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The oxygen abundances in HII regions of the spiral galaxy M101 from the Sloan Digital Sky Survey spectra.
Not Available

An empirical calibration of sulphur abundance in ionised gaseous nebulae
We have derived an empirical calibration of the abundance of S/H as afunction of the S{23} parameter, defined using the bright sulphur linesof [SII] and [SIII]. Contrary to the case for the widely used O{23}parameter, the calibration remains single valued up to the abundancevalues observed in the disk HII regions. The calibration is based on alarge sample of nebulae for which direct determinations of electrontemperatures exist and the sulphur chemical abundances can be directlyderived. ICFs, as derived from the [SIV] 10.52 μ emission line (ISOobservations), are shown to be well reproduced by Barker's formula for avalue of α = 2.5. Only about 30% of the objects in the samplerequire ICFs larger than 1.2. The use of the proposed calibration opensthe possibility of performing abundance analysis with red to IRspectroscopic data using S/H as a metallicity tracer.

Discovery of X-Ray Emission from Supernova 1970G with Chandra: Filling the Void between Supernovae and Supernova Remnants
We report the discovery of X-ray emission from SN 1970G in M101, 35 yrafter its outburst, using deep X-ray imaging with the Chandra X-RayObservatory. The Chandra ACIS spectrum shows that the emission is soft(<~2 keV) and characteristic of the reverse-shock region. The X-rayluminosity, L0.3-2=(1.1+/-0.2)×1037 ergss-1, is likely caused by the interaction of the supernovashock with dense circumstellar matter. If the material was deposited bythe stellar wind from the progenitor, a mass-loss rate ofM˙=(2.6+/-0.4)×10-5 Msolaryr-1(vw/10 km s-1) is inferred.Utilizing the high-resolution Chandra ACIS data of SN 1970G and itsenvironment, we reconstruct the X-ray lightcurve from previous ROSATHRI, PSPC, and XMM-Newton EPIC observations, and find a best-fit linearrate of decline of L~t-s with index s=2.7+/-0.9 over a periodof ~20-35 yr after the outburst. As the oldest supernova detected inX-rays, SN 1970G allows, for the first time, direct observation of thetransition from a supernova to its supernova remnant phase.

Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II Regions
We present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1.

An XMM-Newton view of M101 - II. Global X-ray source properties
We present the global X-ray properties of the point source population inthe grand-design spiral galaxy M101, as seen with XMM-Newton. 108 X-raysources are detected within the D25 ellipse of M101, of which~24 are estimated to be background galaxies. Multiwavelengthcross-correlations show that 20 sources are coincident with HII regionsand/or supernova remnants (SNRs), seven have identified/candidatebackground galaxy counterparts, six are coincident with foreground starsand one has a radio counterpart. While the spectral and timingproperties of the brightest sources were presented by Jenkins et al.,here we apply an X-ray colour classification scheme to split the entiresource population into different types, i.e. X-ray binaries (XRBs),SNRs, absorbed sources, background sources and supersoft sources (SSSs).Approximately 60 per cent of the population can be classified as XRBs,although there is source contamination from background active galacticnuclei (AGN) in this category as they have similar spectral shapes inthe X-ray regime. 15 sources have X-ray colours consistent with SNRs,three of which correlate with known SNR/HII radio sources. Another twoare promising new candidates for SNRs, one is unidentified, and theremainder are a mixture of foreground stars, bright soft XRBs and AGNcandidates. We also detect 14 candidate SSSs, with significantdetections in the softest X-ray band (0.3-1keV) only. 16 sources displayshort-term variability during the XMM-Newton observation, twelve ofwhich fall into the XRB category, giving additional evidence of theiraccreting nature. Using archival Chandra and ROSAT High ResolutionImager data, we find that ~40 per cent of the XMM sources show long-termvariability over a baseline of up to ~10 yr, and eight sources displaypotential transient behaviour between observations. Sources withsignificant flux variations between the XMM and Chandra observationsshow a mixture of softening and hardening with increasing luminosity.The spectral and timing properties of the sources coincident with M101confirm that its X-ray source population is dominated by accreting XRBs.

The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale
We use high signal-to-noise ratio spectra of 20 H II regions in thegiant spiral galaxy M101 to derive electron temperatures for the H IIregions and robust metal abundances over radii R=0.19-1.25R0(6-41 kpc). We compare the consistency of electron temperatures measuredfrom the [O III] λ4363, [N II] λ5755, [S III]λ6312, and [O II] λ7325 auroral lines. Temperatures from[O III], [S III], and [N II] are correlated with relative offsets thatare consistent with expectations from nebular photoionization models.However, the temperatures derived from the [O II] λ7325 line showa large scatter and are nearly uncorrelated with temperatures derivedfrom other ions. We tentatively attribute this result to observationaland physical effects, which may introduce large random and systematicerrors into abundances derived solely from [O II] temperatures. Ourderived oxygen abundances are well fitted by an exponential distributionover six disk scale lengths, from approximately 1.3(O/H)solar in the center to 1/15 (O/H)solar in theoutermost region studied [for solar 12+log(O/H)=8.7]. We measuresignificant radial gradients in N/O and He/H abundance ratios, butrelatively constant S/O and Ar/O. Our results are in approximateagreement with previously published abundances studies of M101 based ontemperature measurements of a few H II regions. However, our abundancesare systematically lower by 0.2-0.5 dex than those derived from the mostwidely used strong-line ``empirical'' abundance indicators, againconsistent with previous studies based on smaller H II region samples.Independent measurements of the Galactic interstellar oxygen abundancefrom ultraviolet absorption lines are in good agreement with theTe-based nebular abundances. We suspect that most of thedisagreement with the strong-line abundances arises from uncertaintiesin the nebular models that are used to calibrate the ``empirical''scale, and that strong-line abundances derived for H II regions andemission-line galaxies are as much as a factor of 2 higher than theactual oxygen abundances. However, other explanations, such as theeffects of temperature fluctuations on the auroral line basedabundances, cannot be completely ruled out. These results point to theneed for direct abundance determinations of a larger sample ofextragalactic H II regions, especially for objects more metal-rich thansolar.

A Metal-poor Compact H II Region with a Small, Isolated, and Dense Young Stellar Cluster
IRAS 04000+5052 appears to be a single, somewhat extended red source onoptical CCD images. From our infrared imaging observations, an infraredcluster consisting of 15 objects is discovered in this region. In thispaper, by discussing and analyzing the observational results ofnear-infrared imaging, optical spectroscopy, and the 12COmolecular line, together with IRAS and NRAO VLA Sky Survey (NVSS) data,we conclude that IRAS 04000+5052 is a compact H II region with a veryyoung dense stellar cluster. The spectrum of the compact H II regionshows that it is a metal-poor nebula, which has the lowest line ratio of[N II]/Hα (~1/16.3) discovered in the H II regions of our Galaxyuntil now. There are three possible explanations. The first is a HerbigBe star model, the second suggests that the material constituting thisnebula could come from some nearby external galaxy from accretion ofgalaxies, and the third suggests that the material was produced in theprimeval phase of our Galaxy and has not been chemically contaminatedmuch. There exists an undetected gap between K=13.6 and the detectionlimit of 16.2 mag. We give two possible interpretations for thisundetected gap; one is based on the star formation itself, and the otheris a possible observational effect. This infrared cluster is a verysmall (0.83pc[north-south]×1.55pc[east-west]) and isolatedhigh-mass protostellar cluster, in which there is a B0.5-type starexciting the compact H II region. The derived heliocentric distance(~4.29 kpc) of IRAS 04000+5052 is twice as large as that of the Perseusarm. It is located far beyond the Perseus arm at the direction(l=150.86).

New light on the search for low-metallicity galaxies - I. The N2 calibrator
We present a simple metallicity estimator based on the logarithmic [Nii]ratio, hereafter N2, which we envisage will become very useful forranking galaxies in a metallicity sequence from redshift survey-qualitydata even for moderately low spectral resolution. We have calibrated theN2 estimator using a compilation of Hii galaxies having accurate oxygenabundances, plus photoionization models covering a wide range ofabundances. The comparison of models and observations indicates thatboth primary and secondary nitrogen are important for the relevant rangeof metallicities. The N2 estimator follows a linear relation withlog(O/H) that holds for the whole abundance range covered by the sample,from approximately to twice the Solar value . We suggest that the ([Sii]ratio (hereafter S2) can also be used as a rough metallicity indicator.Because of its large scatter the S2 estimator will be useful only insystems with very low metallicity, where [Nii] λ 6584 is notdetected or in low-resolution spectra where [Nii] λ 6584 isblended with Hα .

Narrow-band CCD photometry of giant H II regions
We have obtained accurate CCD narrow-band Hβ and Hαphotometry of giant HII regions (GEHRs) in M33, NGC 6822 and M101.Comparison with previous determinations of emission-line fluxes showslarge discrepancies; their probable origins are discussed. Combining ournew photometric data with global velocity dispersion (σ) derivedfrom emission linewidths, we review the L(Hβ)-σ relation. Are-analysis of the properties of the GEHRs included in our sample showsthat age spread and the superposition of components in multiple regionsintroduce a considerable spread in the regression. Combining theinformation available in the literature regarding ages of the associatedclusters, evolutionary footprints on the interstellar medium, andkinematical properties of the knots that build up the multiple GEHRs, wefind that a subsample - which we refer to as young and single GEHRs - dofollow a tight relation in the L-σ plane.

Internal Variation of Electron Density in Galactic and Extragalactic HII Regions
Not Available

The Continuing Radio Evolution of SN 1970G
Using the Very Large Array, we have detected radio emission from thesite of SN 1970G in the Sc galaxy M101. These observations are 31 yrafter the supernova event, making SN 1970G the longest monitored radiosupernova. With flux densities of 0.12+/-0.020 mJy at 6 cm and0.16+/-0.015 mJy at 20 cm, the spectral index of -0.24+/-0.20 appears tohave flattened somewhat when compared with the previously reported valueof -0.56+/-0.11, taken in 1990. The radio emission at 20 cm has decayedsince the 1990 observations with a power-law index ofβ20cm=-0.28+/-0.13. We discuss the radio properties ofthis source and compare them to those of other Type II radio supernovae.

The oxygen abundance distribution in M 101
The well-observed spiral galaxy M 101 was considered. The radialdistributions of oxygen abundances determined in three different ways(with the classic Te-method, with the R23-method,and with the P-method) were compared. It was found that the parameters(the central oxygen abundance and the gradient) of the radial(O/H)P abundance distribution are close to those of the(O/H)T_e abundance distribution. The parameters of the(O/H)R_23 abundance distribution differ significantly fromthose of the (O/H)T_e abundance distribution: the central(O/H)R_23 oxygen abundance is higher by around 0.4 dex andthe gradient is steeper by a factor of around 1.5 as compared to thosevalues in the (O/H)T_e abundance distribution. The dispersionin (O/H)P abundance at fixed radius is rather small, ~ 0.08dex, and is equal to that in (O/H)T_e abundance. Thedispersion in (O/H)R_23 abundance at fixed radius isappreciably larger, ~ 0.16 dex, compared to that in (O/H)T_eabundance. It has been shown that the extra dispersion in(O/H)R_23 abundances is an artifact and reflects scatter inexcitation parameter P at fixed radius.

ROSAT X-Ray Observations of the Spiral Galaxy M81
We present results from the analysis of deep ROSAT HRI and PSPCobservations of the spiral galaxy M81. The inferred total (0.5-2 keVband) luminosity of M81 is ~3×1040 ergs s-1,excluding the contribution from identified interlopers found within theD25 ellipse. The nucleus of the galaxy alone accounts forabout 65% of this luminosity. The rest is due to 26 other X-ray sources(contributing ~10%) and to apparently diffuse emission, which is seenacross much of the galactic disk and is particularly bright in the bulgeregion around the nucleus. Spectral analysis further gives evidence fora soft component, which can be characterized by a two-temperatureoptically thin plasma with temperature at ~0.15 and 0.60 keV and anabsorption of the galactic foreground only. These components, accountingfor ~13% of the X-ray emission from the region, apparently arise in acombination of hot gas and faint discrete sources. We find interestingspatial coincidences of luminous (1037-1040 ergss-1) and variable X-ray sources with shock-heated opticalnebulae. Three of them are previously classified as supernova remnantcandidates. The other one is far off the main body of M81 but isapparently associated with a dense H I concentration produced mostlikely by the tidal interactions of the galaxy with its companions.These associations suggest that such optical nebulae may be powered byoutflows from luminous X-ray binaries, which are comparable to, or moreluminous than, Galactic ``microquasars.''

The chemical evolution of gas-rich dwarf galaxies
In an effort to understand the evolution of N, O and He abundances ingas-rich dwarf galaxies, we investigate the dispersion and mixing ofsupernova ejecta in relation to Hii region evolution and develop anumerical model of chemical evolution based on a double-bursting mode ofstar formation (with an interval of the order of3×107yr between bursts of a pair) which has beendesigned to account for the existence of significant scatter in theN/O-O/H relation. The dependence of the abundances on gas fraction isexplored on the basis of this and similar models, in combination withvarious hypotheses concerning inflow and selective and non-selectiveoutflow. The gas fractions are uncertain within wide limits for bluecompact galaxies, but are more well defined for some dwarf irregulars.Selective winds do not give a good fit to N/O, while closed models andmodels with non-selective winds with or without inflow are all found tobe viable.

Bidimensional Spectroscopy of Nearby Starbursts
Not Available

An empirical calibration of nebular abundances based on the sulphur emission lines
We present an empirical calibration of nebular abundances based on thestrong emission lines of [Sii] and [Siii] in the red part of thespectrum through the definition of a sulphur abundance parameterS23. This calibration presents two important advantagesagainst the commonly used one based on the optical oxygen lines: itremains single-valued up to abundances close to solar and is almostindependent of the degree of ionization of the nebula.

An Ultradeep High-Resolution X-Ray Image of M101: The X-Ray Source Population in a Late-Type Spiral
We have studied the X-ray source population of the face-on spiral galaxyM101 (NGC 5457). Within a field of radius 17' (36 kpc at the distance of7.2 Mpc), covered by an ultradeep (229 ks) ROSAT HRI image, 51 X-raysources are detected with signal-to-noise ratios greater than 3.5. Abouthalf of these sources are associated with the galaxy. The luminosity ofthese galactic sources individually ranges from ~4x10^37 to 2x10^39 ergss^-1 in the 0.5-2 keV band. The average luminosity distribution of thesources can be characterized by a power-law function:dN/dL_X=9.5L^-1.9_X sources per 10^38 ergs s^-1. Combined with archivaldata from the ROSAT PSPC, the Einstein IPC, and the ASCA GIS, we haveexamined spatial, spectral, and timing properties of the X-ray sources.In particular, we have explored the nature of various superluminousX-ray sources with luminosities significantly greater than the Eddingtonlimit (~2x10^38 ergs s^-1) for a ~1.6 M_solar object (a neutron star).These X-ray sources, detected in various ROSAT HRI and PSPCobservations, are not transients and appear to result from recentmassive star formation in outer spiral arms. Three superluminous PSPCsources are associated with giant H II complexes and are clearlyresolved. Two other superluminous ROSAT HRI sources are likelyassociated with shell-like supernova (or more likely hypernova)remnants, which are known to be abnormally luminous in optical and/orradio. We further identify two superluminous sources, which all showhighly absorbed X-ray spectra and time variability during and/or betweenthe observations, as candidates for X-ray binary systems that containblack holes. A comparison of seven nearby spirals shows that their X-raysource luminosity distributions, normalized by total H I masses, arevery similar. But both the number of superluminous X-ray sources and thetotal X-ray luminosity appear to be correlated with the star-formingrate of a galaxy.

Carbon in Spiral Galaxies from Hubble Space Telescope Spectroscopy
We present measurements of the gas-phase abundance ratio C/O in six H IIregions in the spiral galaxies M101 and NGC 2403, based on ultravioletspectroscopy using the Faint Object Spectrograph on the Hubble SpaceTelescope. The ratios of C to O increase systematically with O/H in bothgalaxies, from log C/O~-0.8 at log O/H=-4.0 to log C/O~-0.1 at logO/H=-3.4. C/N shows no correlation with O/H. The rate of increase of C/Ois somewhat uncertain because of uncertainty as to the appropriate UVreddening law and uncertainty in the metallicity dependence on graindepletions. However, the trend of increasing C/O with O/H is clear,confirming and extending the trend in C/O indicated previously fromobservations of irregular galaxies. Our data indicate that the radialgradients in C/H across spiral galaxies are steeper than the gradientsin O/H. Comparing the data to chemical-evolution models for spiralgalaxies shows that models in which the massive star yields do not varywith metallicity predict radial C/O gradients that are much flatter thanthe observed gradients. The most likely hypothesis at present is thatstellar winds in massive stars have an important effect on the yieldsand thus on the evolution of carbon and oxygen abundances. C-to-O andN-to-O abundance ratios in the outer disks of spirals determined to dateare very similar to those in dwarf irregular galaxies. This implies thatthe outer disks of spirals have average stellar-population ages muchyounger than those of the inner disks.

ISO observations of five giant HII regions in M 101.
Not Available

X-ray emission from NGC 4321 (M 100): detection of supernova 1979C
In a 42.8 ks ROSAT HRI X-ray observation of the spiral galaxy NGC 4321(M 100) X-ray emission from the supernova 1979C is discovered, sixteenyears after its outburst, with an (0.1-2.4 keV) X-ray luminosity of L_x= 1.0 x 10(39) erg s(-1) . No X-ray emission is observed from the threeother historical supernovae in NGC 4321 (SN 1901B, SN 1914A and SN1959E). In addition to SN 1979C, seven X-ray point sources are detectedinside the D25 ellipse of the galaxy, with luminositiesranging from 4.2 x 10(38) to 6.5 x 10(39) erg s(-1) . Apart from twobright sources in the nuclear region of NGC 4321, none of the otherpoint-like X-ray sources show any time variability over the observationperiod. An unresolved diffuse emission component fills the entireoptical extent of the galaxy. The total luminosity of the diffusecomponent is 3.5 x 10(40) erg s(-1) . Point sources contribute 1.4 x10(40) erg s(-1) to the total luminosity of 5.5 x 10(40) erg s(-1) .Three archival Einstein HRI observations of NGC 4321 were merged into asingle 41.3 ks observation. Six point-like X-ray sources are detectedinside the D25 ellipse of NGC 4321 with Einstein (0.1-4.5keV) luminosities in the range 1.1 - 5.1 x 10(39) erg s(-1) . Three ofthe sources coincide with the positions of ROSAT sources (the two bulgesources and a southern interarm source). Comparison of the ROSAT andEinstein luminosities show that the sources are variable. Einstein upperlimits are evaluated at the positions of all other ROSAT sources andhistorical supernovae in NGC 4321.

Analysis of the ISOPHOT FIR maps of M51 and M101.
The far-IR ISOPHOT maps of the spiral galaxies M51 and M101 at 60, 100,and at 175μm are analysed. For all areas in these galaxies thespectral energy distributions can be fitted with a single-temperatureemission curve. Colours and dust temperatures were estimated forinteresting sub-fields. The temperatures vary between 33K for HIIregions, and 28K for the disk, arm and interarm regimes.

The Composition Gradient in M101 Revisited. I. H II Region Spectra and Excitation Properties
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1996ApJ...456..504K&db_key=AST

Unresolved Wind-driven Shells and the Supersonic Velocity Dispersion in Giant H II Regions
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...456..264T

Ultraviolet spectral evolution of star clusters in the IUE library.
The ultraviolet integrated spectra of star clusters and H II regions inthe IUE library have been classified into groups based on their spectralappearance, as well as on age and metallicity information from otherstudies. We have coadded the spectra in these groups according to theirS/N ratio, creating a library of template spectra for futureapplications in population syntheses in galaxies. We define spectralwindows for equivalent width measurements and for continuum tracings.These measurements in the spectra of the templates are studied as afunction of age and metallicity. We indicate the windows with a strongmetallicity dependence, at different age stages.

X-ray emission from giant H II regions in M101
We have examined the archival ROSAT Position Sensitive ProportionalCounter (PSPC) and the High-Resolution Imager (HRI) images of the galaxyM101 to study the X-ray properties of its giant H II regions. All fivegiant H II regions, NGC 5447, NGC 5445, NGC 5461, NGC 5462, and NGC5471, show X-ray emission. By observing their basic properties, such assize, hardness ratio, and estimated luminosity, and by comparisons withbetter known regions in the Large Magellanic Cloud (LMC), we are able tosuggest some possible mechanisms for the sources of the X-ray emission.

The form of abundance gradients in three nearby spiral galaxies: M33, M81 and M101
Available emission-line data have been compiled for giant H II regionslocated in three nearby, well-observed spirals M33, M81, and M101 inorder to study the form of interstellar abundance patterns in the disksof these galaxies. Numerous line strength ratios are shown to varysystematically with galactocentric distance. Gradients for stellareffective temperature, ionization parameter, and the abundance ratiosO/H, N/O, and S/O were determined using a sequential photoionizationmodel analysis procedure. The observed behavior of line ratios acrossthe disks of these galaxies is found to be consistent with abundancegradients which are exponential in form and have constant slopes,thereby challenging recent suggestions that gradients flatten in theouter disk. Additionally, the N/O gradient in M101 is found to bemarkedly steeper than in M33 and M81, suggesting that in M101 secondarynitrogen production is relatively more efficient than primary nitrogenproduction.

The IMF and the extinction law in M 101: HST FOS spectra of extragalactic H II regions
We present medium resolution Hubble Space Telescope (HST) Faint ObjectSpectrograph (FOS) spectra of the ionizing star clusters in four giant HII regions in the spiral galaxy M 101, spanning a range in (O/H) from8.2 to 8.9. The observed energy distributions in the wavelength range1150 A to 8200 A are compared to population synthesis models ofextremely young clusters, taking into account the attenuation by dustand the nebular continuum emission. The OB cluster spectra arecompatible with short duration, 3 Myr old star formation bursts governedby a Miller-Scalo type solar neighborhood initial mass function (IMF).The attenuation law towards the targets in M 101 is found to be rathersimilar to the average galactic law but for a much weaker 2175 A bump.Since the observed shape of the composite continua is dominated by theform of the UV extinction, power-law IMFs skewed towards high stellarmasses may also explain the observations with slightly modifiedextinction curves. However, on the basis of the data presented herethere is no need to invoke second order effects such as a variation withmetallicity of the slope of the IMF, or the upper and lower stellar masslimits in the current burst of star formation respectively.

The origin of the far-infrared luminosity within the spiral galaxy M101
High resolution 60 and 100 micron images obtained with the InfaredAstronomical Satellite (IRAS) are compared with H alpha images in orderto investigate the origin of the far-infrared luminosity within thelate-type spiral galaxy M101. There is a good correspondence between thefar-infrared and H-alpha morphology. The far-infrared and H-alphaluminosities have been measured at 129 independent locations on the starforming disk of M101. After correcting the H-alpha luminosity forextinction and extrapolating the IRAS (40-120 microns) luminosity to1000 microns we find that the far-infrared luminosity is commensuratewith that expected from the O and B stars which are required to ionizethe hydrogen gas, at all locations within M101. Additionally, the IRASHiRes 60 and 100 micron images reveal that the dust temperature peakscoincide identically with the location of H II regions. The far-infraredluminosity of M101 is radiated primarily by dust with temperatures wellin excess of that expected for cirrus, but similar to that observed forGalactic and extragalactic H II regions.

Element Abundances in Blue Compact Galaxies
Models of the chemical evolution of blue compact galaxies, where thestar formation is assumed to proceed in short, intense bursts ofactivity, have been computed, with special attention to the evolution ofhelium, nitrogen, oxygen and iron. The contribution to the chemicalenrichment by supernovae (SNe) of the various types (Ia, lb, II) istaken into account, following the most up-to-date prescriptions.Galactic winds powered by SNe are also considered and confirmed as beingable to explain the observed element distributions. In particular, windsreleasing metals in different proportions can explain the high ratio of{DELTA}(He/H)/{DELTA}(O/H) and the distribution of N/O with O/H observedin extragalactic HII regions. No anomalous assumption on the initialmass function (IMF) slope and/or mass cut-offs seems to be required toobtain model predictions in agreement with the data. In particular,Salpeter`s initial mass function is applicable to all these galaxies,although some slight differences cannot be excluded. The [O/Fe] ratiosresulting from these models are always positive, except when many starformation episodes and differential galactic winds are assumed, thussuggesting the latter scenario for systems like the Magellanic Clouds,where the observed [O/Fe] ratios are negative.

Planetary nebulae and the helium-to-metals enrichment ratio
A sample of planetary nebulae (PN), galactic and extragalactic H IIregions and blue compact galaxies (BCG) was used in order to obtain thehelium-to-metals enrichment ratio, dY/dZ. Adopting a simple linearvariation for the helium abundance with metals, and taking into accountthe contamination of the observed helium abundance in PN by the freshhelium produced in their central stars, it is shown that dY/dZ>3, andthat the linear model is limited to low metallicities. Finally, weconclude that the simple model of galactic chemical evolution is unableto explain the observed enrichment ratio unless infall of gaseousmaterial on to the galactic disk is considered.

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Observation and Astrometry data

Constellation:Ursa Major
Right ascension:14h03m01.18s
Apparent magnitude:99.9

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NGC 2000.0NGC 5455

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