Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Time Monitoring Observations of SiO J = 2-1 and J = 3-2 Maser Emission toward Late-Type Stars
We present the results of simultaneous time monitoring observations ofSiO J=2-1 and J=3-2 maser emission for 10 late-type stars (8 Miravariables, 1 OH/IR star, and 1 supergiant) with the 14 m radio telescopeat Taeduk Radio Astronomy Observatory from 1999 January to 2001February. The SiO v=1, J=2-1 and J=3-2 maser emission was detected atalmost all observational epochs. The SiO v=2, J=2-1 maser was detectedfrom 4 late-type stars (VY CMa, R Cas, χ Cyg, R Leo) and the v=2,J=3-2 maser was detected from 7 stars (R Aqr, TX Cam, R Cas, χ Cyg,W Hya, R Leo, IK Tau). The v=3, J=2-1 and J=3-2 masers were alsodetected from χ Cyg and TX Cam, respectively. Based on theseobservational data, line profile and peak velocity variations withrespect to stellar velocity, antenna temperatures, and their ratiovariations as a function of optical phase of central star wereinvestigated. As main results, the line profile and the peak velocityvariation of the v=1, J=3-2 maser with pulsation phase was found todiffer from the v=1, J=2-1 transition. Similarly, the J=2-1 and J=3-2transitions also differ between rovibrational transitions at a givenpulsation phase. However, it is difficult to find significantcorrelations between the peak velocity variation relative to the stellarvelocity of either the J=3-2 or J=2-1 transitions over pulsation phase,due to limited time sampling in these data. The peak and integratedantenna temperature (PT and IT) ratios among rotational ladders andvibrational states are investigated. These ratios between rotationalladders of the v=1, J=2-1, and J=3-2 masers are averaged to be the peakantenna temperature ratio, PT(v=1, J=3-2)/PT(v=1, J=2-1)~0.29, and theintegrated antenna temperature ratio, IT(v=1, J=3-2)/IT(v=1,J=2-1)~0.21, respectively. In the v=2 state, these ratios are PT(v=2,J=3-2)/PT(v=2, J=2-1)~7.94 and IT(v=2, J=3-2)/IT(v=2, J=2-1)~8.50,respectively. The peak and integrated antenna temperature ratios betweenvibrational states are also averaged to be PT(v=2, J=3-2)/PT(v=1,J=3-2)~1.29, IT(v=2, J=3-2)/IT(v=1, J=3-2)~1.02, PT(v=2, J=2-1)/PT(v=1,J=2-1)~0.06, and IT(v=2, J=2-1)/IT(v=1, J=2-1)~0.05, respectively. Theseintensity ratios for the v=2, J=2-1 and v=2, J=3-2 masers suggest thatline overlaps operating in the v=2, J=2-1 transition do not similarlyaffect the v=2, J=3-2 transition.
| Bringing VY Canis Majoris Down to Size: An Improved Determination of Its Effective Temperature
The star VY CMa is a late-type M supergiant with many peculiarities,mostly related to the intense circumstellar environment due to thestar's high mass-loss rate. Claims have been made that would imply thatthis star is considerably more luminous (L~5×105Lsolar) and larger (R~2800 Rsolar) than otherGalactic red supergiants (RSGs). Indeed, such a location in the H-Rdiagram would be well within the ``Hayashi forbidden zone,'' where starscannot be in hydrostatic equilibrium. These extraordinary properties,however, rest on an assumed effective temperature of 2800-3000 K, farcooler than recent work has shown RSGs to be. To obtain a betterestimate, we fit newly obtained spectrophotometry in the optical and NIRwith the same MARCS models used for our recent determination of thephysical properties of other RSGs; we also use V-K and V-J from theliterature to derive an effective temperature. We find that the starlikely has a temperature of 3650 K, a luminosityL~6×104 Lsolar, and a radius of ~600Rsolar. These values are consistent with VY CMa being anordinary, evolved 15 Msolar RSG and agree well with theGeneva evolutionary tracks. We find that the circumstellar dust regionhas a temperature of 760 K, and an effective radius of ~130 AU, ifspherical geometry is assumed for the latter. What causes this star tohave such a high mass loss and large variations in brightness (but withlittle change in color) remains a mystery at present, although wespeculate that perhaps this star and NML Cyg are simply normal RSGscaught during an unusually unstable time.
| Infrared Spectra and Visibilities as Probes of the Outer Atmospheres of Red Supergiant Stars
In the light of the recent results of the stellar interferometry, weexamine the nature of the extra molecular layer outside the photosphereof red supergiant stars, so far studied mostly with the use of theinfrared spectra. Although the visibility data are more direct probes ofthe spatial structure of the outer atmosphere, it is essential that theyare analyzed in combination with the spectral data. In the case of theM2 supergiant μ Cephei, several sets of data, both spectra andvisibilities, strongly suggested the presence of an extra molecularlayer (which we referred to as ``MOLsphere'' for simplicity), and thebasic parameters of the MOLsphere are estimated to be excitationtemperature Tex~1600 K, column densities of CO andH2O molecules Ncol~3.0×1020cm-2, and located at about one stellar radius above thephotosphere or Rin~2.0R*. The result showsreasonable agreement with the one based on the infrared spectra alone,and the model inferred from the spectra is now fully supported with therecent visibility data. In the case of the M2 supergiant αOrionis, the infrared spectra and visibilities show a consistent picturein that its MOLsphere is closer to the photosphere(Rin~1.3R*) with higher gas temperature(Tex~2250 K) and lower gas column density(Ncol~1020 cm-2), compared with that ofμ Cep. Some controversy on the interpretation of the mid-infrareddata of α Orionis can be reconciled. Given that the presence ofthe extra molecular layer is reasonably well established, the majorunsolved problem is how to understand the origin of such a rather warmand dense layer in the outer atmosphere.
| Discovery of an Extraordinarily Massive Cluster of Red Supergiants
We report the discovery of an extraordinarily massive young cluster ofstars in the Galaxy, having an inferred total initial cluster masscomparable to the most massive young clusters in the Galaxy. UsingIRMOS, 2MASS, and Spitzer observations, we conclude that there are 14red supergiants in the cluster, compared with five, in what waspreviously thought to be the richest Galactic cluster of such stars. Weinfer spectral types from near-infrared spectra that reveal deep CObandhead absorption that can only be fit by red supergiants. We identifya gap of ΔKs~4 mag between the stars and the bulk ofthe other stars in the region that can only be fit by models if thebrightest stars in the cluster are red supergiants. We estimate adistance of 5.8 kpc to the cluster by associating an OH maser with theenvelope of one of the stars. We also identify a ``yellow'' supergiantof G6 I type in the cluster. Assuming a Salpeter IMF, we infer aninitial cluster mass of 20,000-40,000 Msolar for cluster agesof 7-12 Myr. Continuing with these assumptions, we find that 80% of theinitial mass and 99% of the number of stars remain at the present time.We associate the cluster with an X-ray source (detected by ASCA andEinstein), a recently discovered very high energy γ-ray source(detected by INTEGRAL and HESS), and several nonthermal radio sources,finding that these objects are likely related to recent supernovae inthe cluster. In particular, we claim that the cluster has produced atleast one recent supernova remnant with properties similar to the CrabNebula.
| V4332 Sgr in `Quiescence'
In 1994 V4332 Sgr underwent a mysterious eruption. Somehow its fastevolution towards a red giant star was, lacking alternativeclassifications, connected to the red variable M31 RV, which had itseruption in 1988. The red eruptive variable V838 Mon draw the attentionback to its 'older twin' V4332 Sgr in February 2002. The new precisephotometry of the progenitor given here shows that the object started torise years before the 1994 event. Post-outburst photometry andspectroscopy from 2002 and 2003 show that the object stopped its declineand seems now to become hotter. The progenitor data and the new highquality spectra provide a supplement and completion to the data aroundthe outburst given by Martini et al. (1999). They thus allow theoriststo give new boundaries for modelling this unusual object.Based on observations collected at the European Southern Observatory, LaSilla and on observations collected at Asiago observatory
| Changes in the Velocity Centroid of SiO Maser Emission
The v=1, J=1-0 silicon monoxide maser emission from 85 stars wasresurveyed in 2005 and 2006. The emission from these stars hadpreviously been measured in 1979 and/or 1983. For the detected stars thevelocity centroids were determined and compared with the previousvalues. If supergiant stars, known binary stars, and S Virginis wereremoved from the data set, 76 sources remained. Seven of these sourceswere not of sufficient flux density to be claimed as detections. Themean difference in the velocity centroids of the remaining sources was0.065 km s-1, and the standard deviation was 2.00 km s-1. It is important to determine the standard deviation of SiOmaser time variations in order to identify the level of the timevariability of the sources, possible binary stars, unusual emissionpatterns, or perhaps planets. The velocity centroid of S Virginisdiffered by 13.5 km s-1 from its 1983 value. This differenceindicates that S Virginis is probably part of a multiple-star system. Anaive analysis of the present data combined with earlier publishedobservations indicates a revolution period of 14.8 yr.
| M33's Variable A: A Hypergiant Star More Than 35 YEARS in Eruption
Variable A in M33 is a member of a rare class of highly luminous,evolved stars near the upper luminosity boundary that show sudden anddramatic shifts in apparent temperature due to the formation ofoptically thick winds in high mass loss episodes. Recent optical andinfrared spectroscopy and imaging reveal that its ``eruption,'' begun in~1950, has ended, having lasted ~45 yr. Our current observations showmajor changes in its wind from a cool, dense envelope to a much warmerstate surrounded by low-density gas with rare emission lines of Ca II,[Ca II], and K I. Its spectral energy distribution has unexpectedlychanged, especially at the long wavelengths, with a significant decreasein its apparent flux, while the star remains optically obscured. Weconclude that much of its radiation is now escaping out of our line ofsight. We attribute this to the changing structure and distribution ofits circumstellar ejecta, corresponding to the altered state of its windas the star recovers from a high mass loss event.
| The Circumstellar Environments of NML Cygni and the Cool Hypergiants
We present high-resolution HST WFPC2 images of compact nebulositysurrounding the cool M-type hypergiants NML Cyg, VX Sgr, and S Per. Thepowerful OH/IR source NML Cyg exhibits a peculiar bean-shaped asymmetricnebula that is coincident with the distribution of its H2Ovapor masers. We show that NML Cyg's circumstellar envelope is likelyshaped by photodissociation from the powerful, nearby association CygOB2 inside the Cygnus X superbubble. The OH/IR sources VX Sgr and S Perhave marginally resolved envelopes. S Per's circumstellar nebula appearselongated in a northeast/southwest orientation similar to that for itsOH and H2O masers, while VX Sgr is embedded in a spheroidalenvelope. We find no evidence for circumstellar nebulosity around theintermediate-type hypergiants ρ Cas, HR 8752, and HR 5171a, noraround the normal M-type supergiant μ Cep. We conclude that there isno evidence for high mass loss events prior to 500-1000 years ago forthese four stars.
| Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. I. Theoretical model - Mass-loss history unravelled in VY CMa
Context: .Mass loss plays a dominant role in the evolution of low massstars while they are on the Asymptotic Giant Branch (AGB). The gas anddust ejected during this phase are a major source in the mass budget ofthe interstellar medium. Recent studies have pointed towards theimportance of variations in the mass-loss history of such objects.Aims: .By modelling the full line profile of low excitation CO linesemitted in the circumstellar envelope, we can study the mass-losshistory of AGB stars. Methods: .We have developed a non-LTEradiative transfer code, which calculates the velocity structure and gaskinetic temperature of the envelope in a self-consistent way. Theresulting structure of the envelope provides the input for the molecularline radiative calculations which are evaluated in the comoving frame.The code allows for the implementation of modulations in the mass-lossrate. This code has been benchmarked against other radiative transfercodes and is shown to perform well and efficiently. Results: .Weillustrate the effects of varying mass-loss rates in case of a superwindphase. The model is applied to the well-studied case of VYCMa. We show that both the observed integrated line strengthsas the spectral structure present in the observed line profiles,unambiguously demonstrate that this source underwent a phase of highmass loss (~3.2 × 10-4 Mȯyr-1) some 1000 yr ago. This phase took place for some 100yr, and was preceded by a low mass-loss phase (~1 ×10-6 Mȯ yr-1) taking some 800 yr.The current mass-loss rate is estimated to be in the order of 8 ×10-5 Mȯ yr-1. Conclusions:.In this paper, we demonstrate that both the relative strength of the COrotational line profiles and the (non)-occurrence of spectral structurein the profile offer strong diagnostics to pinpoint the mass-losshistory.
| Submillimeter vibrationally excited water emission from the peculiar red supergiant VY Canis Majoris
Context: .Vibrationally excited emission from the SiO and H2O moleculesprobes the innermost circumstellar envelopes of oxygen-rich red giantand supergiant stars. VY CMa is the most prolific known emission sourcein these molecules. Aims: .Observations were made to search forrotational lines in the lowest vibrationally excited state of H2O.Methods: .The APEX telescope was used for observations of H2O lines atfrequencies around 300 GHz. Results: .Two vibrationally excitedH2O lines were detected, a third one could not be found. In one of thelines we find evidence for weak maser action, similar to known(sub)millimeter ν2 = 1 lines. We find that the otherline's intensity is consistent with thermal excitation by thecircumstellar infrared radiation field. Several SiO lines were detectedtogether with the H2O lines.
| Full polarization study of SiO masers at 86 GHz
Aims.We study the polarization of the SiO maser emission in arepresentative sample of evolved stars in order to derive an estimate ofthe strength of the magnetic field, and thus determine the influence ofthis magnetic field on evolved stars. Methods: .We madesimultaneous spectroscopic measurements of the 4 Stokes parameters, fromwhich we derived the circular and linear polarization levels. Theobservations were made with the IF polarimeter installed at the IRAM 30m telescope. Results: . A discussion of the existing SiO masermodels is developed in the light of our observations. Under the Zeemansplitting hypothesis, we derive an estimate of the strength of themagnetic field. The averaged magnetic field varies between 0 and 20Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughoutthe circumstellar envelope. As a consequence, the magnetic field mayplay the role of a shaping, or perhaps collimating, agent of thecircumstellar envelopes in evolved objects.
| Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
| Beam Size, Shape and Efficiencies for the ATNF Mopra Radio Telescope at 86-115 GHz
We present data characterising the performance of the Mopra RadioTelescope during the period 2000-2004, including measurements of thebeam size and shape, as well as the overall beam efficiency of thetelescope. In 2004 the full width half maximum of the beam was measuredto be 36+/-3'' at 86GHz, falling to 33+/-2'' at 115GHz. Based on ourobservations of Jupiter we measured the beam efficiency of the Gaussianmain beam to be 0.49+/-0.03 at 86GHz and 0.42+/-0.02 at 115GHz. Sourceswith angular sizes of ~80'' couple well to the main beam, while sourceswith angular sizes between ~80'' and ~160'' couple to the both the mainbeam and inner error beam. Measurements indicate that the inner errorbeam contains approximately one-third the power of the main beam. Wealso compare efficiency corrected spectra to measurements made atsimilar facilities and present standard spectra taken towards themolecular clouds Orion-KL and M17-SW.
| The OH 1612 MHz maser pump rates of stellar, interstellar and post-AGB OH masers
(Pseudo) radiative pumprate of OH 1612 MHz masers is defined for asample of 44 OH/IR sources (infrared sources with OH 1612 MHz maser),irrespective of the real maser pumping mechanisms. The correlationbetween the (pseudo) maser pumprates and the evolutionary status of themaser sources reveals that the radiative pumprates of stellar OH masersare nearly fixed, which agrees with the theoretical prediction forradiatively pumped OH maser. The (pseudo) radiative pumprates ofinterstellar OH masers are not only very small but also varying broadlyover two orders of magnitude, which is argued to be the manifestation ofvarying number of quiet absorbing OH cloudlets and/or various OH maserpumping mechanisms and/or competitive gain between mainline and 1612 MHzOH masers and/or anisotropy of the maser emission. The radiativepumprates of post-AGB OH masers very possibly decrease with increasingIRAS C32 color indices and distribute in an interim regionbetween the stellar and interstellar OH masers in the pumprate-colordiagram.
| Evolution of 1612-MHz maser emission in expanding circumstellar shells
Observations show that 1612-MHz masers of OH/IR stars can fade on atime-scale of a decade. This fading is probably associated with theswitch from rapid mass loss, which is ultimately linked with an internalHe-shell flash, to the much slower mass loss supported by more quiescentconditions. We study the observed maser decay with a compositecomputational model, comprising a time-dependent chemical model of theenvelope, and a radiation transfer model which provides the maserpumping. Our combined model is able to reproduce the rapid decay ofmaser intensity, following a sudden drop in the stellar mass-loss rate.The explanation for the rapid fall in maser emission is not a fall inthe OH number density, or the kinetic temperature in the invertedlayers, but the loss of a radiative pump route which carries populationfrom levels 1 to 4 via levels 16, 15 and 11. The loss of these pumproutes is a result, in turn, of a greatly reduced energy density of53-μm radiation.
| A circular planetary nebula around the OH/IR star OH354.88-0.54 (V1018 Sco)
New deep, high-resolution Hα imagery from the UK Schmidt Telescope(UKST) Unit's Hα survey of the Southern Galactic Plane reveals thepresence of a faint, highly circular, planetary nebula (PN) surroundinga very long-period variable star (now known as V1018 Sco), firstdiscovered as a 1612-MHz OH maser, OH354.88-0.54. The nebular phase-lagdistance, diameter and radial velocity are 3.2 kpc, 0.3 pc and 13 kms-1, respectively. Combining the maser attributes with near-,mid- and far-infrared data and with our optical spectrum of the ring weconclude that the object was an intermediate-mass asymptotic giantbranch (AGB) star (initial stellar mass >= 4Msolar) inwhich the fast wind has recently turned on, ionizing previously shedcircumstellar material. Hence, we speculate that we may be witnessing ahitherto unobserved phase of PN evolution, in which a PN has onlyrecently started to form around a star that is unequivocally still inits AGB phase.
| Magnetic fields around late-type stars using H_2O maser observations .
We present the analysis of the circular polarization, due to Zeemansplitting, of the H_2O masers around a sample of late-type stars todetermine the magnetic fields in their circumstellar envelopes (CSEs).The magnetic field strengths in the H_2O maser regions around the Miravariable stars U Ori and U Her are shown to be several Gauss while thoseof the supergiants S Per, NML Cyg and VY CMa are several hundred mG. Wealso show that large scale magnetic fields permeate the CSE of anevolved star; the polarization of the H_2O masers around VX Sgr revealsa dipole field structure. We shortly discuss the coupling of themagnetic field with the stellar outflow, as such fields could possiblybe the cause of distinctly aspherical mass-loss and the resultingaspherical planetary nebulae.
| Evidence of asymmetry in Mira variable U Ori
Near simultaneous, two high angular resolution observations by lunaroccultation technique at the same wavelength (2.2mu m) but at differentposition angles (PA) result in two unique Uniform Disk (UD) angulardiameters of Mira variable U Ori. UD angular diameter obtained fromobservations at Mt. Abu observatory is 11.9(+/-)0.3 milliarcsecond (mas)at PA 136o while from observations at TIRGO observatory UDvalue obtained is 15.14(+/-)0.05 mas at PA 75o. The sourcebrightness profile derived from a model independent analysis shows anasymmetric spatial structure in both cases. Asymmetric structure of thesource at higher spatial scale was also reported by several authors fromOH and H2O maser distribution at radio wavelengths; thesource is more extended at PA of 30 - 60o. Furthermore,moderate level maximum optical intrinsic polarization of ~1-2% at PAapprox 20o - 40o is also detected. All theevidences bring out the spatial asymmetry in U Ori.
| The Reddening of Red Supergiants: When Smoke Gets in Your Eyes
Deriving the physical properties of red supergiants (RSGs) depends onaccurate corrections for reddening by dust. We use our recent modelingof the optical spectra of RSGs to address this topic. First, we findthat previous broadband studies have underestimated the correction forextinction in the visible, and hence the luminosities (if derived fromV); the shift in the effective wavelengths of the standard B and Vbandpasses necessitates using an effective value of the ratioR'V=4.2 to correct broadband photometry of RSGs ifRV=3.1 for early-type stars viewed through the same dust,where we have assumed the standard reddening law of Cardelli andcoauthors. Use of the Fitzpatrick reddening law would lead toR'V=3.8, as well as slightly lower values ofextinction derived from spectrophotometry, but results in slightlypoorer fits. Second, we find that a significant fraction of RSGs inGalactic OB associations and clusters show up to several magnitudes ofexcess visual extinction compared to OB stars in the same regions; weargue that this is likely due to circumstellar dust around the RSGs. Wealso show that the RSG dust production rate (as indicated by the 12μm excess) is well correlated with bolometric luminosity, contrary towhat has been found by earlier studies. The stars with the highestamount of extra visual extinction also show significant near-UV (NUV)excesses compared to the stellar models reddened by the standardreddening law. This NUV excess is likely due to scattering of the star'slight by the dust and/or a larger average grain size than that typicalof grains found in the diffuse interstellar medium. Similar excesseshave been attributed to circumstellar dust around R Coronae Borealisstars. Finally, we estimate that the RSGs contribute dust grains at therate of 3×10-8Msolar yr-1kpc-2 in the solar neighborhood, comparable to what weestimate for late-type WCs, 1×10-7Msolaryr-1 kpc-2. In the solar neighborhood thisrepresents only a few percent of the dust production (which is dominatedby low-mass AGBs), but we note that in low-metallicity starbursts, dustproduction by RSGs would likely dominate over other sources.
| The Evolution of Supernovae in Circumstellar Wind-Blown Bubbles. I. Introduction and One-Dimensional Calculations
Mass loss from massive stars (>~8 Msolar) can result inthe formation of circumstellar wind-blown cavities surrounding the star,bordered by a thin, dense, cold shell. When the star explodes as acore-collapse supernova (SN), the resulting shock wave will interactwith this modified medium around the star, rather than the interstellarmedium. In this work we first explore the nature of the circumstellarmedium around massive stars in various evolutionary stages. This isfollowed by a study of the evolution of SNe within these wind-blownbubbles. The evolution depends primarily on a single parameter Λ,the ratio of the mass of the dense shell to that of the ejectedmaterial. We investigate the evolution for different values of thisparameter. We also plot approximate X-ray surface brightness plots fromthe simulations. For very small values Λ<<1 the effect ofthe shell is negligible, as one would expect. Values of Λ<~1affect the SN evolution, but the SN ``forgets'' about the existence ofthe shell in about 10 doubling times or so. The remnant density profilechanges, and consequently the X-ray emission from the remnant will alsochange. The initial X-ray luminosity of the remnant is quite low, butinteraction of the shock wave with the dense circumstellar shell canincrease the luminosity by 2-3 orders of magnitude. As the reflectedshock begins to move inward, X-ray images will show the presence of adouble-shelled structure. Larger values result in more SN energy beingexpended to the shell. The resulting reflected shock moves quickly backto the origin, and the ejecta are thermalized rapidly. The evolution ofthe remnant is speeded up, and the entire remnant may appear bright inX-rays. If Λ>>1, then a substantial amount of energy may beexpended in the shell. In the extreme case the SN may go directly fromthe free expansion to the adiabatic stage, bypassing the Sedov stage.Our results show that in many cases the SNR spends a significant amountof time within the bubble. The low density within the bubble can delaythe onset of the Sedov stage and may end up reducing the amount of timespent in the Sedov stage. The complicated density profile within thebubble makes it difficult to infer the mass-loss properties of thepre-SN star by studying the evolution of the resulting SNR.
| The Effective Temperature Scale of Galactic Red Supergiants: Cool, but Not As Cool As We Thought
We use moderate-resolution optical spectrophotometry and the new MARCSstellar atmosphere models to determine the effective temperatures of 74Galactic red supergiants (RSGs). The stars are mostly members of OBassociations or clusters with known distances, allowing a criticalcomparison with modern stellar evolutionary tracks. We find we canachieve excellent matches between the observations and the reddenedmodel fluxes and molecular transitions, although the atomic lines Ca Iλ4226 and Ca II H and K are found to be unrealistically strong inthe models. Our new effective temperature scale is significantly warmerthan those in the literature, with the differences amounting to 400 Kfor the latest type M supergiants (i.e., M5 I). We show that the newlyderived temperatures and bolometric corrections give much betteragreement with stellar evolutionary tracks. This agreement provides acompletely independent verification of our new temperature scale. Thecombination of effective temperature and bolometric luminosities allowsus to calculate stellar radii; the coolest and most luminous stars (KWSgr, Case 75, KY Cyg, HD 206936=μ Cep) have radii of roughly 1500Rsolar (7 AU), in excellent accordance with the largeststellar radii predicted from current evolutionary theory, althoughsmaller than that found by others for the binary VV Cep and for thepeculiar star VY CMa. We find that similar results are obtained for theeffective temperatures and bolometric luminosities using only thedereddened V-K colors, providing a powerful demonstration of theself-consistency of the MARCS models.
| Near-Infrared Water Lines in V838 Monocerotis
V838 Monocerotis had an intriguing, nova-like outburst in 2002 Januarythat has subsequently led to several studies of the object. It is nowrecognized that the outburst of V838 Mon and its evolution are differentfrom those of a classical nova or other classes of well-known eruptivevariables. V838 Mon, along with two other objects that have analogousproperties, appears to make up a new class of eruptive variables. Thereare limited infrared studies of V838 Mon. Here we present near-infraredH-band (1.5-1.75 μm) spectra of V838 Mon from late 2002 to the end of2004. The principal new result from our work is the detection of severalrotation-vibration lines of water in the H-band spectra. The observedwater lines have been modeled to first establish that they are indeeddue to water. Subsequently the temperature and column densities of theabsorbing material, from where the water absorption features originate,are derived. From our analysis, we find that the water features arisefrom a cool, ~750-900 K, region around V838 Mon that appears to begradually cooling with time.
| Radio Photosphere and Mass-Loss Envelope of VY Canis Majoris
We have used the VLA to detect emission from the supergiant VY CMa atradio wavelengths and have constructed 3000-4500 K isothermal outeratmospheres constrained by the data. These models produce a radiophotosphere at 1.5-2 R*. An extrapolation of the model canaccount for the observed total mass-loss rate of the star. We alsopresent mid-infrared imaging of the supergiant which suggests that warmdust is extended in the same direction as the near-infrared reflectionnebula around VY CMa. The origin of the asymmetries in the outflowremains an unsolved problem.
| First Detection of 28Si18O Maser and Thermal Emission from Orion KL
We present the first astronomical detection of28Si18O maser and thermal emission toward Orion KLby using the laboratory-measured line frequencies of28Si18O. The simple model of a28Si18O maser condition adopting a line overlapmechanism leads us to confirm a maser action in spite of its lowestrelative abundance, i.e.,28Si16O/28Si18O~500. Now the28Si18O lines can be used to investigateinterstellar physics and chemistry, especially when the optical depthsof the spectral lines of the main species are high.
| Spectral Observations of the Flare Star HU Del
Results are presented from a study of 27 spectra of the flare star HUDel obtained during 2000-2002 on the 2.6 meter telescope at the ByurakanObservatory. Two flares were detected, along with a weak correlationbetween the equivalent widths of the Hα and Hβ emissionlines. At the time of the photometric maximum of a flare the equivalentwidths of both these lines are minimal and reach their maximum onlysignificantly afterward. A strong increase in the Hβ line intensityis observed which appears to be the consequence of a photometricallyundetected, short-duration flare. A possible relationship between thebinary (multiple) character and the physical activity of this star isexamined briefly in terms of current theoretical models.
| The Young Cluster NGC 2362
An Hα emission survey of the young cluster NGC 2362 resulted inthe detection of 130 Hα emission stars in an11'×11' field approximately centered on thefourth magnitude O9 Ib multiple star τ CMa. The survey was carriedout using the wide-field grism spectrograph on the University of Hawaii2.2 m telescope and the Gemini Multi-Object Spectrograph (GMOS) onGemini North. Deep optical VRCIC (to V~23.0) andnear-infrared (NIR) photometry (JHK) to K~16 were obtained for severalfields within the cluster. Spectra covering the 6000-8000 Å regionat a resolution of R~3000 (adequate for the determination of Li Iλ6708 line strengths) were also acquired for ~200pre-main-sequence (PMS) candidates with GMOS. Ages and masses for theHα emitters in NGC 2362 were inferred from the isochrones andevolutionary tracks of D'Antona & Mazzitelli, as well as those ofBaraffe et al. An estimated cluster age of ~1.8 Myr follows from themodels of D'Antona & Mazzitelli and 3.5-5.0 Myr from those ofBaraffe et al. The fraction of the T Tauri star (TTS) population that iscomposed of weak-line emitters, f(WTTS), is 0.91, compared with 0.43 forthe TTS population of NGC 2264. On the basis of W(Hα) alone, thefraction of TTSs still undergoing accretion is 5%-9%, comparable to theinner disk fraction determined from JHKL-band excesses by Haisch andcoworkers (12%). Approximately 15% of the PMS sample in this studyexhibits possible NIR excess, having EH-K>0.1 mag. Giventhe lack of NIR excess and strong Hα emission from the majority ofcluster members, it is inferred that the inner disk regions of the TTSpopulation have dissipated significantly. The mean level ofchromospheric activity among the WTTS population of NGC 2362 islog(LHα/Lbol)=-3.65, significantly greaterthan that of the low-mass population of the 600 Myr old Hyades cluster,log(LHα/Lbol)=-3.90. The total mass of theHα emitters and the OB stellar population of NGC 2362 defines alower limit for the cluster mass of ~300 Msolar. Allowancefor A- and F-type stars still on the radiative track, multiplicity,outlying members, and the low-mass population lying below thecompleteness limit of the Hα emission survey increases this lowerlimit to well over 500 Msolar. The derived relaxation,disruption, and evaporation timescales for the cluster imply that NGC2362 will likely survive beyond the age of the Pleiades, but statisticsof galactic cluster lifetimes favor its disruption well before the ageof the Hyades.
| 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).
| New Silhouette Disks with Reflection Nebulae and Outflows in the Orion Nebula and M43
We report the detection of several new circumstellar disks seen insilhouette against background nebular light in the outskirts of theOrion Nebula and the neighboring H II region M43. These were detected aspart of our Hα survey of Orion with the Advanced Camera forSurveys on board the Hubble Space Telescope. Several of the disks showbipolar reflection nebulae, microjets, or pronounced temporalvariability of their central stars. The relatively large fraction ofbipolar reflection nebulae and microjets in our sample may be aselection effect caused by the faint nebular background far from theTrapezium. Two disks in our sample are large and particularlynoteworthy: A nearly edge-on disk, d216-0939, is located severalarcminutes northwest of M43 and resembles the famous HH 30 disk/jetsystem in Taurus. It drives the 0.15 pc long bipolar outflow HH 667 andexhibits a remarkable asymmetric reflection nebula caused by the tilt ofthe flared disk. With a diameter of ~2.6" (1200 AU), d216-0939 is aslarge as the giant edge-on silhouette disk d114-426 in the core of theOrion Nebula. The large disk d253-1536 is located in a binary systemembedded within an externally ionized giant proplyd in M43. The diskexhibits distortions that we attribute to tidal interactions with thecompanion star. The bipolar jet HH 668 emerges from the proplydionization front in a direction orthogonal to the disk and can be tracedto the young star embedded within it. A bow shock lies 54" south of thisbinary system along the outflow axis. Proper motions over a 1.4 yrbaseline confirm that these emission knots are indeed moving away fromd253-1536 with speeds as high as ~330 km s-1 in the HH 668microjet and slower motion farther from the star.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS5-26555.
| V4332 Sagittarii revisited
The eruption of V4332 Sgr discovered in February 1994 shows strikingsimilarities to that of V838 Mon started in January 2002. The nature ofthese eruptions is, however, enigmatic and unclear. We present newphotometric and spectroscopic data on V4332 Sgr obtained in April-May2003 at the SAAO. The obtained spectrum shows an unusual emission-linecomponent superimposed on an early M-type stellar spectrum. Theemission-line spectrum is of very low excitation and is dominated bylines from neutral elemets (NaI, FeI, CaI) and molecular bands (TiO,ScO, AlO). We also analyse all the observational data, mainlyphotometric measurements, available for V4332 Sgr. This allows us tofollow the evolution of the effective temperature, radius and luminosityof the object since February 1994 till 2003. We show that the observeddecline of V4332 Sgr can be accounted for by a gravitational contractionof an inflated stellar envelope. The combined optical and infraredphotometry in 2003 shows that apart from the M-type stellar componentthere is a strong infrared excess in the KLM bands. This excess wasabsent in the 2MASS measurements done in 1998 but was probably startingto appear in K in 1999 when the object was observed in the DENIS survey.We interpret the results in terms of a stellar merger scenario proposedby Soker & Tylenda. The infrared excess is likely to be due to adisc-like structure which is either of protostellar nature or has beenproduced during the 1994 eruption and stores angular momentum from themerger event.
| An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars
We present an empirical determination of the mass-loss rate as afunction of stellar luminosity and effective temperature, foroxygen-rich dust-enshrouded Asymptotic Giant Branch stars and redsupergiants. To this aim we obtained optical spectra of a sample ofdust-enshrouded red giants in the Large Magellanic Cloud, which wecomplemented with spectroscopic and infrared photometric data from theliterature. Two of these turned out to be hot emission-line stars, ofwhich one is a definite B[e] star. The mass-loss rates were measuredthrough modelling of the spectral energy distributions. We thus obtainthe mass-loss rate formula log dot{M} = -5.65 + 1.05 log ( L / 10 000{L}_ȯ ) -6.3 log ( T_eff / 3500 K ), valid for dust-enshrouded redsupergiants and oxygen-rich AGB stars. Despite the low metallicity ofthe LMC, both AGB stars and red supergiants are found at late spectraltypes. A comparison with galactic AGB stars and red supergiants showsexcellent agreement between the mass-loss rate as predicted by ourformula and that derived from the 60 μm flux density fordust-enshrouded objects, but not for optically bright objects. Wediscuss the possible implications of this for the mass-loss mechanism.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Cane Maggiore |
| Ascensione retta: | 07h22m58.29s |
| Declinazione: | -25°46'03.5" |
| Magnitudine apparente: | 9.527 |
| Distanza: | 561.798 parsec |
| Moto proprio RA: | 0.9 |
| Moto proprio Dec: | -9.2 |
| B-T magnitude: | 12.229 |
| V-T magnitude: | 9.751 |
Cataloghi e designazioni:
|
