arXiv subjects

Dietrich Baade

Publications and source records attributed to Dietrich Baade.

67 records · Page 4Linked to original sources

arXiv2008

The WFI Halpha spectroscopic survey of the Magellanic Clouds: Be stars in SMC open clusters

At low metallicity, B-type stars show lower loss of mass and, therefore, angular momentum so that it is expected that there are more Be stars in the Magellanic Clouds than in the Milky Way. However, till now, searches for Be stars were only performed in a very small number of open clusters in the Magellanic Clouds. Using the ESO/WFI in its slitless spectroscopic mode, we performed a Halpha survey of the Large and Small Magellanic Cloud. Eight million low-resolution spectra centered on Halpha were obtained. For their automatic analysis, we developed the ALBUM code. Here, we present the observations, the method to exploit the data and first results for 84 open clusters in the SMC. In particular, cross-correlating our catalogs with OGLE positional and photometric data, we classified more than 4000 stars and were able to find the B and Be stars in them. We show the evolution of the rates of Be stars as functions of area density, metallicity, spectral type, and age.

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arXiv2008

Early-type objects in NGC6611 and Eagle Nebula

An important question about Be stars is whether Be stars are born as Be stars or not. It is necessary to observe young clusters to answer this question. Observations of stars in NGC6611 and the star-formation region of Eagle Nebula have been carried out with the ESO-WFI in slitless spectroscopic mode and at the VLT-GIRAFFE. The targets for the GIRAFFE observations were pre-selected from the literature and our catalogue of emission-line stars (ELS) based on the WFI study. GIRAFFE observations allowed us to study accurately the population of the early-type stars with and without emission lines. For this study, we determined the fundamental parameters of OBA stars. We also studied the status of the objects (main sequence or pre-main sequence stars) by using IR data, membership probabilities, and location in HR diagrams. The nature of the early-type ELS in M16 is derived. The slitless observations with the WFI clearly indicate a small number of ELS in M16. We observed with GIRAFFE 101 OBA stars, among them 9 are ELS with circumstellar emission in Halpha. We found that: W080 could be a new He-strong star, like W601. W301 is a possible classical Be star, W503 is a mass-transfer eclipsing binary with an accretion disk, and the other ones are possible Herbig Ae/Be stars. We also found that the rotational velocities of main sequence B stars are 18% lower than those of pre-main sequence B stars, in good agreement with theory about the evolution of rotational velocities. Combining different indications and technics, we found that 27% of the B-type stars are binaries. We also redetermined the age of NGC6611 found equal to 1.2--1.8 Myears in good agreement with the most recent determinations.

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arXiv2007

On the behaviour of B and Be stars at low metallicity

We present new results obtained with the VLT GIRAFFE for a large sample of B and Be stars belonging to the Magellanic Clouds, i.e. at low metallicity. First, we show the effects of the metallicity of the environment on their rotation (linear, angular, and at the ZAMS). Second, we present the analysis of the effects of metallicity and evolution on the appearance of Be stars. We also new present results about the proportions of Be stars to B stars. Third, by cross-correlation with large photometric surveys such as MACHO and OGLE, we report on the detection for the first time of short-term multi-periodicity in 9 Be stars in the Small Magellanic Cloud, which can be interpreted in terms of pulsations.

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arXiv2007

On the nature of early-type emission line objects in NGC6611

The number and the nature of emission line stars in the young open cluster NGC6611 is still the object of debates. Due to the presence of a strong and variable nebulosity in the cluster, the number of emission line stars is highly depending on the technique and the resolution used for the observations. Thanks to observations with the ESO-WFI, in slitless spectroscopic mode, and with the VLT-GIRAFFE we have been able to disentangle the circumstellar and nebular emissions. We confirm the small number of true emission line objects and we precise their nature: mainly Herbig Be stars.

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arXiv2007

ALBUM: a tool for the analysis of slitless spectra and its application to ESO WFI data

ALBUM is a general-purpose tool to visualize and screen large amounts of slitless spectra. It was developed for a search for emission-line stars in SMC and LMC clusters. The observations were obtained with ESO's Wide Field Imager (WFI) and comprise ~8 million low-resolution spectra. The tool as well as the results of its application to the SMC part of the database are presented. The inferred frequency of Be stars is compared to the one in the higher-metallicity environment of the Milky Way.

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arXiv2007

Keck and ESO-VLT View of the Symmetry of the Ejecta of the XRF/SN 2006aj

Nebular-phase spectra of SN 2006aj, which was discovered in coincidence with X-ray flash 060218, were obtained with Keck in 2006 July and the Very Large Telescope in 2006 September. At the latter epoch spectropolarimetry was also attempted, yielding an upper limit of ~ 2% for the polarization. The spectra show strong emission lines of [OI] and MgI], as expected from a Type Ic supernova, but weak CaII lines. The [FeII] lines that were strong in the spectra of SN 1998bw are much weaker in SN 2006aj, consistent with the lower luminosity of this SN. The outer velocity of the line-emitting ejecta is ~ 8000 km/s in July and ~ 7400 km/s in September, consistent with the relatively low kinetic energy of expansion of SN 2006aj. All emission lines have similar width, and the profiles are symmetric, indicating that no major asymmetries are present in the ejecta at the velocities sampled by the nebular lines (v < 8000 km/s), except perhaps in the innermost part. The spectra were modelled with a non-LTE code. The mass of 56Ni required to power the emission spectrum is ~ 0.20 Msun, in excellent agreement with the results of early light curve modelling. The oxygen mass is ~ 1.5 Msun, again much less than in SN 1998bw but larger by ~ 0.7 Msun than the value derived from the early-time modelling. The total ejected mass is ~ 2 Msun below 8000 km/s. This confirms that SN 2006aj was only slightly more massive and energetic than the prototypical Type Ic SN 1994I, but also indicates the presence of a dense inner core, containing ~ 1 Msun of mostly oxygen and carbon. The presence of such a core is inferred for all broad-lined SNe Ic. This core may have the form of an equatorial oxygen-dominated region, but it is too deep to affect the early light curve and too small to affect the late polarization spectrum.

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arXiv2006

Spectropolarimetric diagnostics of thermonuclear supernova explosions

Even at extragalactic distances, the shape of supernova ejecta can be effectively diagnosed by spectropolarimetry. We present here results for 17 Type Ia supernovae that allow a statistical study of the correlation among the geometric structures and other observable parameters of Type Ia supernovae. These observations suggest that their ejecta typically consist of a smooth, central iron rich core and an outer layer with chemical asymmetries. The degree of this peripheral asphericity is correlated with the light-curve decline rate of Type Ia supernovae. These observations lend strong support to delayed-detonation models of Type Ia supernovae.

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arXiv2006

Be stars in open clusters in the Small Magellanic Cloud

We report on the study of the population of B and Be stars in SMC young clusters, performed with the Wide Field Imager in slitless spectroscopic mode at ESO/T2.2m with a filter centered at Halpha. First, we explain the reduction methods we used and our selection of different types of objects. Second, we present results on the proportion of Be stars in SMC clusters, and we compare this proportion to the one observed in the Milky Way. Finally, we also present results on a statistical study of variability of Be stars with OGLE.

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arXiv2005

Pre-Maximum Spectropolarimetry of the Type Ia SN 2004dt

We report observations of SN 2004dt obtained with the Very Large Telescope of the European Southern Observatory on August 13.30, 2004 when the supernova was more than a week before optical maximum. SN 2004dt showed strong lines of \ion{O}{1}, \ion{Mg}{2}, \ion{Si}{2}, and \ion{Ca}{2} with typical velocities of absorption minimum around 17,000 \kms. The line profiles show material moving at velocities as high as 25,000 \kms in these lines. The observations also reveal absorption lines from \ion{S}{2} and \ion{Si}{3} with a velocity of only 11,000 \kms. The highest velocity in the \ion{S}{2} features can be traced no higher than 15,000 \kms, much lower than those of O, Mg, Si, and Ca. SN 2004dt has a polarization spectrum unlike any previously observed. The variation of the polarization across some \ion{Si}{2} lines approaches 2%, making SN 2004dt the most highly polarized SN Ia ever observed. In contrast, the strong line of O I at 777.4 nm shows little or no polarization signature. The degree of polarization points to a richly-structured partially burned silicon layer with substantial departure from spherical symmetry. A geometry that would account for the observations is one in which the distribution of oxygen is essentially spherically symmetric, but with bubbles of intermediate-mass elements with significant opacity within the oxygen substrate.

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arXiv2003

On The Hydrogen Emission From The Type Ia Supernova 2002ic

The discovery of SN 2002ic and subsequent spectroscopic studies have led to the surprising finding that SN 2002ic is a Type Ia supernova with strong ejecta-circumstellar interaction. Here we show that nearly 1 year after the explosion the supernova has become fainter overall, but the H$α$ emission has brightened and broadened dramatically compared to earlier observations. We have obtained spectropolarimetry data which show that the hydrogen-rich matter is highly aspherically distributed. These observations suggest that the supernova exploded inside a dense, clumpy, disk-like circumstellar environment.

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arXiv2003

Spectropolarimetry of SN 2001el in NGC 1448: Asphericity of a Normal Type Ia Supernova

High-quality spectropolarimetry (range 417-860 nm; spectral resolution 1.27 nm and 0.265 nm/pixel) of the SN Ia 2001el were obtained with the ESO Very Large Telescope Melipal (+ FORS1) at 5 epochs. The spectra a week before maximum and around maximum indicate photospheric expansion velocities of about 10,000 km s$^{-1}$. Prior to optical maximum, the linear polarization of the continuum was $\approx 0.2 - 0.3 %$ with a constant position angle, showing that SN 2001el has a well-defined axis of symmetry. The polarization was nearly undetectable a week after optical maximum. The spectra are similar to those of the normally-bright SN 1994D with the exception of a strong double-troughed absorption feature seen around 800 nm (FWHM about 22 nm). This feature could be an important clue to the binary nature of SN Ia, perhaps associated with an accretion disk, or to the nature of the thermonuclear burning. If modeled in terms of an oblate spheroid, the continuum polarization implies a minor to major axis ratio of around 0.9 if seen equator-on; this level of asymmetry would produce an absolute luminosity dispersion of about 0.1 mag when viewed at different viewing angles. If typical for SNe Ia, this would create an RMS scatter of several hundredths of a magnitude around the mean brightness-decline relation.

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arXiv2003

Analysis of the Flux and Polarization Spectra of the Type Ia Supernova SN 2001el: Exploring the Geometry of the High-velocity Ejecta

SN 2001el is the first normal Type Ia supernova to show a strong, intrinsic polarization signal. In addition, during the epochs prior to maximum light, the CaII IR triplet absorption is seen distinctly and separately at both normal photospheric velocities and at very high velocities. The high-velocity triplet absorption is highly polarized, with a different polarization angle than the rest of the spectrum. The unique observation allows us to construct a relatively detailed picture of the layered geometrical structure of the supernova ejecta: in our interpretation, the ejecta layers near the photosphere (v \approx 10,000 km/s) obey a near axial symmetry, while a detached, high-velocity structure (v \approx 18,000-25,000 km/s) with high CaII line opacity deviates from the photospheric axisymmetry. By partially obscuring the underlying photosphere, the high-velocity structure causes a more incomplete cancellation of the polarization of the photospheric light, and so gives rise to the polarization peak and rotated polarization angle of the high-velocity IR triplet feature. In an effort to constrain the ejecta geometry, we develop a technique for calculating 3-D synthetic polarization spectra and use it to generate polarization profiles for several parameterized configurations. In particular, we examine the case where the inner ejecta layers are ellipsoidal and the outer, high-velocity structure is one of four possibilities: a spherical shell, an ellipsoidal shell, a clumped shell, or a toroid. The synthetic spectra rule out the spherical shell model, disfavor a toroid, and find a best fit with the clumped shell. We show further that different geometries can be more clearly discriminated if observations are obtained from several different lines of sight.

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arXiv2002

Spectropolarimetry of the Type Ic SN 2002ap in M74: More Evidence for Asymmetric Core Collapse

High-quality spectropolarimetric data of SN 2002ap were obtained with the ESO Very Large Telescope Melipal (+ FORS1) at 3 epochs that correspond to -6, -2, and +1 days for a V maximum of 9 Feb 2002. The polarization spectra show three distinct broad features at 400, 550, and 750 nm that evolve in shape, amplitude and orientation in the Q-U plane. The continuum polarization grows from nearly zero to ~0.2 percent. The 750 nm feature is polarized at a level > 1 %. We identify the 550 and 750 nm features as Na I D and OI 777.4 nm moving at about 20,000 km/s. The blue feature may be Fe II. We interpret the polarization evolution in terms of the impact of a bipolar flow from the core that is stopped within the outer envelope of a carbon/oxygen core. Although the symmetry axis remains fixed, as the photosphere retreats by different amounts in different directions due to the asymmetric velocity flow and density distribution, geometrical blocking effects in deeper, Ca-rich layers can lead to a different dominant axis in the Q-U plane. We conclude that the features that characterize SN 2002ap, specifically its high velocity, can be accounted for in an asymmetric model with a larger ejecta mass than SN 1994I such that the photosphere remains longer in higher velocity material. The characteristics of "hypernovae" may be the result of orientation effects in a mildly inhomogeneous set of progenitors, rather than requiring an excessive total energy or luminosity. In the analysis of asymmetric events with spherically symmetric models, it is probably advisable to refer to "isotropic equivalent" energy, luminosity, ejected mass, and nickel mass.

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