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X-Ray Study of Herbig Ae/Be Stars
We present ASCA results of intermediate-mass pre-main-sequence (PMS)stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources,we detect 11 plausible X-ray counterparts. X-ray luminosities of thedetected sources in the 0.5-10 keV band are in the range oflogLX~30-32 ergs s-1, which is systematicallyhigher than those of low-mass PMS stars. This fact suggests that thecontribution of a possible low-mass companion is not large. Most of thebright sources show significant time variation; in particular, two HAeBestars-MWC 297 and TY CrA-exhibit flarelike events with long decaytimescales (e-folding time ~10-60 ks). These flare shapes are similar tothose of low-mass PMS stars. The X-ray spectra are successfullyreproduced by an absorbed one- or two-temperature thin-thermal plasmamodel. The temperatures are in the range of kT~1-5 keV, significantlyhigher than those of main-sequence OB stars (kT<1 keV). These X-rayproperties are not explained by wind-driven shocks, but are more likelydue to magnetic activity. On the other hand, the plasma temperaturerises as absorption column density increases or as HAeBe stars ascend toearlier phases. The X-ray luminosity reduces after stellar age of a fewtimes 106 yr. X-ray activity may be related to stellarevolution. The age of the activity decay is apparently near thetermination of jet or outflow activity. We thus hypothesize thatmagnetic activity originates from the interaction of the large-scalemagnetic fields coupled to the circumstellar disk. We also discussdifferences in X-ray properties between HAeBe stars and main-sequence OBstars.
| Astronomical seeing at the Magdalena Ridge Observatory
Not Available
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/141
| Searching for the Molecules of Life in Space
Not Available
| 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 deg2 areain and around the CrA star forming region. With low-resolutionspectroscopy of unidentified ROSAT sources we could find 19 new pre-mainsequence stars, two of which are classical T Tauri stars, the othersbeing weak-lined. The spectral types of these new T Tauri stars rangefrom F7 to M6. The two new classical T Tauri stars are located towardstwo 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 withisochrones for an assumed distance of 130 pc, the distance of the mainCrA dark cloud. The new off-cloud weak-line T Tauri stars may haveformed in similar cloudlets, which have dispersed recently.High-resolution spectra of our new T Tauri stars show that they havesignificantly more lithium absorption than zero-age main-sequence starsof the same spectral type, so that they are indeed young. From thosespectra we also obtained rotational and radial velocities. For somestars we found the proper motion in published catalogs. The directionand velocity of the 3D space motion - south relative to the galaticplane - of the CrA T Tauri stars is consistent with the dark cloud beingformed originally by a high-velocity cloud impact onto the galacticplane, which triggered the star formation in CrA. We also present VRIJHKphotometry for most of the new T Tauri stars to derive theirluminosities, ages, and masses. Partly based on observations collectedat the 1.52 m and 3.5 m telescopes of the European Southern Observatory,Chile, in programs 55.E-0549, 57.E-0646, and 63.L-0023, and onobservations collected at the 0.9 m, 1.5 m, and 4.0 m CTIO telescope.
| Deuterium fractionation and the degree of ionization in the R Coronae Australis molecular cloud core
The fractionation of D and (13) C in HCO(+) was investigated in the RCoronae Australis molecular cloud core. The distributions of H(13) CO(+)and DCO(+) were found to be morphologically similar but their columndensity maxima were found to lie in different locations. The H(13) CO(+)/HC(18) O(+) abundance ratio was found to vary little from 10 within themapped region, in excellent agreement with the (13) CO/C(18) O abundanceratios derived earlier towards the cloud by Harjunpää &Mattila (1996). This corroborates the close relationship between HCO(+)and CO predicted by the chemistry models. The DCO(+) /HCO(+) abundanceratio ranges from 0.006 to 0.04, being lowest towards two locations nearthe embedded infrared source IRS 7 where the kinetic temperature, asderived from methyl acetylene (CH_3CCH) observations, is somewhatelevated. The variation of the degree of deuterium fractionation withinthe core is due to an increase in the kinetic temperature near thecluster of newly born stars. This temperature rise results in twoeffects: Firstly, the reaction H_2D(+) -> H_3(+) becomes faster; andsecondly, an intensified desorption from grain surfaces increases theabundance of neutral atoms and molecules in the gas phase leading to thedestruction of H_3(+) and H_2D(+) ions. Both processes decrease theDCO(+) /HCO(+) abundance ratio. Far from the active region the derivedabundances of neutral species indicate the presence of depletion ontograin surfaces. The observations suggest furthermore that the fractionalelectron abundance, chi (e(-)) , is lowest in the dense clump near IRS7. This region also exhibits a low degree of gas phase depletion. Infact, increased fractional abundances of neutral species such as atomicoxygen and CO lead to a decrease in the [H_3(+]/[) HCO(+]) abundanceratio which is directly proportional to chi (e(-)) .
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Four-colour photometry of eclipsing binaries. XXXIX. Light curves of the pre-main sequence triple system TY Coronae Australis
Complete uvby light curves of the detached Herbig Be eclipsing binary TYCoronae Australis are presented. A total of 1789 photometricmeasurements in each of the four colours were obtained in 1989 and in1992-1994. A detailed analysis of the y light curve obtained in1992-1993 is published separately \cite[(Casey et al. 1997)]{cas97}. Thereflection nebula around the system contributes about 30% of the lightin all four passbands. Here we present and discuss thenon-eclipse-related photometric variability of the system. We suggestthat these variations are the result of variable obscuration, possiblylinked to dust shells physically associated to the system. Based onobservations collected with the Danish 50~cm Strömgren AutomaticTelescope, SAT, at the European Southern Observatory (ESO), La Silla,Chile.
| The Pre-Main-Sequence Eclipsing Binary TY Coronae Australis: Precise Stellar Dimensions and Tests of Evolutionary Models
We analyze new photometric data for the Herbig Be eclipsing binary TYCrA, which securely reveal the secondary eclipse, ~0.03 mag deep in y.From the light-curve solution and our previous spectroscopic data,absolute dimensions of the primary and secondary stars are derived. Themasses are found to be M_1 = 3.16 +/- 0.02 M_ȯ and M_2 = 1.64 +/-0.01 M_ȯ, the radii are R_1 = 1.80 +/- 0.10 R_ȯ and R_2 = 2.08+/- 0.14 R_ȯ, the luminosities are L_1 = 67 +/- 12 L_ȯ and L_2 =2.4 +/- 0.8 L_ȯ, and the effective temperatures are T_1 = 12,000 +/-500 K and T_2 = 4900 +/- 400 K. Here the uncertainties representhigh-confidence limits, not standard deviations. The secondary star is apre-main-sequence star located at the base of the Hayashi tracks. Assuch, it is the least evolved star with a dynamically measured mass.Given higher effective temperatures for the primary (e.g., 12,500 K),the solar-composition 1.64 M_ȯ evolutionary tracks of Swenson etal., Claret, and D'Antona & Mazzitelli are all consistent with theproperties of the TY CrA secondary and suggest an age of order 3 Myr.The radius and projected rotational velocity of the secondary star areconsistent with synchronous rotation. The primary star is located nearthe zero-age main sequence, which, for solar compositions, is consistentwith an age of 3 Myr. However, the primary star is not well representedby any of the 3.16 M_ȯ evolutionary models, which predict somewhathigher effective temperatures than observed.
| X-Ray Sources in Regions of Star Formation. VI. The R CRA Association as Viewed by Einstein.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1544W&db_key=AST
| HIPPARCOS data on Herbig Ae/Be stars: an evolutionary scenario.
Fundamental astrophysical parameters (distance, temperature, luminosity,mass, age) of a sample of 10 Herbig Ae/Be candidates and 3 non-emissionline A and B stars in star forming regions were computed combiningHipparcos parallaxes with data from literature. All genuine Herbig starsin our sample are located between the birthline and the zero-age mainsequence (ZAMS) in the Hertzsprung-Russell diagram (HRD), in accordancewith what is expected for pre-main sequence stars. The region in the HRDclose to the birthline is relatively devoid of stars when compared tothe region closer to the ZAMS, in agreement with the expectedevolutionary time scales. The Herbig Ae/Be stars not associated withstar forming regions were found to be located close to the ZAMS.Additionally we discuss a possible evolutionary scenario for thecircumstellar environment of Herbig stars.
| Star counts in southern dark clouds: Corona Australis and Lupus.
Star counts technique is used towards southern dark globular filamentssituated in the cloud complexes of Corona Australis and Lupus. Tablesand maps of the distribution of visual extinction are presented for eachfilament. Lower limit masses for the filaments and condensations havebeen estimated and the central coordinates of the condensations are alsogiven. R CrA is the most active star forming region among the filamentsstudied in this work whereas Lupus 1, with almost the same lower limitof mass, has only a few T Tauri stars and just one young embeddedobject. The distribution of direction of the magnetic field in thecondensations of Lupus, suggests that the condensation morphologies doesnot have any apparent relation with the magnetic field orientation.
| Meet the Milky Way.
Not Available
| The protoalgol system TY CrA.
Not Available
| The nature of CO and H2O ices in the Corona Australis molecular cloud
Low-resolution spectra at 2.0 to 3.8 micrometers were obtained for 11sources in the R CrA molecular cloud. The 3 micrometers absorption dueto H2O ice was detected in three types of sources including protostars,T Tauri-like stars, and background field stars. There is a tendency thatthe protostars with large H-K have large tau3 micrometers,but no clear linear correlation can be seen between tau3micrometers and H-K as is seen in the Taurus and the rho Ophiuchidark clouds. This scattered distribution suggests the significantcontribution of the circumstellar hot dust to the H-K color and/orsublimation of H2O ice at the inner region of the circumstellar disk dueto heating by protostars. Medium-resolution spectra at 4.7 micrometerswere also obtained for nine sources. The deep absorption at 4.67micrometers due to solid CO was detected in four protostars andmarginally in one background field star, but none in the T Tauri-likestars. Among them, the optical depth of CO ice in IRS 2 is the largestso far detected. It is found that the absorption feature of each sourcehas almost the same central frequency and FWHM which coincide with thecalculated values of small grains with dominant CO mantle. On the otherhand, the column density of CO ice is found to be substantially smallerthan that of H2O ice. This apparent contradiction suggests that thegrain mantle has a layered structure of H2O and CO ices possibly due tothe difference in the conditions of condensation of these molecules ontograin surfaces. Fr om an analysis of previous observations we find thatthe column density ratio of CO ice to H2O ice is 0.25 for backgroundfield stars and suggest that approximately 25% of CO exists in the grainmantles in dark clouds.
| The Accreting Circumstellar Gas Envelope of HD176386 a Young Star in the R-Coronae Star Formation Region
Abstract image available at:http://adsabs.harvard.edu/abs/1993A&A...274..847G
| On the dispersion in direction of interstellar polarization
The spatial distribution of polarization angle is summarized for 15 darkclouds that lack prominent star formation, for four star clustersembedded in dark clouds, for one evolved cluster with little associatedextinction, and for six dark cloud complexes. Most clouds have eitherone well-defined mean direction over their spatial extent or two orthree spatial zones having noticeably different mean directions. Nearlyall regions have a single local maximum in their number distribution ofpolarization angle. Clouds with embedded clusters have a more complexdistribution of polarization direction than do clouds without clusters.It is proposed that the enhanced dispersion of polarization angle inclusters may be more closely associated with young stars and/or withdense gas, than simply with the number of stars in the cluster. Thegreatest dispersion in direction among clouds without clusters, 0.7radians, occurs in L1689 in Ophiuchus.
| OH- and HI-observations of the CrA dark cloud complex
This paper presents OH 1667-MHz observations covering the CrA dark cloudcomplex, as well as HI 21-cm observations in a large region around thecomplex. The OH profiles are of simple structure indicating the presenceof only one component with a mean velocity V(OH) about +5.5 km/s. ThreeOH clumps are apparent, which are coincident with regions of highphotographic extinction. The gas to dust ratio derived from theseobservations is similar to the average value obtained for other darkclouds and shows that OH and dust are well mixed in the CrA dark cloudcomplex. The total mass estimated for the molecular cloud amounts toM(T) about 7000 solar masses. The HI observations show the well-knownridge of local gas of low positive velocities. The accordance betweenthe velocities of molecular and atomic gas in the region suggests thatthe molecular complex is embedded in the local HI-ridge. It is suggestedthat they both are related to Lindblad's feature A (i.e., to Gould'sbelt).
| Interstellar molecular lines in the direction of the young star TY CrA
Echelle data for the CN, CH, and CH(+) lines along the line of sight toTY CrA, a young star in a very young star forming region, are discussed.The molecular abundances of CN and CH are low in comparison to those ofHD 29647. The results suggest that CH(+) is formed in a weak shock thatmay be associated with the local cloud collapse. The large abundances ofCH(+) and CH imply a shock with a velocity of about 10 km/s (seen at anangle from the normal to the shock front of about 70 deg) and aradiation-field-enhancement factor of about 3-5.
| Evidence for discs and jets associated with R and T CR A
Polarization mapping of the reflection nebula NGC 6729 reveals parallelbands of polarization vectors across the premain sequence stars R and TCr A. These bands can be explained by dust discs in which the grains arealigned by toroidal magnetic fields. The dust discs are orientedparallel to each other (in projection and possibly in space) in anorth-south direction, which is orthogonal to the axis of the CO bipolaroutflow from R Cr A observed by Levreault. Optical jets are associatedwith both stars, two with R Cr A and one with T Cr A, which are eitherparallel or antiparallel to each other; however, the optical jets arenot orthogonal to the planes of the discs, but are inclined at about 60deg. A model is suggested in which the optical jets are collimated by asmall inner circumstellar disc, which has decoupled from the magneticfield in outer regions because of ambipolar diffusion, and whoseorientation is determined principally by the angular momentum. The largeouter interstellar disc, in which the grains are aligned by a toroidalmagnetic field, is inclined obliquely to the inner disc and isresponsible for the collimation of the CO bipolar outflow.
| Far-infrared observations of a star-forming region in the Corona Australis dark cloud
A high-resolution far-IR (40-250-micron) survey of a 0.9-sq-deg sectionof the core region of the Corona Australis dark cloud (containing veryyoung stellar objects such as T Tauri stars, Herbig Ae and Be stars,Herbig-Haro objects, and compact H II regions) is presented. Twoextended far-IR sources were found, one associated with the Herbigemission-line star R CrA and the other with the irregular emission-linevariable star TY CrA. The two sources have substantially more far-IRradiation than could be expected from a blackbody extrapolation of theirnear-IR fluxes. The total luminosities of these sources are 145 and 58solar luminosity, respectively, implying that the embedded objects areof intermediate or low mass. The infrared observations of the sourcesassociated with R CrA and TY CrA are consistent with models of theevolution of protostellar envelopes of intermediate mass. However, theTY CrA source appears to have passed the evolutionary stage of expellingmost of the hot dust near the central source, yielding an age of about 1Myr.
| A general study of diffuse galactic nebulae.
Not Available
| Note on the variable nebula in Corona Australis
Not Available
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