arXiv subjects

Eva K. Grebel

Publications and source records attributed to Eva K. Grebel.

214 records · Page 12Linked to original sources

arXiv2000

Hubble Space Telescope Photometry of the Dwarf Spheroidal Galaxy ESO 410-G005

We present HST WFPC2 imaging of the nearby low-surface-brightness dwarf spheroidal galaxy ESO 410-G005, which has been resolved into stars for the first time. The resulting color-magnitude diagram for about 2500 stars shows a red giant branch branch with a tip at I=(22.4+-0.15), which yields a distance of D=(1.9+-0.2) Mpc. ESO 410-G005 is found to be metal-poor with a mean metallicity of (-1.8+-0.4) dex estimated from its red giant branch. Upper asymptotic giant branch stars appear to be present near the center of the galaxy, indicative of a substantial, centrally concentrated intermediate-age population, unless these objects are artifacts of crowding. Previous studies did not detect ESO 410-G005 in H alpha or in HI. ESO 410-G005 is a probable member of the Sculptor group. Its linear separation from the nearest spiral, NGC 55, is 230 kpc on the sky. The deprojected separation ranges from 340 to 615 kpc depending on the assumed distance of NGC 55. ESO 410 G005 appears to be a relatively isolated dSph within the Sculptor group. Its absolute magnitude, Mv = (-12.1+-0.2) mag, its central surface brightness, mu_V = (22.7+-0.1) mag/arcsec^2, and its mean metallicity, [Fe/H] = (-1.8+-0.4) dex, follow the trend observed for dwarf galaxies in the Local Group. (abridged)

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arXiv2000

Keck Studies of M31's Stellar Halo

We present Keck 10-meter/LRIS spectra of candidate red giants in the halo of M31, located at a projected radius of R=19kpc on the minor axis. These spectroscopic targets have been selected using a combination of UBRI-based and morphological screening to eliminate background galaxies. Radial velocity measurements are used to separate M31 halo giants from foreground Milky Way dwarf stars, M31 disk stars, and residual background galaxies. The metallicity of each M31 halo giant is measured using standard photometric and spectroscopic techniques, the latter based on the strength of the CaII triplet. The various [Fe/H] estimates are in rough agreement with one another. The data reveal a large spread (>2dex) in [Fe/H] in M31's halo; there is no strong radial [Fe/H] gradient. LRIS and HIRES spectra are also presented for red giants in five dwarf spheroidal satellites of M31: AndI, AndIII, AndV, AndVI, and AndVII. There appears to be a significant metallicity spread in AndVI and possibly in AndI. The new radial velocity data on these outer dwarfs are used to constrain the total mass of M31: the best estimate is under 10^(12)Msun, somewhat less than the best estimate for the Milky Way.

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arXiv2000

Studies of binary star cluster candidates in the bar of the LMC. II

Binary clusters account for more than 10 % of the cluster population in the Magellanic Clouds. Statistically fewer than 50 % of the found pairs are expected to be chance superpositions. We estimated the cluster encounter rate and suggest that tidal capture is an unlikely formation scenario for the formation of binary clusters. Thus, most true binary clusters can be expected to have formed together. Here we present a study of three binary cluster candidates which are located in the bar of the LMC. NGC 1971 & NGC 1972 are situated in the association LH 59 in the eastern part of the bar. A third star cluster, NGC 1969, is close enough to this pair that all three objects may constitute a triple system. We present the first age determination that is based on CMDs for these star clusters. Our findings suggest that all three clusters are young (40-70 Myr) and may have been formed in the same GMC. It cannot clearly decided whether the clusters are physically interacting or not. NGC 1894 & SL 341 are located at the south-western rim of the LMC bar. This pair is studied in detail for the first time: The isopleths of both clusters reveal an elliptical shape. Whether this might be interpreted as a sign of interaction or is a peculiarity which is shared with a large amount of LMC star clusters which show higher ellipticities than their counterparts in the Milky Way remains unclear. From our age determination we find that both clusters are coeval with an age of 55+/-5 Myr. This makes a formation from the same GMC a likely scenario. SL 385 & SL 387 are a close pair in the western part of the LMC bar. We derived ages of 170+/-30 Myr for SL 385 and >=250 Myr for SL 387. The large age difference makes it unlikely that these two clusters formed in the same GMC.

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arXiv2000

Studies of Binary Star Cluster Candidates in the Bar of the LMC. I: SL 353 & SL 349

We present a detailed study of the cluster pair SL 353 & SL 349. This candidate binary cluster is located at the northwestern rim of the LMC bar. Based on photometric data we find that both clusters are coeval with an age of 550 +/- 100 Myr. We use the Ca II triplet in the spectra of individual red giants to derive radial velocities. Both components of the binary cluster candidate show very similar mean velocities (approx. 274 +/- 10 km/sec for SL 349 and approx. 279 +/- 4 km/sec for SL 353) while the field stars show lower velocities (approx. 240 +/- 19 km/sec). These findings suggest a common origin of the two clusters from the same GMC. In this sense the cluster pair may constitute a true binary cluster. We furthermore investigate the stellar densities in and around the star clusters and compare them with isopleths created from artificial, interacting as well as non-interacting star clusters. Gravitational interaction leads to a distortion which can also be found in the observed pair.

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arXiv2000

The Morphologies of the Small Magellanic Cloud

We compare the distribution of stars of different spectral types, and hence mean age, within the central SMC and find that the asymmetric structures are almost exclusively composed of young main sequence stars. Because of the relative lack of older stars in these features, and the extremely regular distribution of red giant and clump stars in the SMC central body, we conclude that tides alone are not responsible for the irregular appearance of the central SMC. The dominant physical mechanism in determining the current-day appearance of the SMC must be star formation triggered by a hydrodynamic interaction between gaseous components. These results extend the results of population studies (cf. Gardiner and Hatzidimitriou) inward in radius and also confirm the suggestion of the spheroidal nature of the central SMC based on kinematic arguments (Dopita et al; Hardy, Suntzeff & Azzopardi). Finally, we find no evidence in the underlying older stellar population for a ``bar'' or ``outer arm'', again supporting our classification of the central SMC as a spheroidal body with highly irregular recent star formation.

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arXiv1999

Star Clusters in Local Group Galaxies

In about 40% of the Local Group galaxies star clusters have been detected so far, but the census is still incomplete. The properties of these clusters are briefly reviewed, and the impact of galaxy environment on the evolution and survival of star clusters is discussed. Several (but not all) Local Group galaxies seem to share a common epoch of the earliest globular cluster formation. The most massive galaxies show a tendency for rapid enrichment in their oldest cluster population near their centers as compared to clusters at larger galactocentric radii. The galactocentric dependence of cluster sizes and horizontal branch morphology may be intrinsic to the globular cluster formation process rather than to the accretion of dwarf galaxies. The observed properties of globular clusters in the least massive dwarfs suggest that their parent galaxies may originally have been substantially more massive. Cluster destruction mechanisms and time scales are a function of galaxy environment and galaxy mass. The most recent enhancement of star cluster formation in the Magellanic Clouds may have been triggered by their close encounter with each other and the Milky Way.

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arXiv1999

Hubble Space Telescope Photometry of Hodge 301: An "Old" Star Cluster in 30 Doradus

We present Hubble Space Telescope Planetary Camera UVI data for the little-studied cluster Hodge 301 3' northwest of 30 Doradus' central ionizing cluster R136. The average reddening of Hodge 301 is found to be = (0.28+-0.05) mag from published infrared and ultraviolet photometry. Using two different sets of evolutionary models, we derive an age of ~ 20-25 Myr for Hodge 301, which makes it roughly 10 times as old as R136. Hodge 301 is the most prominent representative of the oldest population in the 30 Dor starburst region; a region that has undergone multiple star formation events. This range of ages is an important consideration for the modelling of starburst regions. Hodge 301 shows a widened upper main sequence largely caused by Be stars. We present a list of Be star candidates. The slope of the initial mass function for intermediate-mass main sequence stars ranging from 10 to 1.3 solar masses is found to be -1.4+-0.1 in good agreement with a Salpeter law. There is no indication for a truncation or change of slope of the IMF within this mass range. In accordance with the age of Hodge 301 no obvious pre-main-sequence stars are seen down to 1 solar mass. We estimate that up to 41+-7 stars with more than 12 solar masses may have turned into supernovae since the formation of the cluster. Multiple supernova explosions are the most likely origin of the extremely violent gas motions and the diffuse X-ray emission observed in the cluster surroundings.

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arXiv1998

Evolutionary Histories of Dwarf Galaxies in the Local Group

The star formation histories of Local Group (LG) dwarf galaxies and more distant potential LG members are reviewed. Problems in defining the spatial extent of the LG and membership are briefly discussed. The morphological types found in the LG are presented, and it is suggested that we see continuous evolution from low-mass dwarf irregulars (dIrrs) to dwarf spheroidal galaxies (dSph) in the LG. Star formation histories for LG dwarfs and nearby LG candidates are compiled using population boxes. No two dwarfs, irrespective of morphological type, show the same evolutionary history, and all vary widely in ages of their subpopulations and in their enrichment history. The lack of gas in dSphs and dEs may be explained by a new photoionization scenario. Old populations, often spatially very extended, may be a common property of dwarf galaxies, though their fractions can be very small. Almost all types of dwarf galaxies studied in detail so far show spatial variations in ages and abundances such as radial age/metallicity gradients. The observed star formation histories impose constraints on merger and accretion scenarios. Properties of the Milky Way dSphs are compared to the M31 dSphs and discussed in the framework of the ram pressure/tidal stripping scenario. It is demonstrated that the newly discovered LG dwarfs follow the same relationship for central surface brightness, mean metallicity, and absolute magnitude as the other LG dwarfs. (Shortened)

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arXiv1998

Properties of Two New M31 dSph Companions from Keck Imaging

We have obtained Keck Low Resolution Imaging Spectrograph images in V and I of the newly discovered Local Group dwarf galaxies Pegasus dSph and Cassiopeia dSph and their surrounding field. The first stellar luminosity functions and color-magnitude diagrams are presented for stars with V <~ 25 and I <~ 24. The distances to the new dwarfs are estimated from the apparent I mag of the tip of the red giant branch to be: D_MW (Peg dSph) = 830 +/- 80 kpc and D_MW (Cas dSph) = 760 +/- 70 kpc, consistent with their belonging to the extended M31 satellite system. Both galaxies are dwarf spheroidals (dSphs) with red giant branch morphologies indicating predominantly old stellar populations and estimated mean metallicities, <[Fe/H]>, of -1.3 +/- 0.3 for Peg dSph and -1.4 +/- 0.3 for Cas dSph. Their central surface brightness-<[Fe/H]>-absolute magnitude relationship follows that of other Local Group dSphs, dwarf ellipticals, and dwarf irregulars. In contrast to four out of nine Milky Way dSphs (the four that lie beyond D_MW =100 kpc), none of the six known M31 dSphs appears to have a dominant intermediate-age population.

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arXiv1998

The cluster pair SL 538/ NGC 2006 (SL 537)

We studied in detail the binary cluster candidates SL 538 and NGC 2006 in the Large Magellanic Cloud (LMC). This cluster pair is located in the northwestern part of the large OB association LH 77 in supergiant shell LMC 4. A third star cluster, KMHK 1019, is located within 5' from the cluster pair. Our study is the first age determination of SL 538 and NGC 2006 that is based on CMDs. We derived an age of 18+/-2 Myr for SL 538, 22.5+/-2.5 Myr for NGC 2006, and 16 Myr for KMHK 1019. Thus the three clusters are (nearly) coeval. We identified Be star candidates and find the same ratio N(Be)/N(B) for the components of the binary cluster (12%) while the amount of Be stars detected in KMHK 1019 (5%) and in the surrounding field (2%) is considerably lower. Since Be stars are usually rapid rotators this may indicate intrinsically higher rotational velocities in the components of the cluster pair. Also the IMF derived from the CMDs shows the same slope for both SL 538 and NGC 2006 and is consistent with a Salpeter IMF. An estimation of the cluster masses based on the IMF slopes showed that both clusters have similar total masses. These findings support joint, near-simultaneous formation of the cluster pair in the same giant molecular cloud.

astro-ph↗

arXiv1998

Star Formation Histories of Local Group Dwarf Galaxies

40 galaxies are currently known as, or suspected to be Local Group members within a radius of 1.8 Mpc. 37 of these galaxies are dwarf galaxies with M_B > -18.5 mag. I present a compilation of the star formation histories of dwarf irregulars, dwarf ellipticals, and dwarf spheroidals in the Local Group, and visualize their evolutionary histories through Hodge's population boxes. The study of these star formation histories is a multi-parameter problem: Ages, metallicities, population fractions, and spatial variations must be determined, which depend crucially on the knowledge of reddening and distance. Even the lowest-mass dwarf spheroidal galaxies can show significant spatial variations in star formation, ages, and metallicities. All Local Group dwarf galaxies appear to contain old (>10 Gyr) and intermediate-age (1-10 Gyr) populations irrespective of morphological type. No two galaxies are alike. They vary widely in star formation histories, metallicities, fractions, and ages of their subpopulations even within the same morphological type. Star formation has occurred either in distinct episodes or continuously over long periods of time.

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arXiv1997

The Hourglass Nebulae of Sher 25 and SN 1987 A: Two of a Kind?

We have performed a detailed study of the morphology and kinematics of the hourglass-shaped nebula around the blue supergiant Sher 25 in the galactic giant HII region NGC 3603. Near-infrared high resolution adaptive optics images in the Br gamma line and HST/NICMOS images in the HeI 1.08mu line were compared with iso-velocity maps in the H alpha and [NII] lines. The adaptive optics observations clearly resolved the width of the ring (0.9", i.e., 0.027 pc), yielding delta R / R = 1:8. We show that the H alpha and [NII] lines trace the entire silhouette of the hourglass. The bipolar lobes of the hourglass expand at 70 km/s, whereas the ring around the waist of the hourglass expands at 30 km/s. Both the ring and the bipolar lobes have about the same dynamical age, indicating a common origin and a major outburst and mass-loss event 6630 yr ago. The ionized mass within the hourglass is between 0.3 Mo and 0.6 Mo - quite comparable to the total mass suggested for the expanding (pre-supernova) shell around SN 1987 A. The hourglass structure around Sher 25 is similar to that of SN 1987 A in spatial extent, mass, and velocities. The major differences between these two nebulae might arise from environmental effects. Both internal and external ionization sources are available for Sher 25's nebula. Furthermore, Sher 25 and its hourglass-shaped nebula appear to be moving to the south-west with respect to the ambient interstellar medium, and ram pressure has apparently deformed the hourglass. We conclude that the circumstellar nebulae around SN 1987 A and Sher 25 are very similar and define a new class of nebulae around blue supergiants in their final evolutionary stage.

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arXiv1996

Ring Nebula and Bipolar Outflows Associated with the B1.5 Supergiant Sher #25 in NGC 3603

We have identified a ring-shaped emission-line nebula and a possible bipolar outflow centered on the B1.5 supergiant Sher #25 in the Galactic giant HII region NGC 3603 (distance 6 kpc). The clumpy ring around Sher #25 appears to be tilted by 64 deg against the plane of the sky. Its semi-major axis (position angle approx. 165 deg) is 6.9" long, which corresponds to a ring diameter of 0.4 pc. The bipolar outflow filaments, presumably located above and below the ring plane on either side of Sher #25, show a separation of approx. 0.5 pc from the central star. High-resolution spectra show that the ring has a systemic velocity of V_LSR = +19 km/s and a de-projected expansion velocity of 20 km/s, and that one of the bipolar filaments has an outflow speed of approx. 83 km/s. The spectra also show high [NII]/Halpha ratio, suggestive of strong N enrichment. Sher #25 must be an evolved blue supergiant (BSG) past the red supergiant (RSG) stage. We find that the ratio of equatorial to polar mass-loss rate during the red supergiant phase was approx 16. We discuss the results in the framework of RSG-BSG wind evolutionary models. We compare Sher #25 to the progenitor of SN 1987A, which it resembles in many aspects.

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arXiv1996

Be star surveys with CCD photometry. II. NGC 1818 and its neighbouring cluster in the LMC

As part of an ongoing photometric survey of young Magellanic Cloud clusters we identified Be stars in NGC 1818 and a nearby smaller cluster in the Large Magellanic Cloud. The neighbouring cluster does not contain evolved stars, and its sparsely populated main sequence does not extend to stars as massive as in NGC 1818. Both clusters are younger than the surrounding field population and contain a much higher Be star fraction. We present a table with photometry, astrometric positions, and IAU-conform identifiers of the Be stars. We compare Be star fractions in Magellanic Cloud clusters and Galactic open clusters and discuss possible constraints on Be star theories.

gr-qc↗

arXiv1995

On the nature of the blue giants in NGC 330

The young SMC cluster NGC 330 contains a number of blue stars that lie above the main-sequence turnoff found from our isochrone fitting and below the position of the blue supergiants. We used our own, new spectroscopy and published data on these stars to investigate their possible nature. Problems in interpreting the evolutionary status of the blue giants have been found in several preceding studies. In theoretical H-R diagrams, these stars lie in the rapidly traversed post main-sequence gap, similar to the unexpected concentration found by Fitzpatrick & Garmany (1990) in the H-R diagram of the LMC. We argue that these stars probably are core H burning main-sequence stars that appear as blue stragglers resulting from binary evolution as described in the simulations of Pols \& Marinus (1994) and effects of rapid rotation. Many of the blue stragglers are Be stars and likely rapid rotators. We suggest that there is evidence for the presence of blue stragglers also in NGC 1818, NGC 2004, and NGC 2100. We point out that blue stragglers may be a general phenomenon in the CMDs of young clusters in the Magellanic Clouds and discuss the implications for IMF and age determinations.

astro-ph↗

arXiv1995

Low-mass star formation in CG1: a diffraction limited search for pre-main sequence stars next to NX Puppis

Using adaptive optics at the ESO 3.6m telescope, we obtained diffraction limited JHK-images of the region around the Herbig AeBe star NX Pup. We clearly resolved the close companion (sep. 0.128") to NX Pup -- originally discovered by HST -- and measured its JHK magnitudes. A third object at a separation of 7.0" from NX Pup was identified as a classical T Tauri star so that NX Pup may in fact form a hierarchical triple system. We discuss the evolutionary status of these stars and derive estimates for their spectral types, luminosities, masses and ages.

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