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R. Neuhäuser

Publications and source records attributed to R. Neuhäuser.

At least 91 records · Page 5Linked to original sources
arXiv2008

Searching for substellar companions of young isolated neutron stars

Context: Only two planetary systems around old ms-pulsars are currently known. Young radio pulsars and radio-quiet neutron stars cannot be covered by the usually-applied radio pulse timing technique. However, finding substellar companions around these neutron stars would be of great interest -- not only because of the companion's possible exotic formation but also due to the potential access to neutron star physics. Aims: We investigate the closest young neutron stars to search for substellar companions around them. Methods: Young, thus warm substellar companions are visible in the Near Infrared while the neutron star itself is much fainter. Four young neutron stars are moving fast enough to enable a common proper motion search for substellar companions within few years. Results. For Geminga, RX J0720.4-3125, RX J1856.6-3754, and PSR J1932+1059 we did not find any co-moving companion down to 12, 15, 11, 42 Jupiter masses for assumed ages of 1, 1, 1, 3.1 Myrs and distances of 250, 361, 167, 361 pc, respectively. Near Infrared limits are presented for these four as well as five other neutron stars for which we currently have only observations at one epoch. Conclusions: We conclude that young isolated neutron stars rarely have brown dwarf companions.

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arXiv2008

Direct evidence of a sub-stellar companion around CT Cha

In our ongoing search for close and faint companions around T Tauri stars, we found a very faint (Ks=14.9mag, Ks_0=14.4mag) object, just ~2.67" northwest of the Chamaeleon star-forming region member CT Cha corresponding to a projected separation of ~440AU at 165+/-30 pc. We show that CT Cha A and this faint object form a common proper motion pair from data of the VLT Adaptive Optics (AO) instrument NACO taken in February 2006 and March 2007 and that the companion is by >=4 sigma significance not a stationary background object. Our AO integral field spectroscopy with SINFONI in J, and H+K bands yields a temperature of 2600+/-250K for the companion and an optical extinction of A_V=5.2+/-0.8mag, when compared to spectra calculated from Drift-Phoenix model atmospheres. We demonstrate the validity of the model fits by comparison to several other well-known young sub-stellar objects. Relative flux calibration of the bands was achieved using photometry from the NACO imaging data. We conclude that the CT Cha companion is a very low-mass member of Chamaeleon and very likely a physical companion to CT Cha, as the probability for a by chance alignment is <=0.01. Due to a prominent Pa-Beta emission in the J-band, accretion is probably still ongoing onto the CT Cha companion. From temperature and luminosity (log(Lbol/Lsun)= -2.68+/-0.21), we derive a radius of R=2.20+0.81-0.60 R_Jup. We find a consistent mass of M=17+/-6 MJup for the CT Cha companion from both its luminosity and temperature when placed on evolutionary tracks. Hence, the CT Cha companion is most likely a wide brown dwarf companion or possibly even a planetary mass object.

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arXiv2008

Multiplicity of young stars in and around R Corona Australis

The R CrA star-forming region has a small dark cloud with quite a number of protostars, T Tauri stars, and some Herbig Ae/Be stars, plus a number of weak-line T Tauri stars around the cloud found by ROSAT follow-up observations. We searched for multiples among the young stars in and around the R CrA cloud in order to investigate multiplicity in this region. We performed interferometric and imaging observations with the speckle camera SHARP I at the ESO 3.5m NTT and adaptive optics observation with ADONIS at the ESO 3.6m telescope, all in the near-infrared bands JHK obtained in the years 1995, 2000, and 2001. We found 13 new binaries among the young stars in CrA between 0.13 arcsec (the diffraction limit) and 6 arcsec (set as an upper separation limit to avoid contamination by chance alignments). While most multiples in CrA are binaries, there are also one quadruple (TY CrA), and one triple (HR 7170) which may form a quintuple together with the binary HR 7169. One of the newly detected companions with a large magnitude difference found near the M3-5 type T Tauri star [MR 81] Ha 17 could be a brown dwarf or an infrared companion with an edge-on disk. Among seven Herbig Ae/Be stars in CrA, six are multiple. The multiplicity frequency in CrA is as high as in similar star forming regions. By comparing with the period distribution of main-sequence stars and extrapolating to separations not probed in this survey, we conclude that the companion-star frequency is 95+/-23 %; i.e. the average number of companions per primary is 0.95.

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arXiv2008

Search for extrasolar planets with high-precision relative astrometry

We present our search program for substellar companions using high-precision relative astronomy. Due to its orbital motion around the star, an unseen substellar companion would produce a periodic "wobble" of the host star, which is the astrometric signal of the unseen companion. By measuring the separation between the components of stellar double and triple systems, we want to measure this astrometric signal of a possible unseen companion indirectly as a relative and periodic change of these separations. Using a new observation mode (the "cube-mode") where the frames were directly saved in cubes with nearly no loss of time during the readout, an adaptive optics system to correct for atmospheric noise and an infrared narrow band filter in the near infrared to suppress differential chromatic refraction (DCR) effects we achive for our first target (the double star HD 19994) a relative precision for the separation measurements of about 100...150 micro-arsecond per epoch. To reach a precision in the micro-arcsec-regime, we use a statistical approach. We take several thousand frames per target and epoche and after a verification of a Gaussian distribution the measurement precision can be calculated as the standard deviation of our measurements divided by the square root of the number of Gaussian distributed measurements. Our first observed target is the stellar binary HD 19994 A & B, where the A component has a known radial velocity planet candidate.

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arXiv2008

Confirmation of the binary status of Cha Halpha 2 - a very young low-mass binary in Chamaeleon

Neuhaeuser & Comeron (1998, 1999) presented direct imaging evidence, as well as first spectra, of several young stellar and sub-stellar M6- to M8-type objects in the Cha I dark cloud. One of these objects is Cha Halpha 2, classified as brown dwarf candidate in several publications and suggested as possible binary in Neuhaeuser et al. (2002). We have searched around Cha Halpha 2 for close and faint companions with adaptive optics imaging. Two epochs of direct imaging data were taken with the Very Large Telescope (VLT) Adaptive Optics instrument NACO in February 2006 and March 2007 in Ks-band. We retrieved an earlier image from 2005 from the European Southern Observatory (ESO) Science Archive Facility, increasing the available time coverage. After confirmation of common proper motion, we deduce physical parameters of the objects by spectroscopy, like temperature and mass. We find Cha Halpha 2 to be a very close binary of ~0.16 arcsec separation, having a flux ratio of ~0.91, thus having almost equal brightness and indistinguishable spectral types within the errors. We show that the two tentative components of Cha Halpha 2 form a common proper motion pair, and that neither component is a non-moving background object. We even find evidence for orbital motion. A combined spectrum of both stars spanning optical and near-infrared parts of the spectral energy distribution yields a temperature of 3000+/-100 K, corresponding to a spectral type of M6+/-1 and a surface gravity of log g= 4.0 +0.75-0.5, both from a comparison with GAIA model atmospheres. We derive masses of ~0.110 Msun (>0.070 Msun) and ~0.124 Msun (>0.077 Msun) for the two components of Cha Halpha 2, i.e., probably low-mass stars, but one component could possibly be a brown dwarf.

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arXiv2008

Milli-arcsecond astrophysics with VSI, the VLTI spectro-imager in the ELT era

Nowadays, compact sources like surfaces of nearby stars, circumstellar environments of stars from early stages to the most evolved ones and surroundings of active galactic nuclei can be investigated at milli-arcsecond scales only with the VLT in its interferometric mode. We propose a spectro-imager, named VSI (VLTI spectro-imager), which is capable to probe these sources both over spatial and spectral scales in the near-infrared domain. This instrument will provide information complementary to what is obtained at the same time with ALMA at different wavelengths and the extreme large telescopes.

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arXiv2007

Space cowboys odyssey: beyond the Gould Belt

We present our new advanced model for population synthesis of close-by cooling NSs. Detailed treatment of the initial spatial distribution of NS progenitors and a detailed ISM structure up to 3 kpc give us an opportunity to discuss the strategy to look for new isolated cooling NSs. Our main results in this respect are the following: new candidates are expected to be identified behind the Gould Belt, in directions to rich OB associations, in particular in the Cygnus-Cepheus region; new candidates, on average, are expected to be hotter than the known population of cooling NS. Besides the usual approach (looking for soft X-ray sources), the search in 'empty' $γ$-ray error boxes or among run-away OB stars may yield new X-ray thermally emitting NS candidates.

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arXiv2007

Simultaneous X-ray, radio, near-infrared, and optical monitoring of Young Stellar Objects in the Coronet cluster

Multi-wavelength (X-ray to radio) monitoring of Young Stellar Objects (YSOs) can provide important information about physical processes at the stellar surface, in the stellar corona, and/or in the inner circumstellar disk regions. While coronal processes should mainly cause variations in the X-ray and radio bands, accretion processes may be traced by time-correlated variability in the X-ray and optical/infrared bands. Several multi-wavelength studies have been successfully performed for field stars and approx. 1-10 Myr old T Tauri stars, but so far no such study succeeded in detecting simultaneous X-ray to radio variability in extremely young objects like class I and class 0 protostars. Here we present the first simultaneous X-ray, radio, near-infrared, and optical monitoring of YSOs, targeting the Coronet cluster in the Corona Australis star-forming region, which harbors at least one class 0 protostar, several class I objects, numerous T Tauri stars, and a few Herbig AeBe stars. [...] Seven objects are detected simultaneously in the X-ray, radio, and optical/infrared bands; they constitute our core sample. While most of these sources exhibit clear variability in the X-ray regime and several also display optical/infrared variability, none of them shows significant radio variability on the timescales probed. We also do not find any case of clearly time-correlated optical/infrared and X-ray variability. [...] The absence of time-correlated multi-wavelength variability suggests that there is no direct link between the X-ray and optical/infrared emission and supports the notion that accretion is not an important source for the X-ray emission of these YSOs. No significant radio variability was found on timescales of days.

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arXiv2005

Multiplicity-Study of Exoplanet Host Stars

We carry out a systematic search campaign for wide companions of exoplanet host stars to study their multiplicity and its influence on the long-term stability and the orbital parameters of the exoplanets. We have already found 6 wide companions, raising the number of confirmed binaries among the exoplanet host stars to 20 systems. We have also searched for wide companions of Gl86, the first known exoplanet host star with a white dwarf companion. Our Sofi/NTT observations are sensitive to substellar companions with a minimum-mass of 35 Mjup and clearly rule out further stellar companions with projected separations between 40 and 670AU.

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arXiv2005

XMM-Newton spectroscopy of the metal depleted T Tauri star TWA 5

We present results of X-ray spectroscopy for TWA 5, a member of the young TW Hydrae association, observed with XMM-Newton. TWA~5 is a multiple system which shows Halpha emission, a signature typical of classical T Tauri stars, but no infrared excess. From the analysis of the RGS and EPIC spectra, we have derived the emission measure distribution vs. temperature of the X-ray emitting plasma, its abundances, and the electron density. The characteristic temperature and density of the plasma suggest a corona similar to that of weak-line T Tauri stars and active late-type main sequence stars. TWA 5 also shows a low iron abundance (~0.1 times the solar photospheric one) and a pattern of increasing abundances for elements with increasing first ionization potential reminiscent of the inverse FIP effect observed in highly active stars. The especially high ratio Ne/Fe~10 is similar to that of the classical T Tauri star TW Hya, where the accreting material has been held responsible for the X-ray emission. We discuss the possible role of an accretion process in this scenario. Since all T Tauri stars in the TW Hydrae association studied so far have very high Ne/Fe ratios, we also propose that environmental conditions may cause this effect.

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arXiv2004

Astrometric confirmation of a wide low-mass companion to the planet host star HD89744

In our ongoing deep infrared imaging search for faint wide secondaries of planet-candidate host stars we have confirmed astrometrically the companionship of a low-mass object to be co-moving with HD89744, a companion-candidate suggested already by Wilson et al. (2000). The derivation of the common proper motion of HD89744 and its companion, which are separated by 62.996+-0.035 arcsec, is based on about 5 year epoch difference between 2MASS and our own UKIRT/UFTI images. The companion effective temperature is about 2200 K and its mass is in the range between 0.072 and 0.081 Msun, depending on the evolutionary model. Therefore, HD89744B is either a very low mass stellar or a heavy brown dwarf companion.

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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.

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arXiv2004

A search for wide (sub)stellar companions around extrasolar planet host stars

We present an overview of our ongoing systematic search for wide (sub)stellar companions around the stars known to host rad-vel planets. By using a relatively large field of view and going very deep, our survey can find all directly detectable stellar and massive brown dwarf companions (m>40 Mjup) within a 1000 AU orbit.

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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.

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arXiv2003

A new strongly X-ray flaring M9 dwarf in the solar neighborhood

We report on the discovery of a very low mass (VLM) star in the solar neighborhood, originally identified as an optical counterpart of a flaring X-ray source detected in the ROSAT All-Sky survey. Optical spectroscopy and infrared photometry consistently reveal a spectral type of M9 \pm 0.5 and a distance of ~11 pm 2pc. The optical counterpart of 1RXS J115928.5-524717 shows a large proper motion of 1.08\pm0.06 "/year. 1RXS~J115928.5-524717 is the fourth object among the VLM stars displaying a huge X-ray flare, reaching the unprecedent value of L_X/L_{bol}~0.1.

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arXiv2003

Flare stars in the TW Hydrae association: The HIP 57269 system

We discuss a new member candidate of the TW Hydrae association (TWA) among the stars of the Gershberg et al. (1999) flare star catalog. TWA is one of the closest known associations of young stars at about 60 pc. Three supposedly young flare stars are located in the same region of the sky as TWA. One of them (HIP 57269) shows strong Lithium absorption with spectral type K1/K2V and a high level of chromospheric and coronal activity. It is located at a distance of 48.7\pm6.3 pc in common with the five TWA members observed with Hipparcos (46.7 to 103.9 pc). HIP 57268 A has a wide companion C which also shows Lithium absorption at 6707Åand which has common proper motion with HIP 57269, as well as a close companion resolved visually by Tycho. HIP 57269A&C lie above the main sequence and are clearly pre-main-sequence stars. The UVW-space velocity is more consistent with the star system being a Pleiades super cluster member. The two other flare stars in the TWA sky region do not show Lithium at all and are, hence, unrelated.

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arXiv2003

Rotational periods of very young brown dwarfs and very low-mass stars in ChaI

We have studied the photometric variability of very young brown dwarfs and very low-mass stars (masses well below 0.2 M_sun) in the ChaI star forming region. We have determined photometric periods in the Gunn i and R band for the three M6.5-M7 type brown dwarf candidates ChaHa2, ChaHa3 and ChaHa6 of 2.2 to 3.4 days. These are the longest photometric periods found for any brown dwarf so far. If interpreted as rotationally induced they correspond to moderately fast rotational velocities, which is fully consistent with their v sini values and their relatively large radii. We have also determined periods for the two M5-M5.5 type very low-mass stars B34 and CHXR78C. In addition to the Gunn i and R band data, we have analysed JHK_s monitoring data of the targets, which have been taken a few weeks earlier and confirm the periods found in the optical data. Upper limits for the errors in the period determination are between 2 and 9 hours. The observed periodic variations of the brown dwarf candidates as well as of the T Tauri stars are interpreted as modulation of the flux at the rotation period by magnetically driven surface features, on the basis of a consistency with v sini values as well as (R-i) color variations typical for spots. Furthermore, the temperatures even for the brown dwarfs in the sample are relatively high (>2800K) because the objects are very young. Therefore, the atmospheric gas should be sufficiently ionized for the formation of spots on one hand and the temperatures are too high for significant dust condensation and hence variabilities due to clouds on the other hand.

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