Wolfgang Brandner

Population Gradients in Local Group Dwarf Spheroidal Galaxies

We present a systematic and homogeneous analysis of population gradients for the Local Group dwarf spheroidal galaxies (dSphs) Carina, Sculptor, Sextans, Tucana, and Andromeda I–III, V, and VI. For all of the Milky Way companions studied here, we find significant population gradients. The same is true for the remote dSph Tucana located at the outskirts of the Local Group. Among the M31 dSph companions, only Andromeda I and VI show obvious gradients. In all cases where a horizontal-branch (HB) morphology gradient is visible, the red HB stars are more centrally concentrated. The occurrence of a HB morphological gradient shows a correlation with a morphology gradient in the red giant branch. It seems likely that metallicity is the driver of the gradients in Sextans, Sculptor, Tucana, and Andromeda VI, while age is an important factor in Carina. We find no evidence that the vicinity of a nearby massive spiral galaxy influences the formation of the population gradients.

Membership and Multiplicity among Very Low Mass Stars and Brown Dwarfs in the Pleiades Cluster

We present near-infraredphotometry andoptical spectroscopy of very low mass stars andbrown dwarf candidates in the Pleiades open cluster. The membership status of these objects is assessed using color-magnitude diagrams, lithium and spectral types. Eight objects out of 45 appear to be nonmembers. A search for companions among 34 very low mass Pleiades members in high spatial (M ¹ 0.09 M _ ) resolution images obtainedwith the Hubble Space T elescope (HST ) andthe ad aptive optics system of the Canada-France-Hawaii telescope produced no resolved binaries with separations larger than 0A.2 (a D 27 AU ; P D 444 yr). Nevertheless, we Ðnd evidence for a binary sequence in the color-magnitude diagrams, in agreement with the results of Steele & Jameson for higher mass stars. We apply the lithium test to two objects : CFHT-Pl-16, which lies in the cluster binary sequence but is unresolvedin images obtainedwith the Hubble Space T elescope ; andCFHT-Pl-18, which is binary with separation. The 0A Ðrst object passes the test, but the second object does not. We conclude that CFHT-Pl-16 is an Pleiades brown dwarf binary with separation less than 11 AU and that CFHT-Pl-18 is a foreground system. We compare the multiplicity statistics of the Pleiades very low mass stars and brown dwarfs with that of G- andK-type main-sequence stars in the solar neighborhood . We Ðndthat there is some evid ence for a deÐciency of wide binary systems (separation (27 AU) among the Pleiades very low mass members. We brieNy discuss how this result can Ðt with current scenarios of brown dwarf formation. We correct the Pleiades substellar mass function for the contamination of cluster nonmembers found in this work. We Ðnd a contamination level of 33% among the brown dwarf candidates identiÐed by Bouvier et al. Assuming a power-law IMF across the substellar boundary, we Ðnd a slope dN/dM D M~0.53, implying that the number of objects per mass bin is still rising but the contribution to the total mass of the cluster is declining in the brown dwarf regime. Subject headings : binaries : general E open clusters and associations : individual (Pleiades) E stars : evolution E stars : formation E stars : low mass, brown dwarfs E stars : luminosity function, mass function On-line material : color Ðgure

Galactic Starburst NGC 3603 from X-Rays to Radio

NGC 3603 is the most massive and luminous visible starburst region in the Galaxy. We present the first Chandra ACIS-I X-ray image and spectra of this dense, exotic object, accompanied by a deep centimeter-wavelength Australia Telescope Compact Array radio image at similar 1 spatial resolution and Hubble Space Telescope/ground-based optical data. At the S/N > 3 level, Chandra detects several hundred X-ray point sources (compared to the three distinct sources seen by ROSAT). At least 40 of these sources are definitely associated with optically identified cluster O- and W-R-type members, but most are not. A diffuse X-ray component is also seen out to ~2 (4 pc) from the center, probably arising mainly from the large number of merging/colliding hot stellar winds and/or numerous faint cluster sources. The point-source X-ray fluxes generally increase with increasing bolometric brightnesses of the member O/W-R stars, but with very large scatter. Some exceptionally bright stellar X-ray sources may be colliding wind binaries. The radio image shows (1) two resolved sources, one definitely nonthermal, in the cluster core near where the X-ray/optically brightest stars with the strongest stellar winds are located, (2) emission from all three known proplyd-like objects (with thermal and nonthermal components), and (3) many thermal sources in the peripheral regions of triggered star formation. Overall, NGC 3603 appears to be a somewhat younger and hotter scaled-down version of typical starbursts found in other galaxies.

Hokupa'a-Gemini Discovery of Two Ultracool Companions to the Young Star HD 130948

We report the discovery of two faint ultracool companions to the nearby (d ~ 17.9 pc) young G2 V star HD 130948 (HR 5534, HIP 72567) using the Hokupaa adaptive optics (AO) instrument mounted on the Gemini North 8 m telescope. Both objects have the same common proper motion as the primary star as seen over a 7 month baseline and have near-IR photometric colors that are consistent with an early L classification. Near-IR spectra taken with the NIRSPEC AO instrument on the Keck II telescope reveal K I lines, FeH, and H2O band heads. Based on these spectra, we determine that both objects have a spectral type of dL2 with an uncertainty of two spectral subclasses. The position of the new companions on the H-R diagram in comparison with theoretical models is consistent with the young age of the primary star (<0.8 Gyr) estimated on the basis of X-ray activity, lithium abundance, and fast rotation. HD 130948B and C likely constitute a pair of young contracting brown dwarfs with an orbital period of about 10 yr and will yield dynamical masses for L dwarfs in the near future.

HST/WFPC2 and VLT/ISAAC Observations of Proplyds in the Giant H II Region NGC 3603

We report the discovery of three proplyd-like structures in the giant H II region NGC 3603. The emission nebulae are clearly resolved in narrowband and broadband HST/WFPC2 observations in the optical and broadband VLT/ISAAC observations in the near-infrared. All three nebulae are tadpole shaped, with the bright ionization front at the head facing the central cluster and a fainter ionization front around the tail pointing away from the cluster. Typical sizes are 6000 AU × 20,000 AU The nebulae share the overall morphology of the proplyds (PROto PLanetarY DiskS) in Orion, but are 20 to 30 times larger in size. Additional faint filaments located between the nebulae and the central ionizing cluster can be interpreted as bow shocks resulting from the interaction of the fast winds from the high-mass stars in the cluster with the evaporation flow from the proplyds. Low-resolution spectra of the brightest nebula, which is at a projected separation of 1.3 pc from the cluster, reveal that it has the spectral excitation characteristics of an ultra compact H II region with electron densities well in excess of 104 cm-3. The near-infrared data reveal a point source superposed on the ionization front. The striking similarity of the tadpole-shaped emission nebulae in NGC 3603 to the proplyds in Orion suggests that the physical structure of both types of objects might be the same. We present two-dimensional radiation hydrodynamical simulations of an externally illuminated star-disk-envelope system, which was still in its main accretion phase when first exposed to ionizing radiation from the central cluster. The simulations reproduce the overall morphology of the proplyds in NGC 3603 very well, but also indicate that mass-loss rates of up to 10-5 M☉ yr-1 are required in order to explain the size of the proplyds. We report the discovery of three proplyd-like structures in the giant H II region NGC 3603. The emission nebulae are clearly resolved in narrowband and broadband HST/WFPC2 observations in the optical and broadband VLT/ISAAC observations in the near-infrared. All three nebulae are tadpole shaped, with the bright ionization front at the head facing the central cluster and a fainter ionization front around the tail pointing away from the cluster. Typical sizes are 6000 AU × 20,000 AU. The nebulae share the overall morphology of the proplyds (PROto PLanetarY DiskS) in Orion, but are 20 to 30 times larger in size. Additional faint filaments located between the nebulae and the central ionizing cluster can be interpreted as bow shocks resulting from the interaction of the fast winds from the high-mass stars in the cluster with the evaporation flow from the proplyds. Low-resolution spectra of the brightest nebula, which is at a projected separation of 1.3 pc from the cluster, reveal that it has the spectral excitation characteristics of an ultra compact H II region with electron densities well in excess of 104 cm−3. The near-infrared data reveal a point source superposed on the ionization front. The striking similarity of the tadpole-shaped emission nebulae in NGC 3603 to the proplyds in Orion suggests that the physical structure of both types of objects might be the same. We present two-dimensional radiation hydrodynamical simulations of an externally illuminated star-disk-envelope system, which was still in its main accretion phase when first exposed to ionizing radiation from the central cluster. The simulations reproduce the overall morphology of the proplyds in NGC 3603 very well, but also indicate that mass-loss rates of up to 10−5 M☉ yr−1 are required in order to explain the size of the proplyds. Due to these high mass-loss rates, the proplyds in NGC 3603 should only survive ≈105 yr. Despite this short survival time, we detect three proplyds. This indicates that circumstellar disks must be common around young stars in NGC 3603 and that these particular proplyds have only recently been exposed to their present harsh UV environment.

Hubble Space Telescope NICMOS Detection of a Partially Embedded, Intermediate-Mass, Pre-Main-Sequence Population in the 30 Doradus Nebula

We present the detection of an intermediate-mass pre-main-sequence population embedded in the nebular filaments surrounding the 30 Doradus region in the Large Magellanic Cloud (LMC) using HST/NICMOS. In addition to four previously known luminous Class I infrared “protostars,” the NICMOS data reveal 20 new sources with intrinsic infrared excess similar to Galactic pre-main sequence stars. Based on their infrared brightness, these objects can be identified as the LMC equivalent of Galactic pre-main sequence stars. The faintest LMC Young Stellar Objects in the sample have colors similar to T Tauri and have about the same brightness as T Tauri if placed at the distance of the LMC. We find no evidence for a lower-mass cut-off in the initial mass function. Instead, the whole spectrum of stellar masses from pre-main sequence stars with �1.5M⊙ to massive O stars still embedded in dense knots appears to be present in the nebular filaments. The majority of the young stellar objects can be found to the north of the central starburst cluster R136. This region is very likely evolving into an OB association. The observations provide further evidence that star formation in the 30 Doradus region is very similar to Galactic star formation, and confirm the presence of sequential star formation in 30 Doradus, with present-day star formation taking place in the arc of molecular gas to the north and west of the starburst cluster. Subject headings: (galaxies:) Magellanic Clouds — galaxies: stellar content — stars: formation — stars: pre-main sequence

Subarcsecond Mid-Infrared Structure of the Dust Shell around IRAS 22272+5435*

We report sub-arcsecond imaging of extended mid-infrared emission from a proto-planetary nebula (PPN), iras 22272+5435, performed at the MMT observatory with its newly upgraded 6.5 m aperture telescope and at the Keck observatory. The mid-infrared emission structure is resolved into two emission peaks separated by

Infrared spectrum and proper motion of the brown dwarf companion of HR 7329 in Tucanae

0arcsec.5 - 0arcsec.6

A Search for Jovian Planets around Hot White Dwarfs

in the MMT 11.7

The Peak Brightness and Spatial Distribution of Asymptotic Giant Branch Stars Near the Nucleus of M32

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