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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.
| 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.
| On the MBM 12 Young Association
I present a comprehensive study of the MBM 12 young association (MBM12A). By combining infrared (IR) photometry from the Two-Micron All-SkySurvey (2MASS) survey with new optical imaging and spectroscopy, I haveperformed a census of the MBM 12A membership that is complete to 0.03Msolar (H~15) for a 1.75d×1.4d field encompassing theMBM 12 cloud. I find five new members with masses of 0.1-0.4Msolar and a few additional candidates that have not beenobserved spectroscopically. From an analysis of optical and IRphotometry for stars in the direction of MBM 12, I identify M dwarfs inthe foreground and background of the cloud. By comparing the magnitudesof these stars to those of local field dwarfs, I arrive at a distancemodulus 7.2+/-0.5 (275 pc) to the MBM 12 cloud; it is not the nearestmolecular cloud and is not inside the local bubble of hot ionized gas ashad been implied by previous distance estimates of 50-100 pc. I havealso used Li strengths and H-R diagrams to constrain the absolute andrelative ages of MBM 12A and other young populations; these dataindicate ages of 2+3-1 Myr for MBM 12A and ~10 Myrfor the TW Hya and η Cha associations. MBM 12A may be a slightlyevolved version of the aggregates of young stars within the Taurus darkclouds (~1 Myr) near the age of the IC 348 cluster (~2 Myr).
| 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.}
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| 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.
| 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.
| 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.
| Lunar occultation summary. II
Results of a second series of 196 two-color photoelectric lunaroccultation observations made within the interval from May 1972 toDecember 1973 are reported. Each observation has been employed toestimate the minimum magnitude difference between the observed objectand possible undetected companions. The study contains the analyses of143 disappearances and 53 reappearances. Eighteen of these eventsmanifest some degree of multiplicity. Included in the study are 18occultation events for Pleiades members.
| Observations of occultations of stars by the moon
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974A&AS...13..395H&db_key=AST
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