arXiv subjects

Ralph Neuhaeuser

Publications and source records attributed to Ralph Neuhaeuser.

53 records · Page 3Linked to original sources

arXiv2002

Radial-velocity survey of members and candidate members of the TW Hydrae association

We report our spectroscopic observations of stars belonging to the young nearby group known as the TW Hydrae association, as well as of a number of potential members of the association identified in kinematic and X-ray surveys. Multiple radial velocity measurements were obtained for each object, several of which turn out to be multiple systems. Orbital solutions are presented for 3 double-lined binaries, one single-lined binary, and a double-lined triple system, all with short periods. Effective temperatures and projected rotational velocities are presented for each visible object. None of the candidate members of the association in our sample is confirmed as a true member. The large fraction of close binaries among the candidate members has to do with their selection based on X-ray emission from ROSAT, which tends to favor the inclusion of tidally-locked systems that are active but not necessarily young.

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arXiv2002

Infrared imaging and spectroscopy of companion candidates near the young stars HD 199143 and HD 358623 in Capricornius

We present JHK images of the young (~20 Myrs) nearby (~48 pc) stars HD 199143 and HD 358623 to search for (sub-)stellar companions around them. The images were obtained in JHK with the speckle camera SHARP-I in July 2001 and in H with SofI in Dec 2000, July 2001, and Dec 2001, all at the ESO 3.5m NTT. We present a companion candidate with a 2 arc sec offset being almost 2 mag fainter than HD 358623 with proper motion consistent with the known proper motion of the primary star HD 358623 A. Then, we obtained a spectrum in the H-band (with SofI at the NTT) of this companion showing that it has spectral type M1-3, consistent with being a companion of HD 358623 A (K7-M0). Also, a companion candidate with a 1 arcsecond offset being 2 mag fainter than HD 199143 is detected. The JHK colors of this candidate are consistent with a spectral type M0-2. We determine the ages and masses of all four objects from theoretical tracks and isochrones, all four stars appear to be co-eval with ~20 Myrs. Our limits for additional detectable but undetected companions are such that we would have detected all stellar companions with separations > 0.5 arc sec (24 AU at 48 pc).

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arXiv2002

VLT spectra of the companion candidate Cha Ha 5/cc 1

We obtained optical and infrared spectra of Cha Ha 5/cc 1, a faint possibly sub-stellar companion candidate next to the M6-type brown dwarf candidate Cha Ha 5 in Cha I, using FORS1 and ISAAC at the VLT. The VRIJHK colors of Cha Ha 5/cc 1 are consistent with either an L-type companion or a K-type background giant. Our spectra show that the companion candidate actually is a background star.

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arXiv2002

HST, VLT, and NTT imaging search for wide companions to bona-fide and candidate brown dwarfs in the Cha I dark cloud

We present results from a deep imaging search for companions around the young bona-fide and candidate brown dwarfs Cha Ha 1 to 12 in the Cha I dark cloud, performed with HST WFPC2 (R, I, Ha), VLT FORS1 (VRI), and NTT SofI (JHK). We find 16 faint companion candidates around five primaries with separations between 1.5" and 7" and magnitudes in R & I from 19 to 25 mag, i.e. up to 8 mag fainter than the primaries. While most of these companion candidates are probably unrelated background objects, there is one promising candidate, namely 1.5" SW off the M6-dwarf Cha Ha 5. This candidate is 3.8 to 4.7 mag fainter than the primary and its colors are consistent with an early- to mid-L spectral type. Assuming the same distance (140 pc) and absorption (0.47 mag in I) as towards the primary, the companion candidate has log (L(bol)/L(odot) = -3.0 +- 0.3. At the age of the primary (1 to 5 Myrs), the faint object would have a mass of 3 to 15 Jupiter masses according to Burrows et al. (1997) and Chabrier & Baraffe (2000) models. The probability for this companion candidate to be an unrelated fore- or background object is smaller than 0.7%, its colors are marginally consistent with a strongly reddened background K giant. One other companion candidate has infrared colors consistent with an early T-dwarf. In addition, we present indications for Cha Ha 2 being a close (0.2") binary with both components very close to the sub-stellar limit. Our detection limits are such that we should have detected all companions above 1 Jup with separations above 2" (320 AU) and all above 5 Jup at 0.35" (50 AU).

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arXiv2002

Imaging and spectroscopy of DENIS-P J104814-395606

We obtained deep H- and K-band images of DENIS-P J104814-395606 using SofI and the speckle camera SHARP-I at the ESO-3.5m-NTT as well as QUIRC at the Mauna Kea 2.2m telescope between December 2000 and June 2001. The target was recently discovered as nearby M9-dwarf among DENIS sources (Delfosse et al. 2001). We detect parallactic motion on our images and determine the distance to be 4.6 pm 0.3 pc, more precise than previously known. From the available colors, the distance, and the spectral type, we conclude from theoretical models that the star has a mass of ~0.075 to 0.09 M_sun and an age of ~1 to 2 Gyrs. We also obtained H- and K-band spectra of this star with ISAAC at the VLT. A faint companion candidate is detected 6 arc sec NNW of the star, which is 6.4 pm 0.5 mag fainter in H. However, according to another image taken several month later, the companion candidate is not co-moving with the M9-dwarf. Instead, it is a non-moving background object. Limits for undetected companion candidates are such that we can exclude any stellar companions outside of ~0.25 arc sec (1 AU), any brown dwarf companions (above the deuterium burning mass limit) outside of ~2 arc sec (9 AU), and also any companion down to ~40 M_jup with \ge 0.15 arc sec (0.7 AU) separation, all calculated for an age of 2 Gyrs. Our observations show that direct imaging of sub-stellar companions near the deuterium burning mass limit in orbit around nearby ultra-cool dwarfs is possible, even with separations that are smaller than the semi-major axis of the outermost planet in our solar system, namely a few tens of AU.

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arXiv2001

Spectroscopic binaries in a sample of ROSAT X-ray sources south of the Taurus molecular clouds

We report the results of our radial-velocity monitoring of spectroscopic binary systems in a sample of X-ray sources from the ROSAT All Sky Survey south of the Taurus-Auriga star-forming region. The original sample of approximately 120 sources by Neuhaeuser et al. was selected on the basis of their X-ray properties and the visual magnitude of the nearest optical counterpart, in such a way as to promote the inclusion of young objects. Roughly 20% of those sources have previously been confirmed to be very young. We focus here on the subset of the original sample that shows variable radial velocities (43 objects), a few of which have also been flagged previously as being young. New spectroscopic orbits are presented for 42 of those systems. Two of the binaries, RXJ0528.9+1046 and RXJ0529.3+1210, are indeed weak-lined T Tauri stars likely to be associated with the Lambda Orionis region. Most of the other binaries are active objects of the RS CVn-type, including several W UMa and Algol systems. We detect a strong excess of short-period binaries compared to the field, and an unusually large fraction of double-lined systems, as well as an overall high frequency of binaries out of the original sample. These results can be understood as selection effects. A short description of the physical properties of each binary is provided, and a comparison with evolutionary tracks is made using the stellar density as a distance-independent measure of evolution (abridged).

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arXiv2001

Discovery of new embedded Herbig-Haro objects in the rho Ophiuchi dark cloud

We report here the discovery of a 30"-chain of embedded Herbig-Haro (HH) objects in the rho Ophiuchi dark cloud. These HH objects were first detected during a deep K_S-band observation (completeness magnitude for point source~19) made with NTT/SOFI. We confirm their nature with follow-up observations made with H_2 v=1-0 S(1) narrow-band filter. We argue that they belong to two different jets emanating from two Class I protostars: the main component of the recently resolved subarcsecond radio binary YLW15 (also called IRS43), and IRS54. We propose also to identify the [S II] knot HH224NW1 (Gomez et al 1998) as emanating from a counterjet of YLW15. The alignment between these HH objects and the thermal jet candidate found in YLW15 by Girart et al. (2000) implies that this jet is not precessing at least on timescale ~(2-4)x1E4 yr.

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arXiv2001

Towards a Mass and Radius Determination of the Nearby Isolated Neutron Star RX J185635-3754

We discuss efforts to determine the mass, radius, and surface composition of the nearby compact object RX J185635-3754 from its multi-wavelength spectral energy distribution. We compute non-magnetized model atmospheres and emergent spectra for selected compositions and gravities, and discuss efforts to fit existing and new observational data from ROSAT, EUVE and the HST. The spectral energy distribution matches that expected from a heavy-element dominated atmosphere, but not from a uniform temperature blackbody. Non-magnetic light element atmospheres cannot be simultaneously reconciled with the optical and X-ray data. We extend previous studies, which were limited to one fixed neutron star mass and radius. For uniform temperature models dominated by heavy elements, the redshift z is constrained to be 0.3 < z < 0.4 and the best-fit mass and radius are M approx 0.9 solar masses and R approx 6 km (for a 61 pc distance). These values for M and R together are not permitted for any plausible equation of state, including that of a self-bound strange quark star. A simplified two-temperature model allows masses and radii up to about 50% larger, or a factor of 2 in the case of a black body. The observed luminosity is consistent with the thermal emission of an isolated neutron star no older than about 1 million years, the age inferred from available proper motion and parallax information

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arXiv2001

Observing the planet formation time-scale by ground-based direct imaging of planetary companions to young nearby stars: Gemini/Hokupa'a image of TWA-5

Many extra-solar planets and a few planetary systems have been found indirectly by small periodic radial velocity variations around old nearby stars. The orbital characteristics of most of them are different from the planets in our solar system. Hence, planet formation theories have to be revised. Therefore, observational constraints regarding young planets would be very valuable. We have started a ground-based direct imaging search for giant planets in orbit around young nearby stars. Here, we will motivate the sample selection and will present our direct imaging observation of the very low-mass (15 to 40 Jupiter masses) brown dwarf companion TWA-5 B in orbit around the nearby young star TWA-5 A, recently obtained with the 36-element curvature-sensing AO instrument Hokupa'a of the University of Hawai'i at the 8.3m Gemini-North telescope on Mauna Kea. We could achieve a FWHM of 64 mas and 25 % Strehl. We find significance evidence for orbital motion of B around A.

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arXiv2001

Radial-Velocity Monitoring of Members and Candidate Members of the TW Hydrae Association

We present our spectroscopic measurements of the radial velocity, effective temperature, and projected rotational velocity of several of the known members of the TW Hya association, as well as measurements for candidate members selected on the basis of their X-ray or kinematic properties. A number of our targets turn out to be binaries, but most are non-members. The radial velocities for some of the other candidates support the conclusion that they are kinematically associated with the group, although further observations are required to show that they are indeed pre-main sequence objects.

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arXiv2001

Ground-based exoplanet near-infrared search by imaging and spectroscopy: 3 new companion candidates in TWA

We report first results from our ground-based infrared imaging search for sub-stellar companions (brown dwarfs and giant planets) of young (up to 100 Myrs) nearby (up to 100 pc) stars, where companions should be well separated from the central stars and still relatively bright due to ongoing accretion and/or contraction. Our observations are performed mainly with SOFI and SHARP at the ESO 3.5m NTT on La Silla (imaging) and with ISAAC at the ESO 8.2m Antu (VLT-UT1) on Cerro Paranal (imaging and spectroscopy), all in the H- and K-bands. Here, we present new companion candidates around three T Tauri stars (TWA-8 A, RXJ1121.1-3845, and RXJ1121.3-3447 N) in the TW Hya group, which would have been sub-stellar if at the same distance and age as the T Tauri stars, but are found to be background stars by spectroscopy.

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arXiv2001

The proper motion of the neutron star RXJ1856.5-3754 as measured by optical and X-ray imaging

The proper motion of the faint (V=25.6 mag) neutron star RXJ1856.5-3754 as measured consistently with both the FORS1 optical imager on the ESO Very Large Telescope (VLT) as well as with the ROSAT High Resolution Imager is presented. With two VLT images obtained with a 0.95 year epoch difference in Apr 1999 and Apr 2000, a proper motion of mu_alpha = 0.326 \pm 0.064" per year to the east and no detectable motion in declination is found, consistent with simultaneous findings by Walter (2001) using three HST observations. Also, by comparing the positional differences between the strong X-ray source RXJ1856.5-3754 and other X-ray sources detected in the same ROSAT High Resolution Imager field (observed in Oct 1994 and Oct 1997), a proper motion of 0.34 \pm 0.12" to the east is detected. This may be the first star, where a proper motion is clearly detected in X-ray images. Additional VLT images taken in half-year intervalls give an upper limit to the parallax of about 51 mas. Furthermore, in archived VLT observations, the first B-band detection of this object is found with B=25.14 \pm 0.41 mag.

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arXiv2000

Search for young stars among ROSAT All-Sky Survey X-ray sources in and around the R CrA dark cloud

We present the ROSAT All-Sky Survey data in a 126 deg^2 area in and around the CrA star forming region. With low-resolution spectroscopy of unidentified ROSAT sources we could find 19 new pre-main sequence stars, two of which are classical T Tauri stars, the others being weak-lined. The spectral types of these new T Tauri stars range from F7 to M6. The two new classical T Tauri stars are located towards two small cloud-lets outside of the main CrA cloud. They appear to be ~10 Myrs old, by comparing their location in the H-R diagram with isochrones for an assumed distance of 130 pc, the distance of the main CrA dark cloud. The new off-cloud weak-line T Tauri stars may have formed in similar cloud-lets, which have dispersed recently. High-resolution spectra of our new T Tauri stars show that they have significantly more lithium absorption than zero-age main-sequence stars of the same spectral type, so that they are indeed young. From those spectra we also obtained rotational and radial velocities. For some stars we found the proper motion in published catalogs. The direction and velocity of the 3D space motion - south relative to the galatic plane - of the CrA T Tauri stars is consistent with the dark cloud being formed originally by a high-velocity cloud impact onto the galactic plane, which triggered the star formation in CrA. We also present VRIJHK photometry for most of the new T Tauri stars to derive their luminosities, ages, and masses.

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arXiv1999

Medium-resolution optical spectroscopy of young stellar and sub-stellar M-dwarfs in the Cha I dark cloud

We obtained medium-resolution spectra of the bona-fide brown dwarf Cha H$α$ 1, the five brown dwarf candidates Cha H$α$ 2 to 6, two additional late M-type brown dwarf candidates, all originally selected by H$α$ emission, and four previously known T Tauri stars, all located in Chamaeleon I. The spectral types of our targets range down to M8. We show their spectra and also list their IJK magnitudes from DENIS. All objects have radial velocities consistent with kinematic membership to Cha I and show Li 6708Åabsorption. Our Cha I brown dwarf candidates with lithium are young or sub-stellar or both. Cha H$α$ 1, 3, 6, and 7 are certainly brown dwarfs: Either they are as old or older than the Pleiades and should have burned all their original lithium if they were late M-type stars, or, if they are younger than the Pleiades, $\le 125$ Myrs, they are sub-stellar because of the young age and the late spectral types ($\ge$ M7), according to three different sets of evolutionary tracks and isochrones. To classify Cha H$α$ 2, 4, 5, and 8 as either stellar or sub-stellar, evolutionary tracks of higher reliability are needed.

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arXiv1998

On the number of accreting and cooling isolated neutron stars detectable with the ROSAT All-Sky Survey

We present limits to the log N - log S curve for isolated neutron stars, both cooling and accreting neutron stars, which are not active as radio pulsars, as observed with the ROSAT All-Sky Survey and compare it with theoretical expectations. So far, only one isolated neutron star is identified optically among ROSAT sources, namely RXJ185635-3754 (Walter & Matthews 1997). Three more promising candidates have been suggested. In addition, several upper limit estimates are available on the space density of such neutron stars from different optical follow-up studies. We show that the log N - log S curve according to the current observations, including the identified neutron star, the three additional candidates, and the upper limits, lies between the theoretical expectations for middle-aged cooling neutron stars and old accreting neutron stars. At least one of the neutron star candidates found so far with ROSAT may be cooling instead of accreting. We suggest that the fact that more accreting isolated old neutron stars were expected (e.g., Madau & Blaes 1994) than observed is mostly due to the velocity distribution used in those calculations. More recent radio observations indicate that there are fewer slow neutron stars, ie., fewer accreting X-ray bright old neutron stars. At the X-ray bright end of the log N - log S curve, however, the ROSAT observations agree well with the theoretical expectations.

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arXiv1998

Search for X-ray emission from bona-fide and candidate brown dwarfs

Following the recent classification of the X-ray detected object V410 x-ray 3 with a young brown dwarf candidate (Briceno et al. 1998) and the identification of an X-ray source in Chamaeleon as young bona-fide brown dwarf (Neuhaeuser & Comeron 1998), we investigate all ROSAT All-Sky Survey and archived ROSAT PSPC and HRI pointed observations with bona-fide or candidate brown dwarfs in the field of view with exposure times ranging from 0.13 to 221 ks, including dedicated 64 ks and 42 ks deep ROSAT HRI pointed observations on the low-mass star BRI0021-0214 and the brown dwarf Calar 3, respectively. Out of 26 bona-fide brown dwarfs, one is newly detected in X-rays, namely rho Oph GY 202. Also, four out of 57 brown dwarf candidates studied here are detected in X-rays, namely the young Taurus brown dwarf candidates MHO-4, MHO-5, V410 Anon 13, and V410 x-ray 3. The M9.5-type star BRI0021-0214 is not detected. In the appendix, we also present catalogued, but as yet unnoticed B- and R-band data for some of the objects studied here.

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arXiv1997

HIPPARCOS results for ROSAT-discovered young stars

Out of about 500 Lithium-rich ROSAT counterparts, which are presumed to be low-mass pre-main sequence stars, 21 stars have been observed by HIPPARCOS. We study their parallaxes, proper motions, and photometric data. For 7 out of 10 Taurus and Lupus stars in our sample, proper motions and parallaxes are not inconsistent with membership to these associations, while most of the stars in Chamaeleon and Scorpius appear to be young foreground stars. Combined with ground based photometry and spectroscopy, HIPPARCOS parallaxes allow us to place 15 stars on an H-R diagram. All these 15 stars are indeed pre-main sequence stars with ages from 1 to 15 Myr. Only two of the stars are located on the Hayashi-tracks, whereas the other 13 are post-T Tauri stars located on radiative tracks. Although this sample is admittedly small, containing only 3% of the total sample of Lithium-rich ROSAT counterparts, it does not confirm recent predictions by other authors: We find no stars in the age range from 20 to 100 Myr. The foreground pre-main sequence stars may have been ejected towards us, or they belong to the Gould Belt system, a plane filled with young stars.

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