Doradus dominates the eastern side of the LMC's diffuse UV radiation Ãeld ; the presence of this "mini-starburst" region may be felt as far south as -72 deh. Data for this region would be of great value in constraining the LMC's structure and geometry.
[edit] Misselt at al., THE ASTROPHYSICAL JOURNAL, 515:128-139, 1999
The UV extinction is analyzed using IUE data. Differences between the average extinction curves of the 30 Dor region and the rest of the LMC are reduced compared with previous studies. Since large variations in UV extinction are seen within both the LMC and the Galaxy, global parameters such as metallicity cannot be directly responsible for the observed variations from galaxy to galaxy as has been suggested (e.g., Clayton & Martin 1985). However, one effect of decreased metallicity in the LMC is that the typical molecular cloud is bigger but more diffuse than those in the Galaxy (Pak et al. 1998). Hence, dust grains in the LMC may not spend as much time in dense, shielded environments as grains in the Galaxy. The lack of time in dense environments may contribute to the apparent small size of the LMC grains as indicated by the low values of R_V measured in this study. In addition, the weak and narrow bump lines of sight in the LMC all lie near the 30 Dor star-forming region, w hich has no analog in the Galaxy. The dust along these sight lines has probably been affected by the proximity to the harsh environment of the copious star formation associated with 30 Dor. The processing history of dust grains (i.e., coagulation and mantling in dense cloud environments and exposure to strong shocks and radiation Ãeld outside of clouds) is probably quite different in these three galaxies owing to the different molecular cloud environments and the varying intensity of star formation. The interplay between at least these two factors likely plays an important role in determining the form the UV extinction. The fact that starburst galaxies appear to have SMC type dust regardless of metallicity (Calzetti et al. 1994; Gordon et al. 1997) implies that the star formation history of a galaxy plays an important role in determining the extinction properties. However, the complicated relationship between extinction properties in the UV and environment implied by the Ga lactic and Magellanic Cloud data suggests that great care must be taken in assuming the form of the UV extinction in external galaxies.
[edit] Vuillemin, A., A&A Supp. Ser., 72, 249-257 (1988)
Near UV image of the LMC has been taken by the author during the second Skylab mission. The mirror coating had a 350 \AA bandpass (FWHM) centered at 2600 \AA. The main result includes integrated fluxes corrected for reddening at 2600 \AA in the Column of table II of the article. The paper also gives the absolute UV magnitude and B and V magnitudes (non-corrected for reddening).
