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Yeni bir Makale Öner
|The Radial Velocity Experiment (RAVE): First Data Release|
We present the first data release of the Radial Velocity Experiment(RAVE), an ambitious spectroscopic survey to measure radial velocitiesand stellar atmosphere parameters (temperature, metallicity, and surfacegravity) of up to one million stars using the Six Degree Fieldmultiobject spectrograph on the 1.2 m UK Schmidt Telescope of theAnglo-Australian Observatory. The RAVE program started in 2003,obtaining medium-resolution spectra (median R=7500) in the Ca-tripletregion (8410-8795 Å) for southern hemisphere stars drawn from theTycho-2 and SuperCOSMOS catalogs, in the magnitude range 9
|What Is the Total Deuterium Abundance in the Local Galactic Disk?|
Analyses of spectra obtained with the Far Ultraviolet SpectroscopicExplorer (FUSE) satellite, together with spectra from the Copernicus andinterstellar medium absorption profile spectrograph (IMAPS) instruments,reveal an unexplained, very wide range in the observeddeuterium/hydrogen (D/H) ratios for interstellar gas in the Galacticdisk beyond the Local Bubble. We argue that spatial variations in thedepletion of deuterium onto dust grains can explain these localvariations in the observed gas-phase D/H ratios. We present a variabledeuterium depletion model that naturally explains the constant measuredvalues of D/H inside the Local Bubble, the wide range of gas-phase D/Hratios observed in the intermediate regime [logN(HI)=19.2-20.7], and thelow gas-phase D/H ratios observed at larger hydrogen column densities.We consider empirical tests of the deuterium depletion hypothesis: (1)correlations of gas-phase D/H ratios with depletions of the refractorymetals iron and silicon, and (2) correlation with the H2rotational temperature. Both of these tests are consistent withdeuterium depletion from the gas phase in cold, not recently shockedregions of the ISM, and high gas-phase D/H ratios in gas that has beenshocked or otherwise heated recently. We argue that the mostrepresentative value for the total (gas plus dust) D/H ratio within 1kpc of the Sun is >=23.1+/-2.4(1σ) parts per million (ppm).This ratio constrains Galactic chemical evolution models to have a verysmall deuterium astration factor, the ratio of primordial to total (D/H)ratio in the local region of the Galactic disk, which we estimate to befd<=1.19+0.16-0.15(1σ) or<=1.12+/-0.14(1σ) depending on the adopted light-elementnuclear reaction rates.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by the Johns HopkinsUniversity under NASA contract NAS5-32985.
|Astrophysics in 2005|
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
|Forty Years of Progress in Long-Baseline Optical Interferometry: 2005 Robert Ellery Lecture|
The development of long-baseline optical interferometry in Australiafrom the Narrabri Stellar Intensity Interferometer (NSII) to the SydneyUniversity Stellar Interferometer (SUSI) and the resulting technical andscientific achievements are described. Three examples of results fromthe SUSI programme, for a single star, a double-lined spectroscopicbinary, and a Cepheid variable, are presented to illustrate the advancesmade in the past four decades. The leading role that Australia hasplayed in the development of the field worldwide is discussed from apersonal viewpoint. Long-baseline optical interferometry has promisedmuch, has been slow to deliver, and has been restricted to black-beltinterferometrists, but it has now matured to the point where it isbecoming an observational technique for astronomers in general.
|First Results from the CHARA Array. VII. Long-Baseline Interferometric Measurements of Vega Consistent with a Pole-On, Rapidly Rotating Star|
We have obtained high-precision interferometric measurements of Vegawith the CHARA Array and FLUOR beam combiner in the K' bandat projected baselines between 103 and 273 m. The measured visibilityamplitudes beyond the first lobe are significantly weaker than expectedfor a slowly rotating star characterized by a single effectivetemperature and surface gravity. Our measurements, when compared tosynthetic visibilities and synthetic spectrophotometry from a Roche-vonZeipel gravity-darkened model atmosphere, provide strong evidence forthe model of Vega as a rapidly rotating star viewed very nearly pole-on.Our best-fitting model indicates that Vega is rotating at ~91% of itsangular break-up rate with an equatorial velocity of 275 kms-1. Together with the measured vsini, this velocity yieldsan inclination for the rotation axis of 5deg. For this modelthe pole-to-equator effective temperature difference is ~2250 K, a valuemuch larger than previously derived from spectral line analyses. A polareffective temperature of 10,150 K is derived from a fit to ultravioletand optical spectrophotometry. The synthetic and observed spectralenergy distributions are in reasonable agreement longward of 140 nm,where they agree to 5% or better. Shortward of 140 nm, the model is upto 10 times brighter than observed. The model has a luminosity of ~37Lsolar, a value 35% lower than Vega's apparent luminositybased on its bolometric flux and parallax, assuming a slowly rotatingstar. Our model predicts the spectral energy distribution of Vega asviewed from its equatorial plane, and it may be employed in radiativemodels for the surrounding debris disk.
|AmFm Stars as a Test of Rotational Mixing Models|
Stellar evolution models have been calculated for stars of 1.7-2.5Msolar with both the Geneva-Toulouse and Montreal codes. Inthe Geneva-Toulouse code, the internal evolution of angular momentum iscalculated self-consistently along with the transport of a few species.In the Montreal code, the transport of 24 species is treated in detail,taking into account radiative accelerations, thermal diffusion, andgravitational settling, along with the turbulent diffusion coefficientscalculated in the Geneva-Toulouse code. It is verified that the twocodes lead to very similar internal structure for a given mass. Thecalculated surface abundances are compared to abundance anomaliesobserved on AmFm stars. It is found that with approximately the sameparameters as used for other types of stars, the Geneva-Toulouse codeleads to turbulent transport coefficients that produce abundanceanomalies consistent with the observed ones for HD 73045, HD 23610, andSirius. Taking into account the effect of the anisotropy of turbulenceon vertical transport plays an important role, although the level ofanisotropy in stellar envelopes is very uncertain; this effect isusually neglected in calculations. The stabilizing effect of the meanmolecular weight gradient can also be important. The current level ofaccuracy of observed abundances only permits to choose within aone-parameter family of models. To distinguish between turbulent modelswith different interior profiles, an accuracy of 0.03 dex is required ofabundance determinations, a level of accuracy that is not currentlyachieved. It is also shown that taking into account thepre-main-sequence evolution of the rotation profile leads to animportant reduction in the Ω dependence of turbulent transport forslow rotators.
|Simulating observable comets. III. Real stellar perturbers of the Oort cloud and their output|
Context: .This is the third of a series of papers on simulating themechanisms acting currently on the Oort cloud and producing the observedlong-period comets.Aims.In this paper we investigate the influence ofcurrent stellar perturbers on the Oort cloud of comets under thesimultaneous galactic disk tide. We also analyse the past motion of theobserved long-period comets under the same dynamical model to verify thewidely used definition of dynamically new comets. Methods.The action ofnearby stars and the galactic disk tide on the Oort cloud was simulated.The original orbital elements of all 386 long-period comets of qualityclasses 1 and 2 were calculated, and their motion was followednumerically for one orbital revolution into the past, down to theprevious perihelion. We also simulated the output of the close futurepass of GJ 710 through the Oort cloud. Results.The simulated flux of theobservable comets resulting from the current stellar and galacticperturbations, as well as the distribution of perihelion direction, wasobtained. The same data are presented for the future passage of GJ 710.A detailed description is given of the past evolution of aphelion andperihelion distances of the observed long-period comets. Conclusions. Weobtained no fingerprints of the stellar perturbations in the simulatedflux and its directional structure. The mechanisms producing observablecomets are highly dominated by galactic disk tide because all currentstellar perturbers are too weak. Also the effect of the close passage ofthe star GJ 710 is very difficult to recognise on the background of theGalactic-driven observable comets. For the observed comets we found only45 to be really dynamically "new" according to our definition based onthe previous perihelion distance value.
|Subarcsecond Mid-Infrared Imaging of Dust in the Bipolar Nebula Hen 3-401|
We present high-resolution, nearly diffraction-limited narrow- andbroadband mid-IR images of bipolar reflection nebula Hen 3-401. Thedeconvolved images yield a pixel-limited spatial resolution of 0.09",demonstrating the superior imaging quality of the Gemini telescope. Theinfrared image of Hen 3-401 consists of a prominent core region of size1" and extended emissions along the walls of the bipolar lobes. We findthat the distribution of aromatic infrared band (AIB) emission isdifferent from that of the continuum emission and that the star hasundergone multiple ejection of the AIB emission feature carrier at theend stages of its evolution. From the observed temperature gradient inthe core, we suggest that the core has a flared disk geometry.This paper is based on observations obtained at the Gemini Observatory,which is operated by the Association of Universities for Research inAstronomy, Inc., under a cooperative agreement with the NSF on behalf ofthe Gemini partnership: the National Science Foundation (US), theParticle Physics and Astronomy Research Council (UK), the NationalResearch Council (Canada), CONICYT (Chile), the Australian ResearchCouncil (Australia), CNPq (Brazil), and CONICET (Argentina).
|San Pedro Mártir Mid-Infrared Photometric System|
With the goal of defining the San Pedro Martir Mid-infrared PhotometricSystem (SPM-MIR), observations of well-defined calibration stars werecarried out on 9 photometric runs from 2000 to 2005. The mid-infrared (2- 28 μm) camera CID-BIB of the Observatorio AstronómicoNacional was used. A set of 9 filters, the ``silicate'' series SiN, SiO,SiP, SiQ, SiR, SiS, the broad-band N (10.8 μm) filter, and thenarrow-band QH2 (17.15 μm) and Q2 (18.7 μm), was used to derivethe extinction coefficients and zero magnitude points. Atmosphericextinction corrections were carried out making use of Padéapproximants and we found a linear dependence on the parameters with thelow air mass extinction coefficient. The atmospheric transmission curvesand extinction coefficients of SPM were compared with those obtained forthe Mauna Kea site. Color terms were derived using a set of IRAS LSRsources observed with the CID-BIB.
|The Puzzle of the Metallic Line Stars|
In the puzzle of the metallic line (Am) stars, there still seem to bemissing pieces. While the ``normal'' A stars have elemental abundancesclose to solar, the classical Am stars show stronger absorption linesfor most heavy elements in their spectra. Elements with ionizationpotentials that nearly agree with those of hydrogen or helium havereduced abundances. The Ca II and Sc II lines are especially weak. TheAm stars have no ultraviolet emission lines. They are binaries that,with very few exceptions, have rotational velocities vsini lower than100 km s-1. Of the main-sequence A stars, 20% to 30% are Amstars. Here we rediscuss previous suggestions that tried to explain thepeculiar line strengths in the Am star spectra. In particular, wecompare the well-studied properties of Hyades A and Am stars in order toidentify reasons that can or cannot explain the differences. We findthat accretion of interstellar material by A stars with distortedmagnetic fields, which are weaker than those in peculiar A (Ap) stars,has the best chance of explaining the main characteristics of thepeculiar heavy-element abundances in Am star photospheres.Charge-exchange reactions also seem to be important.
|Georges Lemaitre, fondateur de la cosmologie moderne.|
|An Infrared Imaging Study of the Bipolar Proto-Planetary Nebula IRAS 16594-4656|
High-resolution mid-infrared images have been obtained in N and Q bandfor the proto-planetary nebula IRAS 16594-4656. A bright equatorialtorus and a pair of bipolar lobes can clearly be seen in the infraredimages. The torus appears thinner at the center than at the edges,suggesting that it is viewed nearly edge-on. The infrared lobescorrespond to the brightest lobes of the reflection nebula seen in theHubble Space Telescope (HST) optical image, but with no sign of thepoint-symmetric structure seen in the visible image. The lobe structureshows a close correspondence with a molecular hydrogen map obtained withHST, suggesting that the dust emission in the lobes traces thedistribution of the shocked gas. The shape of the bipolar lobes showsclearly that the fast outflow is still confined by the remnantcircumstellar envelope of the progenitor asymptotic giant branch (AGB)star. However, the nondetection of the dust outside of the lobessuggests that the temperature of the dust in the AGB envelope is too lowfor it to be detected at 20 μm.The paper is based on observations obtained at the Gemini Observatory.The Gemini Observatory is operated by the Association of Universitiesfor Research in Astronomy, Inc., under a cooperative agreement with theNational Science Foundation (NSF) on behalf of the Gemini partnership:the NSF (United States), the Particle Physics and Astronomy ResearchCouncil (United Kingdom), the National Research Council (Canada),CONICYT (Chile), the Australian Research Council (Australia), CNPq(Brazil), and CONICET (Argentina).
|Heliospheric Response to Different Possible Interstellar Environments|
At present, the heliosphere is embedded in a warm, low-densityinterstellar cloud that belongs to a cloud system flowing through thelocal standard of rest with a velocity near ~18 km s-1. Thevelocity structure of the nearest interstellar material (ISM), combinedwith theoretical models of the local interstellar cloud (LIC), suggestthat the Sun passes through cloudlets on timescales of<=103-104 yr, so the heliosphere has been, andwill be, exposed to different interstellar environments over time. Bymeans of a multifluid model that treats plasma and neutral hydrogenself-consistently, the interaction of the solar wind with a variety ofpartially ionized ISM is investigated, with the focus on low-densitycloudlets such as are currently near the Sun. Under the assumption thatthe basic solar wind parameters remain/were as they are today, a rangeof ISM parameters (from cold neutral to hot ionized, with variousdensities and velocities) is considered. In response to differentinterstellar boundary conditions, the heliospheric size and structurechange, as does the abundance of interstellar and secondary neutrals inthe inner heliosphere, and the cosmic-ray level in the vicinity ofEarth. Some empirical relations between interstellar parameters andheliospheric boundary locations, as well as neutral densities, areextracted from the models.
|Spectral Irradiance Calibration in the Infrared. XVI. Improved Accuracy in the Infrared Spectra of the Secondary and Tertiary Standard Calibration Stars|
We have increased the spectral resolution and accuracy of the absoluteinfrared spectral flux for a subset of stellar standards created byCohen and his collaborators in previous papers in this series. Wecombined the moderate-resolution (λ/Δλ~400)spectroscopy obtained on infrared standard stars by the Short WavelengthSpectrometer on the Infrared Space Observatory with high-qualityphotometry that is tied to our recent absolute calibration from theMidcourse Space Experiment. New spectra were created for 33 stars, 9 ofwhich are Cohen et al. secondary standards and another 20 of which aretertiary standards for which Cohen adopted spectral templates. Theeffective temperatures and angular diameters for the stars are derivedfrom the absolute spectra and compare favorably with independentmeasures of these quantities in the literature. The present spectradisplay systematic differences with those of Cohen and colleagues inthat they have 4%-7% lower fluxes in the 1-4 μm spectral region. Ourspectra remove the A-K star calibration bias recently noted in thecalibration of the Spitzer Space Telescope camera.
|Computed Hβ indices from ATLAS9 model atmospheres|
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
|Hubble Space Telescope spectroscopy of the Balmer lines in Sirius B*|
Sirius B is the nearest and brightest of all white dwarfs, but it isvery difficult to observe at visible wavelengths due to the overwhelmingscattered light contribution from Sirius A. However, from space we cantake advantage of the superb spatial resolution of the Hubble SpaceTelescope (HST) to resolve the A and B components. Since the closestapproach in 1993, the separation between the two stars has becomeincreasingly favourable and we have recently been able to obtain aspectrum of the complete Balmer line series for Sirius B using the HSTSpace Telescope Imaging Spectrograph (STIS). The quality of the STISspectra greatly exceeds that of previous ground-based spectra, and canbe used to provide an important determination of the stellar temperature(Teff= 25193 K) and gravity (logg= 8.556). In addition, wehave obtained a new, more accurate, gravitational redshift of 80.42 +/-4.83 km s-1 for Sirius B. Combining these results with thephotometric data and the Hipparcos parallax, we obtain newdeterminations of the stellar mass for comparison with the theoreticalmass-radius relation. However, there are some disparities between theresults obtained independently from logg and the gravitational redshiftwhich may arise from flux losses in the narrow 50 × 0.2arcsec2 slit. Combining our measurements of Teffand logg with the Wood evolutionary mass-radius relation, we obtain abest estimate for the white dwarf mass of 0.978 Msolar.Within the overall uncertainties, this is in agreement with a mass of1.02 Msolar obtained by matching our new gravitationalredshift to the theoretical mass-radius relation.
|Absolute Calibration of the Infrared Array Camera on the Spitzer Space Telescope|
The Infrared Array Camera (IRAC) on the Spitzer Space Telescope isabsolutely calibrated by comparing photometry of a set of A stars nearthe north ecliptic pole to predictions based on ground-basedobservations and a stellar atmosphere model. The brightness of pointsources is calibrated to an accuracy of 3%, relative to models forA-star stellar atmospheres, for observations performed and analyzed inthe same manner as for the calibration stars. This includes correctionsfor the location of the star in the array and the location of thecentroid within the peak pixel. Long-term stability of the IRACphotometry was measured by monitoring the brightness of A dwarfs and Kgiants (near the north ecliptic pole) observed several times per month;the photometry is stable to 1.5% (rms) over a year.Intermediate-timescale stability of the IRAC photometry was measured bymonitoring at least one secondary calibrator (near the ecliptic plane)every 12 hr while IRAC was in nominal operations; the intermediate-termphotometry is stable, with a 1% dispersion (rms). One of the secondarycalibrators was found to have significantly time-variable (5%)mid-infrared emission, with a period (7.4 days) matching the opticallight curve; it is possibly a Cepheid variable.
|The Detection and Measurement of Telluric Ozone from Stellar Spectra|
``One man's noise is another man's signal.'' Never has that(mis)quotation been more apt than in this instance, in which theannoying features of telluric origin, bypassed and avoided by theastronomer, need to be salvaged by the atmospheric scientist from datadetritus. This paper describes the first phase of a project designed todetermine telluric O3 column densities from historic stellarspectra, and demonstrates the validity of the concept by comparing theresults with almost contemporary measurements made by satellite.
|An Atlas of Spectrophotometric Landolt Standard Stars|
We present CCD observations of 102 Landolt standard stars obtained withthe Ritchey-Chrétien spectrograph on the Cerro TololoInter-American Observatory 1.5 m telescope. Using stellar atmospheremodels, we have extended the flux points to our six spectrophotometricsecondary standards, in both the blue and the red, allowing us toproduce flux-calibrated spectra that span a wavelength range from 3050Å to 1.1 μm. Mean differences between UBVRI spectrophotometrycomputed using Bessell's standard passbands and Landolt's publishedphotometry were determined to be 1% or less. Observers in bothhemispheres will find these spectra useful for flux-calibrating spectra,and through the use of accurately constructed instrumental passbands,will be able to compute accurate corrections to bring instrumentalmagnitudes to any desired standard photometric system (S-corrections).In addition, by combining empirical and modeled spectra of the Sun,Sirius, and Vega, we calculate and compare synthetic photometry toobserved photometry taken from the literature for these three stars.
|The open-cluster initial-final mass relationship and the high-mass tail of the white dwarf distribution|
Recent studies of white dwarfs in open clusters have provided newconstraints on the initial-final mass relationship (IFMR) formain-sequence stars with masses in the range 2.5-6.5Msolar.We re-evaluate the ensemble of data that determines the IFMR and arguethat the IFMR can be characterized by a mean IFMR about which there isan intrinsic scatter. We investigate the consequences of the IFMR forthe observed mass distribution of field white dwarfs using populationsynthesis calculations. We show that while a linear IFMR predicts a massdistribution that is in reasonable agreement with the recent resultsfrom the Palomar-Green survey, the data are better fitted by an IFMRwith some curvature. Our calculations indicate that a significant (~28)percentage of white dwarfs originating from a single star evolution hasmasses in excess of ~0.8Msolar, obviating the necessity forpostulating the existence of a dominant population of high-mass whitedwarfs that arise from binary star mergers.
|Observations of Titan's Mesosphere|
We have recorded spectra of Titan's ν4 band ofCH4 at a higher resolving power (R=70,000) than priormeasurements to better constrain the thermal structure of Titan'satmosphere from 100 to 600 km altitude. Radiative transfer analyses ofthe spectra indicate a temperature profile below 300 km that isconsistent with past measurements. The high resolving power of ourobservations provides the first infrared measurement of Titan's thermalstructure between 300 and 600 km. We detect the presence of amesosphere, with a drop of temperature above380+50-100 km altitude of at least 15 K,consistent with radiative cooling of the atmosphere by emission fromhydrocarbons.
|A Spectroscopic Study of HL Canis Majoris|
We present optical spectroscopy of the dwarf nova HL Canis Majoris overa span of 4 years. The observations were made during standstill,outburst, and quiescence. We determine an orbital period of0.2167867+/-0.0000017 days, based on radial velocities determined fromHα, Hβ, and He I λ 5876 emission. We also presentequivalent widths of the spectral features in outburst and inquiescence.Based on observations obtained at the MDM Observatory, operated byDartmouth College, Columbia University, Ohio State University, and theUniversity of Michigan.
|Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres|
We search the Hubble Space Telescope (HST) archive for previouslyunanalyzed observations of stellar H I Lyα emission lines, ourprimary purpose being to look for new detections of Lyα absorptionfrom the outer heliosphere and to also search for analogous absorptionfrom the astrospheres surrounding the observed stars. The astrosphericabsorption is of particular interest because it can be used to studysolar-like stellar winds that are otherwise undetectable. We find andanalyze 33 HST Lyα spectra in the archive. All the spectra weretaken with the E140M grating of the Space Telescope Imaging Spectrograph(STIS) instrument on board HST. The HST STIS spectra yield four newdetections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD165185) and seven new detections of astrospheric absorption (EV Lac, 70Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling theprevious number of heliospheric and astrospheric detections. Whencombined with previous results, 10 of 17 lines of sight within 10 pcyield detections of astrospheric absorption. This high detectionfraction implies that most of the ISM within 10 pc must be at leastpartially neutral, since the presence of H I within the ISM surroundingthe observed star is necessary for an astrospheric detection. Incontrast, the detection percentage is only 9.7% (3 out of 31) for starsbeyond 10 pc. Our Lyα analyses provide measurements of ISM H I andD I column densities for all 33 lines of sight, and we discuss someimplications of these results. Finally, we measure chromosphericLyα fluxes from the observed stars. We use these fluxes todetermine how Lyα flux correlates with coronal X-ray andchromospheric Mg II emission, and we also study how Lyα emissiondepends on stellar rotation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
|The epoch of the constellations on the Farnese Atlas and their origin in Hipparchus's lost catalogue|
|Numerical Simulations of the Hyades Dynamics and the Nature of the Moving Hyades Cluster|
We present our numerical simulations of the dynamical evolution of theHyades open cluster. The simulations were performed usinga modifiedNBODY6 algorithm that included tidal forces and a realistic orbit of thecluster in a gravitational field described by the Miyamoto-Nagaipotential. Our goal was to study the nature of movingclu sters. We showthat the stars that were earlier cluster members could be lateridentified within a sphere of 50 pc in diameter around the Sun. Thenumber of such stars for the chosen initial mass and virial radius ofthe cluster does not exceed ten. The maximum space velocity of thesestars relative to the core of the current cluster does not exceed 3 kms-1. Our numerical simulations confirm the assumption that some of themoving clusters near the Sun could consist of stars that have escapedfrom open clusters in the course of their dynamical evolution.
|Infrared portrait of the nearby massive star-forming region IRAS 09002-4732|
We present high-resolution near-infrared and mid-infrared imaging,mid-infrared spectroscopy and millimetre-wavelength continuumobservations of the nearby massive star-forming complex IRAS 09002-4732.A rich cluster of young stars displaying near-infrared excess emissionis detected. We identify the ionising source of the ultracompact H IIregion G268.42-0.85 and show that this star is the dominant heating andilluminating source of the region. Spectral type estimates based ondifferent methods are consistent with a star of spectral type O9. Thecombination of the new observations with literature data allows us toset up the first structural model for the region. We argue that theultracompact H II region is embedded in the rear side of the southern CSclump. Additionally, we detect several interesting objects. Among theseobjects are a network of dark dust filaments, an elongated, externallyheated object with strong infrared excess inside the H II region andobjects seen as silhouettes in the foreground of the large southernreflection nebulosity. The filamentary structures may play an importantrole in the star formation process.
|Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey I|
We present a quantitative atlas and catalog of dark clouds derived byusing the optical database ``Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts.
|Predicting accurate stellar angular diameters by the near-infrared surface brightness technique|
I report on the capabilities of the near-infrared (near-IR) surfacebrightness technique to predict reliable stellar angular diameters asaccurate as <~2 per cent using standard broad-band Johnson photometryin the colour range -0.1 <= (V-K)O<= 3.7 includingstars of A, F, G, K spectral type. This empirical approach is fast toapply and leads to estimated photometric diameters in very goodagreement with recent high-precision interferometric diametermeasurements available for non-variable dwarfs and giants, as well asfor Cepheid variables. Then I compare semi-empirical diameters predictedby model-dependent photometric and spectrophotometric (SP) methods withnear-IR surface brightness diameters adopted as empirical referencecalibrators. The overall agreement between all these methods is withinapproximately +/-5 per cent, confirming previous works. However, on thesame scale of accuracy, there is also evidence for systematic shiftspresumably as a result of an incorrect representation of the stellareffective temperature in the model-dependent results. I also comparemeasurements of spectroscopic radii with near-IR surface brightnessradii of Cepheids with known distances. Spectroscopic radii are found tobe affected by a scatter as significant as >~9 per cent, which is atleast three times greater than the formal error currently claimed by thespectroscopic technique. In contrast, pulsation radii predicted by theperiod-radius (PR) relation according to the Cepheid period result aresignificantly less dispersed, indicating a quite small scatter as aresult of the finite width of the Cepheid instability strip, as expectedfrom pulsation theory. The resulting low level of noise stronglyconfirms our previous claims that the pulsation parallaxes are the mostaccurate empirical distances presently available for Galactic andextragalactic Cepheids.
|Disk Evolution in the Orion OB1 Association|
We analyze multiband photometry of a subsample of low-mass stars in theassociations Ori OB1a and 1b discovered during the Centro deInvestigaciones de Astronomía (CIDA) Orion Variability Survey,which have ages of 7-10 and 3-5 Myr, respectively. We obtainedUBVRCIC photometry at Mount Hopkins for sixclassical T Tauri stars (CTTSs) and 26 weak T Tauri stars (WTTSs) in OriOB1a and for 21 CTTSs and two WTTSs in Ori OB1b. We also obtained L-bandphotometry for 14 CTTSs at Mount Hopkins and 10 and 18 μm photometrywith OSCIR at Gemini for six CTTSs; of these, all six were detected at10 μm, whereas only one was detected at 18 μm. We estimate massaccretion rates from the excess luminosity at U and find that they areconsistent with determinations for a number of other associations, withor without high-mass star formation. The observed decrease of massaccretion rate with age is qualitatively consistent with predictions ofviscous evolution of accretion disks, although other factors can alsoplay a role in slowing accretion rates. We compare the excesses overphotospheric fluxes in H-K, K-L, and K-N with the younger sample ofTaurus and find an overall decrease of disk emission from Taurus to OriOB1b to Ori OB1a. This decrease implies that significant grain growthand settling toward the midplane has taken place in the inner disks ofOri OB1. We compare the spectral energy distribution of the stardetected at both 10 and 18 μm with disk models for similar stellarand accretion parameters. We find that the low fluxes shortward of 18μm of this Ori OB1b star cannot be due to the smaller disk radiusexpected from viscous evolution in the presence of the far-ultravioletradiation fields from the OB stars in the association. Instead, we findthat the disk of this star is essentially a flat disk, with little ifany flaring, indicating a significant degree of dust settling toward themidplane, as expected from dust evolution in protoplanetary disks.
|Joint VLBA/VLTI Observations of the Mira Variable S Orionis|
We present the first coordinated Very Long Baseline Array (VLBA) / VeryLarge Telescope Interferometer (VLTI) measurements of the stellardiameter and circumstellar atmosphere of a Mira variable star.Observations of the v=1, J=1-0 (43.1 GHz) and v=2, J=1-0 (42.8 GHz) SiOmaser emission toward the Mira variable S Ori were conducted using theVLBA. Coordinated near-infrared K-band measurements of the stellardiameter were performed using VLTI-VINCI closely spaced in time to theVLBA observations. Analysis of the SiO maser data recorded at a visualvariability phase 0.73 shows the average distance of the masers from thecenter of the distribution to be 9.4 mas for the v=1 masers and 8.8 masfor the v=2 masers. The velocity structure of the SiO masers appears tobe random, with no significant indication of global expansion/infall orrotation. The determined near-infrared, K-band, uniform disk (UD)diameters decreased from ~10.5 mas at phase 0.80 to ~10.2 mas at phase0.95. For the epoch of our VLBA measurements, an extrapolated UDdiameter of ΘKUD=10.8+/-0.3 mas wasobtained, corresponding to a linear radius ofRKUD=2.3+/-0.5 AU orRKUD=490+/-115 Rsolar. Our coordinatedVLBA/VLTI measurements show that the masers lie relatively close to thestellar photosphere at a distance of ~2 photospheric radii, consistentwith model estimates. This result is virtually free of the usualuncertainty inherent in comparing observations of variable stars widelyseparated in time and stellar phase.
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