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W. Brandner

Publications and source records attributed to W. Brandner.

At least 217 records · Page 12Linked to original sources
arXiv2006

Resolved Hubble Space spectroscopy of ultracool binary systems

Using the low-resolution mode of the Space Telescope Imaging Spectrograph (STIS) aboard the \emph{Hubble Space Telescope} (HST), we have obtained spatially resolved spectra of 20 ultracool dwarfs. 18 of them belong to 9 known very low-mass binary systems with angular separations in the range 0.37-0.098 arcseconds. We have derived spectral types in the range dM7.5 to dL6 from the PC3 index, and by comparing our STIS spectra with ground-based spectra of similar spectral resolution from Mart{\'ı}n et al. (1999). We have searched for H$_α$ emission in each object but it was clearly detected in only 2 of them. We find that the distribution of H$_α$ emission in our sample is statistically different from that of single field dwarfs, suggesting an intriguing anticorrelation between chromospheric activity and binarity for M7--M9.5 dwarfs. We provide measuments of the strength of the main photospheric features and the PC3 index, and we derive calibrations of spectral subclasses versus F814W and K-band absolute magnitudes for a subset of 10 dwarfs in 5 binaries that have known trigonometric parallaxes.

physics.space-ph↗
arXiv2006

Discovery of a Very Nearby Brown Dwarf to the Sun: A Methane Rich Brown Dwarf Companion to the Low Mass Star SCR 1845-6357

We present VLT/NACO SDI images of the very nearby star SCR 1845-6357 (hereafter SCR 1845). SCR 1845 is a recently discovered (Hambly et al. 2004) M8.5 star just 3.85 pc from the sun (Henry et al. 2006). Using the capabilities of the unique SDI device, we discovered a substellar companion to SCR 1845 at a separation of 4.5 AU (1.170''+-0.003'' on the sky) and fainter by 3.57+-0.057 mag in the 1.575 um SDI filter. This substellar companion has an H magnitude of 13.16+0.31-0.26 (absolute H magnitude of 15.30+0.31-0.26), making it likely the brightest mid-T dwarf known. The unique Simultaneous Differential Imager (SDI) consists of 3 narrowband filters placed around the 1.6 um methane absorption feature characteristic of T-dwarfs (Teff < 1200 K). The flux of the substellar companion drops by a factor of 2.7+-0.1 between the SDI F1(1.575 um) filter and the SDI F3(1.625 um) filter, consistent with strong methane absorption in a substellar companion. We estimate a spectral type of T5.5+-1 for the companion based on the strength of this methane break. The chances that this object is a background T dwarf are vanishing small -- and there is no isolated background T-dwarf in this part of the sky according to 2MASS. Thus, it is a bound companion, hereafter SCR 1845-6357B. For an age range of 100 Myr - 10 Gyr and spectral type range of T4.5-T6.5, we find a mass range of 9 - 65 MJup for SCR 1845B from the Baraffe et al. 2003 COND models. SCR 1845AB is the 24th closest stellar system to the Sun (at 3.85 pc); the only brown dwarf system closer to the Sun is Eps Indi Ba-Bb (at 3.626 pc). In addition, this is the first T-dwarf companion discovered around a low mass star.

astro-ph↗
arXiv2005

Multiplicity of very low-mass objects in the Upper Scorpius OB association: a possible wide binary population

We report the initial results of a VLT/NACO high spatial resolution imaging survey for multiple systems among 58 M-type members of the nearby Upper Scorpius OB association. Nine pairs with separations below 100 have been resolved. Their small angular separations and the similarity in the brightness of the components (DMagK <1 for all of them), indicate there is a reasonable likelihood several of them are true binaries rather than chance projections. Follow-up imaging observations with WHT/LIRIS of the two widest binaries confirm that their near-infrared colours are consistent with physical very low mass binaries. For one of these two binaries, WHT/LIRIS spectra of each component were obtained. We find that the two components have similar M6-M7 spectral types and signatures of low-gravity, as expected for a young brown dwarf binary in this association. Our preliminary results indicate a possible population of very low-mass binaries with semimajor axis in the range 100 AU 150 AU, which has not been seen in the Pleiades open cluster. If these candidates are confirmed (one is confirmed by this work), these results would indicate that the binary properties of very low-mass stars and brown dwarfs may depend on the environment where they form.

astro-ph↗
arXiv2005

The low-mass pre-main sequence population of the stellar association LH 52 in the Large Magellanic Cloud discovered with Hubble Space Telescope WFPC2 Observations

We report on the serendipitous discovery of ~ 500 low-mass candidate PMS stars in the vicinity of the stellar association LH 52 in the Large Magellanic Cloud. We present evidence that the red faint sequence of these stars seen in the CMD of LH 52 from HST/WFPC2 observations belongs only to the association and follows almost perfectly isochrone models for PMS stars of masses down to ~ 0.3 M_solar. We find that this feature has a Galactic counterpart and that the mass spectrum of the candidate PMS stars in LH 52 seems to correspond to a Salpeter IMF with a slope Gamma ~ -1.26 in the mass range 0.8 - 1.4 M_solar.

astro-ph↗
arXiv2005

On the dynamics of the ABDoradus system

We present an astrometric analysis of the binary systems ABDorA /ABDorC and ABDorBa / ABDorBb. These two systems of well-known late-type stars are gravitationally associated and they constitute the quadruple ABDoradus system. From the astrometric data available at different wavelengths, we report: (i) a determination of the orbit of ABDorC, the very low mass companion to ABDorA, which confirms the mass estimate of 0.090Msun reported in previous works; (ii) a measurement of the parallax of ABDorBa, which unambiguously confirms the long-suspected physical association between this star and ABDorA; and (iii) evidence of orbital motion of ABDorBa around ABDorA, which places an upper bound of 0.4Msun on the mass of the pair ABDorBa / ABDorBb (50% probability). Further astrometric monitoring of the system at all possible wavelengths would determine with extraordinary precision the dynamical mass of its four components.

astro-ph↗
arXiv2005

A Hubble Space Telescope ACS Search for Brown Dwarf Binaries in the Pleiades Open Cluster

We present the results of a high-resolution imaging survey for brown dwarf binaries in the Pleiades open cluster. The observations were carried out with the Advance Camera for Surveys onboard the Hubble Space Telescope. Our sample consists of 15 bona-fide brown dwarfs. We confirm 2 binaries and detect their orbital motion, but we did not resolve any new binary candidates in the separation range between 5.4AU and 1700AU and masses in the range 0.035--0.065~Msun. Together with the results of our previous study (Martin et al., 2003), we can derive a visual binary frequency of 13.3$^{+13.7}_{-4.3}$\% for separations greater than 7~AU masses between 0.055--0.065~M$_{\sun}$ and mass ratios between 0.45--0.9$<q<$1.0. The other observed properties of Pleiades brown dwarf binaries (distributions of separation and mass ratio) appear to be similar to their older counterparts in the field.

astro-ph↗
arXiv2005

AB Doradus C: Age, Spectral Type, Orbit, and Comparison to Evolutionary Models

We expand upon the results of Close et al. 2005 regarding the young, low-mass object AB Dor C and its role as a calibration point for theoretical tracks. We present an improved spectral reduction and a new orbital solution with two additional epochs. Our improved analysis confirms our spectral type of M8 (+/- 1) and mass of 0.090+/-0.003 solar masses for AB Dor C. Comparing the results for AB Dor C with other young, low-mass objects with dynamical masses we find a general trend where current evolutionary models tend to over-predict the temperature (or under-predict the mass) for low mass stars and brown dwarfs. Given our precision, there is a ~99% chance that the mass of AB Dor C is underestimated by the DUSTY tracks in the HR diagram.

astro-ph↗
arXiv2005

The low-mass initial mass function of the field population in the Large Magellanic Cloud with Hubble Space Telescope WFPC2 Observations

We present V- and I-equivalent HST/WFPC2 stellar photometry of an area in the Large Magellanic Cloud (LMC), located on the western edge of the bar of the galaxy, which accounts for the general background field of its inner disk. The WFPC2 observations reach magnitudes as faint as V=25 mag, and the large sample of more than 80,000 stars allows us to determine in detail the Present-Day Mass Function (PDMF) of the detected main-sequence stars, which is identical to the Initial Mass Function (IMF) for masses M <~ 1 M_solar. The low-mass main-sequence mass function of the LMC field is found not to have a uniform slope throughout the observed mass range, i.e. the slope does not follow a single power law. This slope changes at about 1 M_solar to become more shallow for stars with smaller masses down to the lowest observed mass of ~ 0.7 M_solar, giving clear indications of flattening for even smaller masses. We verified statistically that for stars with M <~ 1 M_solar the IMF has a slope Gamma around -2, with an indicative slope Gamma =~ -1.4 for 0.7 <~ M/M_solar <~ 0.9, while for more massive stars the main-sequence mass function becomes much steeper with Gamma =~ -5. The main-sequence luminosity function (LF) of the observed field is in very good agreement with the Galactic LF as it was previously found. Taking into account several assumptions concerning evolutionary effects, which should have changed through time the stellar content of the observed field, we reconstruct qualitatively its IMF for the whole observed mass range (0.7 <~ M/M_solar <~ 2.3) and we find that the number of observed evolved stars is not large enough to have affected significantly the form of the IMF, which thus is found almost identical to the observed PDMF.

astro-ph↗
arXiv2005

GRAVITY: The AO-Assisted, Two-Object Beam-Combiner Instrument

We present the proposal for the infrared adaptive optics (AO) assisted, two-object, high-throughput, multiple-beam-combiner GRAVITY for the VLTI. This instrument will be optimized for phase-referenced interferometric imaging and narrow-angle astrometry of faint, red objects. Following the scientific drivers, we analyze the VLTI infrastructure, and subsequently derive the requirements and concept for the optimum instrument. The analysis can be summarized with the need for highest sensitivity, phase referenced imaging and astrometry of two objects in the VLTI beam, and infrared wavefront-sensing. Consequently our proposed instrument allows the observations of faint, red objects with its internal infrared wavefront sensor, pushes the optical throughput by restricting observations to K-band at low and medium spectral resolution, and is fully enclosed in a cryostat for optimum background suppression and stability. Our instrument will thus increase the sensitivity of the VLTI significantly beyond the present capabilities. With its two fibers per telescope beam, GRAVITY will not only allow the simultaneous observations of two objects, but will also push the astrometric accuracy for UTs to 10 micro-arcsec, and provide simultaneous astrometry for up to six baselines.

astro-ph↗
arXiv2005

The Initial Mass Function toward the low-mass end in the Large Magellanic Cloud with HST/WFPC2 Observations

We present V and I equivalent HST/WFPC2 photometry of two areas in the Large Magellanic Cloud: The southern part of the stellar association LH 52, located on the western edge of the super-shell LMC 4, and a field between two associations, which is located on the southwestern edge of the shell, and which accounts for the general background field of the galaxy. The HST/WFPC2 observations reach magnitudes as faint as V=25 mag, much deeper than have been observed earlier in stellar associations in the LMC. We determine the MF for main-sequence stars in the areas. Its slope (Gamma) in both areas is steeper for stars with masses M < 2 M_solar (-4 < Gamma < -6), compared with stars of M > 2 M_solar (-1 < Gamma < -2). Thus, as far as the field of the LMC concerns the MF does not have a uniform slope throughout its observed mass range. The MF of the general field of the LMC was found previously to be steeper than the MF of a stellar association for massive stars with M > 5 M_solar. We conclude that this seems to be also the case toward lower masses down to M ~ 1 M_solar. Our data allow to construct the field-subtracted, incompleteness-corrected, main-sequence MF of the southwestern part of the young stellar association LH 52, which accounts for the Initial Mass Function (IMF) of the system. Its mean slope is found to be comparable, but still more shallow than a typical Salpeter IMF (Gamma = -1.12 +/- 0.24) for masses down to ~ 1 M_solar. We found indications that the IMF of the association probably is ``top-heavy'', due to the large number of intermediate-mass stars in the field of the system, while the general LMC field is found to be responsible for the low-mass population, with M < 2 M_solar, observed in both fields.

astro-ph↗
arXiv2005

A NICMOS Direct Imaging Search for Giant Planets around the Single White Dwarfs in the Hyades

We report preliminary results from our search for massive giant planets (6-12 Jupiter masses) around the known seven single white dwarfs in the Hyades cluster at sub-arcsec separations. At an age of 625 Myr, the white dwarfs had progenitor masses of about 3 solar masses, and massive gaseous giant planets should have formed in the massive circumstellar disks around these ex-Herbig A0 stars, probably at orbital separations similar or slightly larger than that of Jupiter. Such planets would have survived the post-Main-Sequence mass loss of the parent star and would have migrated outward adiabatically to a distance of about 25 AU. At the distance of the Hyades (45 pc) this corresponds to an angular separation of 0.5 arcsec. J and H magnitudes of these giants are in the range of 20.5-23.3 mag, which can be resolved with NICMOS. The achieved sensitivities and contrast ratios agree well with simulations. Preliminary evaluation of the NICMOS data set did not reveal any evidence for neither planetary mass companions with masses down to about 10 Jupiter masses nor brown dwarfs around any of the seven white dwarfs for separations larger than 0.5 arcsec.

astro-ph↗
arXiv2005

The sub-arcsecond dusty environment of Eta Carinae

The core of the nebula surrounding Eta Carinae has been observed with the VLT Adaptive Optics system NACO and with the interferometer VLTI/MIDI to constrain spatially and spectrally the warm dusty environment and the central object. In particular, narrow-band images at 3.74 and 4.05 micron reveal the butterfly shaped dusty environment close to the central star with unprecedented spatial resolution. A void whose radius corresponds to the expected sublimation radius has been discovered around the central source. Fringes have been obtained in the Mid-IR which reveal a correlated flux of about 100Jy situated 0.3" south-east of the photocenter of the nebula at 8.7 micron, which corresponds with the location of the star as seen in other wavelengths. This correlated flux is partly attributed to the central object, and these observations provide an upper limit for the SED of the central source from 2.2 to 13.5 micron. Moreover, we have been able to spectrally disperse the signal from the nebula itself at PA=318 degree, i.e. in the direction of the bipolar nebula 310 degree) within the MIDI field of view of 3". A large amount of corundum (Al2O3) is discovered, peaking at 0.6-1.2" south-east from the star, whereas the dust content of the Weigelt blobs is dominated b silicates. We discuss the mechanisms of dust formation which are closely related to the geometry of this Butterfly nebulae.

astro-ph↗
arXiv2004

A possible third component in the L dwarf binary system DENIS-P J020529.0-115925 discovered with the Hubble Space Telescope

We present results showing that the multiple system DENIS-P J020529.0-115925 is likely to be a triple system of brown dwarfs. The secondary of this previously known binary system shows a clear elongation on six images obtained at six different epochs. Significant residuals remain after PSF subtraction on these images, characteristic of multiplicity, and indicating that the secondary is probably a double itself. Dual-PSF fitting shows that the shape of the secondary is consistent with that of a binary system. These measurements show that the probability that DENIS-P J020529.0-115925 is a triple system is very high. The photometric and spectroscopic properties of the system are consistent with the presence of three components with spectral types L5, L8 and T0.

astro-ph↗
arXiv2004

First determination of the dynamical mass of a binary L dwarf

We present here the results of astrometric, photometric and spectroscopic observations leading to the determination of the orbit and dynamical masses of the binary L dwarf 2MASSW J0746425+2000321. High angular resolution observations spread over almost 4 years and obtained with the Hubble Space Telescope (HST), the ESO Very Large Telescope (VLT), and a the W. M. Keck Observatory (Keck) allow us to cover 36% of the period, corresponding to 60% of the orbit, and, for the first time, to derive a precise estimate of the total and individual masses of such a late-type object. We find an orbital period of 3850.9$^{+904}_{-767}$ days. The corresponding total mass is 0.146$^{+0.016}_{-0.006}$ M$_{\sun}$, with uncertainties depending on the distance. Spatially resolved low resolution optical (550--1025 nm) spectra have been obtained with HST/STIS, allowing us to measure the spectral types of the two components (L0$\pm$0.5 for the primary and L1.5$\pm$0.5 for the secondary). We also present precise photometry of the individual components measured on the high angular resolution images obtained with HST/ACS and WFPC2 (visible), VLT/NACO (J, H and Ks bands) and Keck I (Ks) band). These spectral and photometric measurements enable us to estimate their effective temperatures and mass ratio, and to place the object accurately in a H--R diagram. The binary system is most likely formed by a primary with a mass of 0.085$\pm$0.010 M$_{\sun}$ and a secondary with a mass of 0.066$\pm$0.006 M$_{\sun}$, thus clearly substellar, for an age of approximately 300$\pm$150 Myr. H$α$ variability indicates chromospheric and/or magnetic activity.

astro-ph↗
arXiv2004

A young binary Brown Dwarf in the R-CrA star formation region

We present imaging and spectroscopic observations with HST (WFPC2, ACS/HRC and STIS), VLT (FORS2) and Keck (HIRES) of the dM8 ultra-cool dwarf DENIS-P J185950.9-370632, located in the R-CrA region. The presence of lithium absorption at 670.8 nm and the strong Halpha emission indicate a young age and a sub-stellar mass. Our diffraction-limited images resolve a companion at the separation limit of HST/ACS (~0.06"). The 2.1 mJy flux in the LW2 filter (5.0-8.5 microns) of the Infrared Space Observatory (see 1999A&A...350..883) likely corresponds to an infrared excess, suggesting the presence of circumstellar material. Proper motion and photometric measurements, as well as the Halpha activity, confirm membership in the R-CrA star forming region. If confirmed by further observations, DENIS-P J185950.9-370632 would be the first accreting sub-stellar multiple system observed to date.

astro-ph↗
arXiv2004

Search for substellar companions using AO - first results obtained with NAOS-CONICA

We present first results of our NAOS-CONICA search for close substellar companions around young nearby stars. This program was started only a few months ago. We have obtained 1st epoch images of several targets which are unpublished young stars (age<100 Myrs), hence ideal targets to look for planetary companions. For one target star we could even take a 2nd epoch image. By comparing both images we could look for co-moving companions of the target star. Those data show clearly that the detection of planetary companions m <13 Mjup inward a saturn-like orbit (r<10 AU) is feasible with NAOS-CONICA and in addition that the astrometric confirmation of those companions is doable with only a few weeks of epoch difference.

astro-ph↗
arXiv2004

HD 77407 and GJ 577: two new young stellar binaries detected with the Calar Alto Adaptive Optics system ALFA

We present the first results from our search for close stellar and sub-stellar companions to young nearby stars on the northern sky. Our infrared imaging observations are obtained with the 3.5 m Calar Alto telescope and the AO system ALFA. With two epoch observations which were separated by about one year, we found two co-moving companion candidates, one close to HD 77407 and one close to GJ 577. For the companion candidate near GJ 577, we obtained an optical spectrum showing spectral type M4.5; this candidate is a bound low-mass stellar companion confirmed by both proper motion and spectroscopy. We estimate the masses for HD 77407 B and GJ 577 B to be ~0.3 to 0.5 Msun and ~0.16 to 0.2 Msun, respectively. Compared to Siess al.(2000) models, each of the two pairs appears co-eval with HD 77407 A,B being 10 to 40 Myrs old and GJ 577 A,B being older than 100 Myrs. We also took multi-epoch high-resolution spectra of HD 77407 to search for sub-stellar companions, but did not find any with 3 Mjup as upper mass (msin(i)) limit (for up to 4 year orbits); however, we detected a long-term radial velocity trend in HD 77407 A, consistent with a ~ 0.3 Msun companion at ~ 50 AU separation, i.e. the one detected by the imaging. Hence, HD 77407 B is confirmed to be a bound companion to HD 77407 A. We also present limits for undetected, but detectable companions using a deep image of HD 77407 A and B, also observed with the Keck NIRC2 AO system; any brown dwarfs were detectable outside of 0.5 arcsec (17 AU at HD 77407), giant planets with masses from ~ 6.5 to 12 Mjup were detectable at > 1.5 arcsec.

astro-ph↗