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Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| A Spectroscopic and Photometric Survey of Stars in the Field of L1457: A New Distance Determination
We present a spectroscopic and photometric survey of a sample of fieldstars in the region of the molecular cloud L1457. High-qualitycoudé feed spectra, together with five-band photometry in theSloan Digital Sky Survey system and near-infrared archival data from theTwo Micron All Sky Survey, are used to derive color excesses anddistances for the stars. Based on these data, a new distance estimate of360+/-30 pc is derived for the cloud, supporting recent results by K. L.Luhman. The data further indicate that the north-south velocity gradientseen in the millimeter-wave CO data is mirrored in a distance gradient,with the northern part of the cloud being closer to us. A second, lessopaque, layer of extinction is detected at ~80 pc. This distance isconsistent with the earlier distance estimates to the cloud, based on NaI absorption. We identify this layer with the wall of the hot LocalBubble. Hence, the dense cloud is not, as previously thought, associatedwith the Local Bubble.
| Photometric Investigation of the MBM 12 Molecular Cloud Area in Aries. II. Cloud Distance
Photoelectric magnitudes and color indices in the Vilnius seven-colorsystem for 152 stars are used to investigate the interstellar extinctionin the area of the Aries molecular cloud MBM 12, coinciding with theL1454 and L1457 dust clouds. Spectral types, absolute magnitudes, colorexcesses, interstellar extinctions and distances of the stars aredetermined. The plot of interstellar extinction A_V versus distanceshows that the dust cloud is situated at a distance of 325 pc, at 180 pcfrom the Galactic plane, and its true diameter is about 11 pc. Theinterstellar extinction law in the area is found to be normal, typicalfor the diffuse dust. Ten peculiar or unresolved binary stars and someheavily reddened stars are detected.
| Photometric Investigation of the MBM 12 Molecular Cloud Area in Aries. I. Photoelectric Photometry
The results of photoelectric photometry in the Vilnius seven-colorsystem are given for 152 stars down to 12.2 mag in the area of themolecular cloud MBM 12 and the dust clouds L1454 and L1457 in Aries. Theresults of photometric classification of stars are also given. Theinvestigation of interstellar extinction in the area is described in thenext paper.
| The distance to the nearest star-forming clouds: MBM12 and MBM20
We present high-resolution spectra (R ~ 49,000) of stars that haveparallax measurements from the Hipparcos satellite and are projectedalong the line of sight to the two nearest known star forming clouds tothe Sun: MBM12 and MBM20. The spectra were obtained with the FOCESEchelle Spectrograph at the 2.2 meter telescope in Calar Alto, Spain andthe wavelength range was chosen to include the interstellar Na I D linesat lambda 5889.950 Ä and lambda 5895.924 Ä. Since the starsare at a range of distances, we use their spectra along with theirparallaxes from Hipparcos to determine the distance to the moleculargas. The stars in front of the cloud do not show interstellar Na I Dabsorption features while the stars behind the cloud do showinterstellar absorption features. We find that both clouds are somewhatmore distant than previously estimated. The revised distance to MBM12 is58+/-5 pc < d < 90+/-12 pc and the distance to MBM20 is 112+/-15pc < d < 161+/-21 pc.
| ROSAT PSPC observations of T Tauri stars in MBM12 PSPC observations of T Tauri stars in MBM12
We present the ROSAT PSPC pointed and ROSAT All-Sky Survey (RASS)observations and the results of our low and high spectral resolutionoptical follow-up observations of the T Tauri stars (TTS) and X-rayselected T Tauri star candidates in the region of the high galacticlatitude dark cloud MBM12 (L1453-L1454, L1457, L1458). Previousobservations have revealed 3 ``classical'' T Tauri stars and 1``weak-line'' T Tauri star along the line of sight to the cloud. Becauseof the proximity of the cloud to the sun, all of the previously knownTTS along this line of sight were detected in the 25 ks ROSAT PSPCpointed observation of the cloud. We conducted follow-up opticalspectroscopy at the 2.2-meter telescope at Calar Alto to look forsignatures of youth in additional X-ray selected T Tauri starcandidates. These observations allowed us to confirm the existence of 4additional TTS associated with the cloud and at least 2 young mainsequence stars that are not associated with the cloud and place an upperlimit on the age of the TTS in MBM12 ~ 10 Myr. The distance to MBM12 hasbeen revised from the previous estimate of 65+/-5 pc to 65+/-35 pc basedon results of the Hipparcos satellite. At this distance MBM12 is thenearest known molecular cloud to the sun with recent star formation. Weestimate a star-formation efficiency for the cloud of 2-24%. We havealso identified a reddened G9 star behind the cloud with A_v ~ 8.4-8.9mag. Therefore, there are at least two lines of sight through the cloudthat show larger extinctions (A_v > 5 mag) than previously thoughtfor this cloud. This higher extinction explains why MBM12 is capable ofstar-formation while most other high-latitude clouds are not. Table~4 isonly available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html.}
| Radial velocities. Measurements of 2800 B2-F5 stars for HIPPARCOS
Radial velocities have been determined for a sample of 2930 B2-F5 stars,95% observed by the Hipparcos satellite in the north hemisphere and 80%without reliable radial velocity up to now. Observations were obtainedat the Observatoire de Haute Provence with a dispersion of 80Ä,mm(-1) with the aim of studying stellar and galactic dynamics.Radial velocities have been measured by correlation with templates ofthe same spectral class. The mean obtained precision is 3.0 km s(-1)with three observations. A new MK spectral classification is estimatedfor all stars. Based on observations made at the Haute ProvenceObservatory, France and on data from The Hipparcos Catalogue, ESA.Tables 4, 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.htm
| The local distribution of NA I interstellar gas
We present high-resolution absorption measurements (lambda/Delta lambdaapproximately 75,000) of the interstellar Na I D lines at 5890 A toward80 southern hemisphere early-type stars located in the localinterstellar medium (LISM). Combining these results with other sodiummeasurements taken from the literature, we produce galactic maps of thedistribution of neutral sodium column density for a total of 293 starsgenerally lying within approximately 250 pc of the Sun. These mapsreveal the approximate shape of the mid-plane contours of the rarefiedregion of interstellar space termed the Local Bubble. Its shape is seenas highly asymmetric, with a radius ranging from 30 to 300 pc, and withan average radius of 60 pc. Similar plots of the Galactic mid-planedistribution of sources emitting extreme ultraviolet radiation show thatthey also trace out similar contours of the Local Bubble derived from NaI absorption measurements. We conclude that the Local Bubble absorptioninterface can be represented by a hydrogen column density,NuETA = 2 x 1019 cm-2, which explainsboth the local distribution of Na I absorption and the observed galacticdistribution of extreme ultraviolet sources. The derived mid-planecontours of the Bubble generally reproduce the large-scale featurescarved out in the interstellar medium by several nearby galactic shellstructures.
| Further high-resolution NA I observations of the local interstellar medium
High-resolution absorption measurements of the interstellar Na I D linesat 5890 A observed toward 27 early-type stars in the local interstellarmedium (LISM) are presented. These results are combined with otherhigh-resolution sodium measurements to map the space distribution ofneutral sodium column density for some 118 stars out to less than 200pc. These measurements indicate an upper limit to the neutral sodiumcolumn density of log N(Na I) less than 10.0/sq cm can be inferred outto a distance of 50 pc in most directions in the LISM. Also, therarefield region of the Local Bubble may extend beyond 60 pc in at least35 percent of the directions sampled thus far. Evidence is shown for aubiquitous, comoving vector for neutral NaI gas clouds in the LISM whichis in a different direction to LISM vectors previously reported for moreionized local gas clouds. A comparison of the measured sodium columnswith those of interstellar Ca II for a sample of 12 stars within 95 pcresults in a ratio of Na I/Ca II less than 0.5 for most stars. Thisvalue implies that there could be warm, neutral gas with T of about12,500 K beyond 50 pc in the LISM.
| Flamsteed stars not contained in the Yale "Catalogue of Bright Stars"
Not Available
| On the nearest molecular clouds. II - MBM 12 and 16
The paper presents echelle spectra recorded at the D lines of Na I forthree stars projected on the high-latitude molecular cloud MBM 16 at l =172 deg, b = -38 deg. The A stars HD 21142 at about 95 pc and HD 21134at about 240 pc show strong D-line absorption at the same velocities asthe CO emission observed at these positions. The distance to MBM 16therefore is in the range of 60 to 95 pc. MBM 16 is only 11 deg awayfrom MBM 12, previously placed by the same method at distance of about65 pc. Consideration is given to the relationship between clouds 12 and16 and the local hot low-density interstellar gas.
| UBV photometry of stars whose positions are accurately known.
Photometric indices V, B-V, and U-B were measured for about 560 stars ofthe AGK3R and NPZT catalogs between BD declinations 11 deg and 23 deg,using the 40-cm Cassegrain telescope of the Kvistaberg Observatory fromApril 1986 to May 1987. The observation procedure and the reductiontechnique were the same as in the earlier papers of this series by Oja(1984, 1985, 1986, and 1987). The mean errors were calculated from theinternal consistency of the data. The accuracy was found to be the sameas for the earlier parts of the survey.
| A molecular cloud in the local, hot interstellar medium
Echelle spectra recorded at the D lines of Na I are reported for nine Aor F stars. Lying at approximate distances ranging from 25 to 230 pc,the stars are projected on or near the high-latitude molecular cloud MBM12 at l = 159 deg, b = -34 deg. Among a subgroup of five of these starsseparated by no more than 1.2 deg on the sky, four which are located atdistances d more than 70 pc show strong interstellar D line absorptionnear the radial velocity of the CO emission observed in this generaldirection. The fifth star, at roughly 60 pc, shows no detectableabsorption. MBM 12 therefore probably lies at roughly 65 pc, within thelocal region filled primarily by very hot, low-density gas, a conclusionsupported by the large internal velocity dispersion of the molecularcloud complex.
| Positions et Mouvements propres de 357 Etoiles de Repère de la Zone + 16° à + 18° pour l'époque et l'équinoxe 1950,0
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