Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
A Spitzer Space Telescope Study of Disks in the Young σ Orionis Cluster
We report new Spitzer Space Telescope observations, using the IRAC andMIPS instruments, of the young (~3 Myr) σ Orionis cluster. Weidentify 336 stars as members of the cluster, using optical andnear-infrared color-magnitude diagrams. Using the spectral energydistribution slopes in the IRAC spectral range, we place objects intoseveral classes: non-excess stars, stars with optically thick disks(such as classical T Tauri stars), class I (protostellar) candidates,and stars with ``evolved disks'' the last exhibit smaller IRAC excessesthan optically thick disk systems. In general, this classificationagrees with the location expected in IRAC-MIPS color-color diagrams forthese objects. We find that the evolved disk systems are mostly acombination of objects with optically thick but nonflared disks,suggesting grain growth and/or settling, and transition disks, systemsin which the inner disk is partially or fully cleared of small dust. Inall, we identify seven transition disk candidates and three possibledebris disk systems. As in other young stellar populations, the fractionof disks depends on the stellar mass, ranging from ~10% for stars in theHerbig Ae/Be mass range (>2 Msolar) to ~35% for those inthe T Tauri mass range (1-0.1 Msolar). The IRAC infraredexcesses found in stellar clusters and associations with and withoutcentral high-mass stars are similar, suggesting that externalphotoevaporation is not very important in many clusters. Finally, wefind no correlation between the X-ray luminosity and the disk infraredexcess, suggesting that the X-rays are not strongly affected by diskaccretion.
| Spitzer Observations of NGC 2362: Primordial Disks at 5 Myr
We present results from a mid-infrared imaging survey of the ~5 Myr oldcluster NGC 2362 carried out with the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope. The archival mid-infrared data weremerged with extant Hα emission data, optical and near-infraredphotometry, and moderate-resolution optical spectroscopy to identify theremnant disk-bearing population of the cluster and to estimate thefraction of stars that still retain primordial circumstellar disks. Theprincipal sample of 232 suspected cluster members with masses rangingfrom ~10 to 0.3 Msolar (B2-M5 spectral types) was drawn fromknown Hα emission stars, X-ray-detected stars from a single 100 ksarchival Chandra observation, and established lithium-rich stars. Asecond sample of 153 stars over a similar mass range whose membershipstatus was based on optical photometry alone was also examined. Measuredfluxes in the optical and infrared passbands were fitted with synthetic,low-resolution spectra created using the NextGen atmospheric models,permitting the detection of infrared excesses relative to predictedstellar photospheric fluxes. Using the measured slope of the stellarspectral energy distribution through the four IRAC channels tocharacterize disk emission for the 195 out of 232activity/lithium-selected stars and the 105 out of 153 photometricmembership candidates having complete IRAC photometry, we derive anupper limit for the primordial, optically thick disk fraction of NGC2362 of ~7%+/-2%, with another ~12%+/-3% of suspected members exhibitinginfrared excesses indicative of weak or optically thin disk emission.The presence of circumstellar disks among candidate members of NGC 2362is strongly mass-dependent, such that no stars more massive than ~1.2Msolar exhibit significant infrared excess shortward of 8μm. An upper limit for the fraction of stars hosting primordial,optically thick disks peaks near 10.7%+/-4% for stars with massesbetween 1.05 and 0.6 Msolar, but the Spitzer IRAC survey issensitivity-limited below ~0.3 Msolar. From Hαemission-line strengths, an upper limit for the accretion fraction ofthe cluster is estimated at ~5%, with most suspected accretorsassociated with primordial, optically thick disks identified withSpitzer. The presence of primordial disk-bearing stars in NGC 2362, someof which are suspected of still experiencing gaseous accretion, mayimply that even within dense cluster environments, sufficient numbers ofinner disks survive to ages consistent with core accretion models ofgiant planet formation to account for the observed frequency ofexoplanets within 5 AU of all FGKM-type stars.
| Young Stellar Groups around Herbig Ae/Be Stars: A Low-Mass YSO Census
We present near-IR and mid-IR observations of eight embedded youngstellar groups around Herbig Ae/Be stars (HAEBEs) using archived SpitzerIRAC data and 2MASS data. These young stellar groups are nearby (<=1kpc) and still embedded within their molecular clouds. In order toidentify the young stellar objects in our sample, we use the color-colordiagram of J-[3.6] versus Ks-[4.5]. The Spitzer images of oursample show that the groups around HAEBEs, spectral types earlier thanB8, are usually associated with bright infrared nebulosity. Within this,there are normally 10-50 young stars distributed close to the HAEBEs(<1 pc). Not only are there young stars around the HAEBEs, there arealso young stellar populations throughout the whole cloud, some of whichare distributed and some of which are clumped. The groups around theHAEBEs are substructures of the large young population within themolecular cloud. The sizes of groups are also comparable with thosesubstructures seen in massive clusters. Young stars in groups aroundHAEBEs have generally larger SED slopes compared to those outside, whichsuggests that the young stars in groups are probably younger than thedistributed systems. This might imply that there is usually a higher andmore continuous star-forming rate in groups, that the formation ofgroups initiates later, or that low-mass stars in groups form slowerthan those outside. Finally, there is no obvious trend between the SEDslopes and the distance to the HAEBEs for those young stars within thegroups. This suggests that the clustering of young stars dominates overthe effect of massive stars on the low-mass young stars at the scale ofour study.
| The Spitzer c2d Survey of Nearby Dense Cores. IV. Revealing the Embedded Cluster in B59
Infrared images of the dark cloud core B59 were obtained with the Spitzer Space Telescope as part of the ``Cores to Disks'' Legacy Scienceproject. Photometry from 3.6-70 μm indicates at least 20 candidatelow-mass young stars near the core, more than doubling the previouslyknown population. Out of this group, 13 are located within ~0.1 pc inprojection of the molecular gas peak, where a new embedded source isdetected. Spectral energy distributions span the range from smallexcesses above photospheric levels to rising in the mid-infrared. Oneother embedded object, probably associated with the millimeter sourceB59-MMS1, with a bolometric luminosity Lbol~2Lsolar, has extended structure at 3.6 and 4.5 μm, possiblytracing the edges of an outflow cavity. The measured extinction throughthe central part of the core is AV>~45 mag. The B59 coreis producing young stars with a high efficiency.
| SiO and CH_3CCH abundances and dust emission in high-mass star-forming cores
Aims.We determine the fractional SiO abundance in high-mass star-formingcores, and investigate its dependence on physical conditions, to provideconstraints on the chemistry models of the formation of SiO in the gasphase or via grain mantle evaporation. The work addresses also CH3CCHchemistry, as the kinetic temperature is determined using thismolecule. Methods: .We estimate the physical conditions of 15high-mass star-forming cores and derive the fractional SiO and CH3CCHabundances using spectral line and dust continuum observations with theSEST. Results: .The kinetic temperatures as derived from CH3CCHrange from 25 to 39 K, the average being 33 K. The average gas densityin the cores is 4.5 × 106 cm-3. The SiOemission regions are extended and typically half of the integrated lineemission comes from the velocity range traced out by CH3CCH emission.The upper limit of SiO abundance in this "quiescent" gas component is~10-10. The average CH3CCH abundance is about 7 ×10-9. It shows a shallow, positive correlation with thetemperature, whereas SiO shows the opposite tendency. Conclusions:.We suggest that the high CH3CCH abundance and its possible increasewhen the clouds become warmer is related to the intensified desorptionof the chemical precursors of the molecule from grain surfaces. Incontrast, the observed tendency of SiO does not support the idea thatthe evaporation of Si-containing species from the grain mantles would beimportant, and it contradicts models where neutral reactions withactivation barriers dominate SiO production. A possible explanation forthe decrease is that warmer cores represent more evolved stages of coreevolution with fewer high-velocity shocks and thus less efficient SiOreplenishment.
| Who is eating the outflow? High-angular resolution study of an intermediate-mass protostar in L1206
Context: .Up to now only a few intermediate-mass molecular outflows havebeen studied with enough high-angular resolution. Aims: .The aimof this work is to study in detail the intermediate-mass YSO IRAS22272+6358A, which is embedded in L1206, and its molecular outflow, toinvestigate the interaction of the outflow with the dense protostellarmaterial and to compare their properties with those of lower masscounterparts. Methods: .We carried out OVRO observations of the2.7 mm continuum emission, CO (J=1→0), C18O(J=1→0),and HC3N (J=12→11) to map the core of L1206 with high-angularresolution and to derive the properties of the dust emission, themolecular outflow, and the dense protostellar envelope. Results:.The 2.7 mm continuum emission has been resolved into four sources,labeled OVRO 1, 2, 3, and 4. The intermediate-mass Class 0/I object OVRO2, with a mass traced by the dust emission of 14.2 Mȯ,is the source associated with IRAS 22272+6358A. The CO (J=1→0)observations have revealed a very collimated outflow driven by OVRO 2,at a PA ≃ 140°, that has a very weak southeastern red lobe anda much stronger northwestern blue lobe. Photodissociation toward the redlobe produced by the ionization front coming from the bright-rimmeddiffuse Hii region could be responsible for the morphology of theoutflow. The spatial correlation between the outflow and the elongateddense protostellar material traced by HC3N (J=12→11) suggests aninteraction between the molecular outflow and the protostellar envelope.Shocks produced by the molecular outflow, and possibly by the shockfront preceding the ionization front, could account for the southernenhancement of HC3N. The properties of the intermediate-mass protostarOVRO 2 and the molecular outflow are consistent with those of lower masscounterparts. The C18O abundance relative to molecularhydrogen estimated toward OVRO 2 is 3×10-8, a value ~6to 13 times lower than typical abundances estimated toward molecularclouds. The most plausible explanation for such a difference is COdepletion toward OVRO 2.
| Disk Evolution in Cep OB2: Results from the Spitzer Space Telescope
We present the results of an infrared imaging survey of two clusters inthe Cep OB2 Association, Tr 37 and NGC 7160, using the IRAC and MIPSinstruments on board the Spitzer Space Telescope. Our observations coverthe wavelength range from 3.6 to 24 μm, allowing us to detect diskemission over a typical range of radii ~0.1 to ~20 AU from the centralstar. In Tr 37, with an age of about 4 Myr, about 48% of the low-massstars exhibit detectable disk emission in the IRAC bands. Roughly 10% ofthe stars with disks may be ``transition'' objects, with essentiallyphotospheric fluxes at wavelengths <=4.5 μm but with excesses atlonger wavelengths, indicating an optically thin inner disk. The medianoptically thick disk emission in Tr 37 is lower than the correspondingmedian for stars in the younger Taurus region; the decrease in infraredexcess is larger at 6-8 μm than at 24 μm, suggesting that graingrowth and/or dust settling has proceeded faster at smaller disk radii,as expected on general theoretical grounds. Only about 4% of thelow-mass stars in the 10 Myr old cluster NGC 7160 show detectableinfrared disk emission. We also find evidence for 24 μm excessesaround a few intermediate-mass stars, which may represent so-called``debris disk'' systems. Our observations provide new constraints ondisk evolution through an important age range.
| VLBI Observations of the Water Masers near LkHα 234 and BD +40 4124
I present observations made with the National Radio AstronomyObservatory's Very Large Array and Very Long Baseline Array of the watermasers near the candidate Herbig Ae/Be star V1318 Cygni S (V1318S) inthe vicinity of the Herbig Ae/Be star BD +40 4124 and near the HerbigAe/Be star LkHα 234. The water masers are not found to beassociated with either of the Herbig Ae/Be stars dominant in the opticalbut with other embedded sources. The masers appear to form in theoutflows from these sources, some of which have been detected in radiocontinuum by other authors. The water masers near BD +40 4124 are foundto be associated with the star V1318S, itself a candidate Herbig Ae/Bestar. The water masers imaged with very long baseline interferometryresolutions near LkHα 234 are found to be associated with thecontinuum sources LkHα 234-VLA 2 and LkHα 234-VLA 3B. I donot detect maser emission from LkHα 234-VLA1. It is likely thatthe water maser emission originally thought to be associated with HerbigAe/Be stars is in fact hosted by embedded young stellar objects, islinked to their outflowing jets, and is not found in their disks.
| Detection of a hot core in the intermediate-mass Class 0 protostar NGC 7129-FIRS 2
We report high angular resolution ({HPBW}˜ 0.6'' × 0.5'' at1.3 mm) observations of the Class 0 intermediate-mass (IM) protostar NGC7129-FIRS 2 using the Plateau de Bure Interferometer. Our observationsshow the existence of an intense unresolved source in the continuum at1.3 mm and 3 mm at the position of the Class 0 object. In addition,compact CH3CN emission is detected at this position. The high rotationaltemperature derived from the CH3CN lines (T_rot ≈ 50 K), as well asthe enhanced CH3CN fractional abundance (X(CH3CN) 7.0×10-9), shows the existence of a hot core in this IM youngstellar object. This is to our knowledge the first IM hot core detectedso far. Interferometric maps of the region in the CH3OH5{kk'}→4{kk'} and D2CO 4{04}→3{03} lines are also presented inthis paper. The methanol emission presents two condensations, oneassociated with the hot core, which was very intense in the high upperstate energy lines (E_u>100 K), and the other associated with thebipolar outflow which dominates the emission in the low excitationlines. Enhanced CH3OH abundances (X(CH3OH) 3× 10-8- afew 10-7) were measured in both components. While intenseD2CO 404 → 303 emission was detected towardsthe hot core, the N2D+ 3→ 2 line was not detected in ourinterferometric observations. The different behaviors of D2CO andN2D+ emissions suggest different formation mechanisms for thetwo species and different deuteration processes for H2CO andN2H+ (surface and gas-phase chemistry, respectively).Finally, the spectrum of the large bandwidth correlator shows a forestof lines at the hot core position, revealing that this object isextraordinarily rich in complex molecules. For deeper insight into thechemistry of complex molecules, we compared the fractional abundances ofthe complex O- and N- bearing species in FIRS 2 with those in hotcorinos and massive hot cores. Within the large uncertainty involved infractional abundance estimates towards hot cores, we did not detect anyvariation in the relative abundances of O- and N-bearing molecules ([CH3CN] /[ CH3OH] ) with the hot core luminosity. However, the O-bearingspecies H2CO and HCOOH seemed to be more abundant in low andintermediate mass stars than in massive star-forming regions. We proposethat this could be the consequence of a different grain mantlecomposition in low and massive star-forming regions.
| The Initial Configuration of Young Stellar Clusters: A K-Band Number Counts Analysis of the Surface Density of Stars
We present an analysis of stellar distributions for the young stellarclusters GGD 12-15, IRAS 20050+2720, and NGC 7129, which range in far-IRluminosity from 227 to 5.68×103 Lsolar andare all still associated with their natal molecular clouds. The dataused for this analysis include near-IR data obtained with FLAMINGOS onthe MMT and newly obtained wide-field 850 μm emission maps from SCUBAon the JCMT. Cluster size and azimuthal asymmetry are measured viaazimuthal and radial averaging methods, respectively. To quantify thedeviation of the distribution of stars from circular symmetry, we definean azimuthal asymmetry parameter, and we investigate the statisticalproperties of this parameter through Monte Carlo simulations. Thedistribution of young stars is compared to the morphology of themolecular gas using stellar surface density maps and the 850 μm maps.We find that two of the clusters are not azimuthally symmetric and showa high degree of structure. The GGD 12-15 cluster is elongated and isaligned with newly detected filamentary structure at 850 μm. IRAS20050+2720 is composed of a chain of three subclusters, in agreementwith Chen and coworkers, although our results show that two of thesubclusters appear to overlap. Significant 850 μm emission isdetected toward two of the subclusters but is not detected toward thecentral subcluster, suggesting that the dense gas may already be clearedthere. In contrast to these two highly embedded subclusters, we find ananticorrelation of the stars and dust in NGC 7129, indicating that muchof the parental gas and dust has been dispersed. The NGC 7129 clusterexhibits a higher degree of azimuthal symmetry, a lower stellar surfacedensity, and a larger size than the other two clusters, suggesting thatthe cluster may be dynamically expanding following the recent dispersalof natal molecular gas. These analyses are further evidence thatembedded, forming clusters are often not spherically symmetricstructures but can be elongated and clumpy and that these morphologiesmay reflect the initial structure of the dense molecular gas.Furthermore, this work suggests that gas expulsion by stellar feedbackresults in significant dynamical evolution within the first 3 Myr ofcluster evolution. We estimate peak stellar volume densities and discussthe impact of these densities on the evolution of circumstellar disksand protostellar envelopes.
| Number ratios of young stellar objects in embedded clusters
Embedded clusters usually contain young stellar objects in differentevolutionary stages. We investigate number ratios of objects in theseclasses in the star-forming regions ρ Ophiuchi, Serpens, Taurus,Chamaeleon I, NGC 7129, IC 1396A and IC 348. They are compared to thetemporal evolution of young stars in numerical simulations ofgravoturbulent fragmentation in order to constrain the models and todetermine the evolutionary stage of the clusters. Serpens is theyoungest and IC 348 the most evolved cluster, although the time when theobservations are best represented by the models varies stronglydepending on the model. We find an inverse correlation of the starformation efficiency (SFE) of the models with the Mach number. However,the observational SFE values cannot be reproduced by the currentisothermal models. This argues for models that take into accountprotostellar feedback processes and/or the effects of magnetic fields.
| A cold globule with a Class 0/I embedded source
Infrared observations of the dark cloud DC 303.8-14.2 and the embeddedpoint source IRAS 13036-7644 have been made with the ISOPHOT instrumentaboard ISO at 7.7 μm, 60 μm, 100 μm and 200 μm. We havecompared the properties of dust in DC 303.8-14.2 and the ThumbprintNebula (TPN), a morphologically similar globule but without starformation. The dust temperature at the center of DC 303.8-14.2 has aminimum of 14.6 ± 1 K, similar to that in the TPN. A comparisonof far-infrared radial optical depth distributions between these cloudsat angular scales of ~1.5'-3' shows no difference. The observationssuggest that there is a bright rim of 7.7 μm emission that peaks justoutside the optical bright rim of the cloud, indicating a halo of verysmall dust particles or PAHs around the cloud. The bolometric luminosityof the IRAS source is estimated to be about 1.0 Lȯ. Theheating of the cloud by the IRAS source is negligible; the thermalstructure of the cloud is dominated by external heating. We have studiedthe evolutionary status of the IRAS source and find it to be locatedbetween Class 0 and Class I, at a late accretion phase. The source wasdetected at 7.7 μm and we suggest that this emission is due to the7.7 μm UIR band. We also give a compilation of small molecular cloudswhose extended FIR emission has been mapped with ISO.
| Chemical evolution in the environment of intermediate mass young stellar objects
We have carried out a molecular survey of the Class 0 IM protostar NGC7129 - FIRS 2 (hereafter FIRS 2) and the Herbig Be star LkHα 234with the aim of studying the chemical evolution of the envelopes ofintermediate-mass (IM) young stellar objects (YSOs). The two objectshave similar luminosities (~500 L_ȯ) and are located in the samemolecular cloud which minimizes the chemical differences due todifferent stellar masses or initial cloud conditions. Moreover, sincethey are located at the same distance, we have the same spatialresolution in both objects. A total of 17 molecular species (includingrare isotopes) have been observed in both objects and the structure oftheir envelopes and outflows has been determined with unprecedentdetail. Our results show that the protostellar envelopes are dispersedand warmed up during the evolution of the YSO into a pre-main sequencestar. In fact, the envelope mass decreases by a factor >5 from FIRS 2to LkHα 234, while the kinetic temperature increases from ~13 K to28 K. On the other hand, there is no molecular outflow associated withLkHα 234. The molecular outflow seems to stop before the starbecomes visible. These physical changes strongly affect the chemistry oftheir envelopes. The N2H+ and NH3 abundances seem to be quitesimilar in the two objects. However, the H13CO+abundance is a factor of ~3 lower in the densest part of FIRS 2 than inLkHα 234, very likely because of depletion. In contrast, the SiOabundance is larger by a factor of ~100 in FIRS 2 than in LkHα234. CS presents complex behavior since its emission arises in differentenvelope components (outflow, cold envelope, hot core) and could alsosuffer from depletion. The CH3OH and H2CO column densities are verysimilar in FIRS 2 and LkHα 234 which implies that thebeam-averaged abundances are a factor >5 larger in LkHα 234than in FIRS 2. The same is found for the PDR tracers CN and HCN whichhave similar column densities in both objects. Finally, complex behavioris found for the deuterated compounds. While theDCO^+/H13CO+ ratio decreases by a factor of ~4from FIRS 2 to LkHα 234, the D2CO/H2CO ratios is within a factor1.5 in both objects. The detection of a warm CH3CN component withTk >63 K shows the existence of a hot core in FIRS 2. Thusfar, only a handful of hot cores have been detected in low andintermediate mass stars. Based on our results in FIRS 2 and LkHα234, we propose some abundance ratios that can be used as chemicalclocks for the envelopes of IM YSOs. The SiO/CS, CN/N2H^+, HCN/N2H^+,DCO^+/HCO+ and D2CO/DCO+ ratios are gooddiagnostics of the protostellar evolutionary stage.Tables 1-3 are only available in electronic form athttp://www.edpsciences.org
| Which Are the Youngest Protostars? Determining Properties of Confirmed and Candidate Class 0 Sources by Broadband Photometry
We searched the literature to obtain a complete list of known Class 0sources. A list of 95 confirmed or candidate objects was compiled. Tothe best of our knowledge, all published broadband observations from 1μm to 3.5 mm have been collected and are assembled in a catalog.These data were used to determine physical properties (Tbol,Lbol, Lsmm/Lbol, Menv) andfor a uniform classification. Fifty sources possess sufficientobservational data and are classified as Class 0 or Class 0/1 objects.The source properties are compared with different evolutionary models toinfer ages and masses, and their correlations are investigated. About25% of the sources are found to be in a quiet accretion phase or possessa significantly different time evolution of the accretion rate than theaverage. In Taurus, with its isolated star formation mode, this seemsespecially to be the case.
| Star formation from gravoturbulent fragmentation:. mass accretion and evolution of protostars
We present numerical simulations of the fragmentation and collapse ofturbulent, self-gravitating molecular clouds. Protostellar massaccretion rates are highly time-variable and are correlated with thefinal stellar mass as well as with the turbulent flow velocity of theenvironment. Combining the accretion rates with an evolutionary schemeallows us to determine T_bol, L_bol, and M_env and generate evolutionarytracks for the model protostars. The distribution of our model stars inthe T_bol-L_bol-M_env parameter space is compared with that of a sampleof all known Class 0 sources. Finally, we investigate the number ratiosof young stars belonging to different evolutionary classes in thenumerical models and compare them to those in observed star-formingclusters.
| Search for HH Objects and Emission Stars in Star Formation Regions. III. PMS Stars in NGC 7129
Slitless spectroscopy is used to discover 22 emission stars in thecentral and northwest regions of the cluster located in the NGC 7129nebula. 16 of them are found for the first time. This sample isessentially complete up to V 20.0. The emission stars are distributednonuniformly over the field of the cluster and are concentrated towardits center. Photometry in the V, R, and I bands is conducted on morethan a hundred stars in the cluster. This yields an average absorptioncoefficient A V=1.7±0.27 for this region. Based on theirpositions in color diagrams for the optical and near IR ranges, most ofthe emission stars can be regarded as T Tau objects.
| The 24 Micron View of Embedded Star Formation in NGC 7129
We present observations of the star formation region NGC 7129 taken withthe Multiband Imaging Photometer for Spitzer (MIPS). A significantpopulation of sources, likely pre-main-sequence members of the youngstellar cluster, are revealed outside the central photoionizationregion. Combining Infrared Array Camera (IRAC) and ground-basednear-infrared images, we have obtained colors and spectral energydistributions for some 60 objects. The [3.6]-[4.5] versus [8]-[24]color-color plane shows sources clustered at several different loci,which roughly correspond to the archetypal evolutionary sequence Class0, I, II, and III. We obtain preliminary classifications for 36 objectsand find significant numbers of both Class I and II objects. Most of thepre-main-sequence candidates are associated with the densest part of themolecular cloud surrounding the photoionization region, indicatingactive star formation over a broad area outside the central cluster. Wediscuss three Class II candidates that exhibit evidence of inner diskclearing, which would be some of the youngest known examples of atransition from accretion to optically thin quiescent disks.
| The NGC 7129 Young Stellar Cluster: A Combined Spitzer, MMT, and Two Micron All Sky Survey Census of Disks, Protostars, and Outflows
We present the analysis of seven-band (1.2 to 8 μm) ground- andspace-based imaging of the NGC 7129 young stellar cluster from FLAMINGOSon MMT, Two Micron All Sky Survey (2MASS), and the Infrared Array Camera(IRAC) on Spitzer. An analysis of the H-[4.5] versus J-H colors reveals84 objects with circumstellar disks. Of these, 42 are located within thecluster core, a 0.5 pc (100") radius region of enhanced stellar surfacedensity. From a luminosity- and extinction-limited sample of the starswithin the cluster core boundary we have determined that 54%+/-14% havecircumstellar disks. Finally, we report the detection of severalresolved outflows in the IRAC 4.5 μm mosaic.
| Initial Results from the Spitzer Young Stellar Cluster Survey
We report initial results from IRAC observations of four young stellarclusters. These regions are part of a larger Spitzer Space Telescopesurvey of 31 young stellar groups and clusters within 1 kpc of the Sun.In each of the four clusters, there are between 39 and 85 objects withcolors inconsistent with reddened stellar photospheres. We identifythese objects as young stars with significant emission fromcircumstellar dust. Applying an analysis developed in a companion paper,we classify these objects as either pre-main-sequence stars with disks(Class II) or protostellar objects (Class I). These show that the sitesof recent star formation are distributed over multiparsec size scales.In two clusters, Cepheus C and S140, we find protostars embedded infilamentary dark clouds seen against diffuse emission in the IRAC bands.
| Infrared Array Camera (IRAC) Colors of Young Stellar Objects
We compare the infrared colors predicted by theoretical models ofprotostellar envelopes and protoplanetary disks with initialobservations of young stellar objects made with the Infrared ArrayCamera (IRAC) on the Spitzer Space Telescope. Disk and envelope modelscharacterized by infall and/or accretion rates found in previous studiescan quantitatively account for the range of IRAC colors found in fouryoung embedded clusters: S140, S171, NGC 7129, and Cep C. The IRACcolor-color diagram ([3.6]-[4.5] vs. [5.8]-[8.0]) can be used to helpdistinguish between young stars with only disk emission and protostarswith circumstellar envelopes.
| Excitation of Molecular Material near the Young Stellar Object LkHα 234 in NGC 7129
With the Spitzer Infrared Spectrograph (IRS) we have obtained the firstmid-IR spectroscopy of NGC 7129, in the unusually strong outflow and ina ridge of H2 emission near the Herbig Be star LkHα234. The UV radiation field strength is estimated from polycyclicaromatic hydrocarbon (PAH) band intensities in the H2 ridgeand found to be comparable to that of NGC 7023. From the rotationalH2 emission lines we have deduced aperture average excitationtemperatures and column densities in the two regions, finding theH2 ridge values to be consistent with pumping by UVfluorescence, but also comparable to warm-gas regions of Cep A that formH2 in nondissociative C-shocks. The H2 emission inthe outflow is consistent with formation by collisional excitation inJ-shocks, with shock velocities of 10-30 km s-1. Aphotodissociating component may be present in the outflow, by similarityof S(0) line intensities in both regions. There is no indication of warmdust in the outflow. We also present the first 16 μm imaging of aGalactic nebula using the unique imaging capabilities of the IRS,combined with ground-based 2.12 μm [H2 1-0 S(1)] imaging.Candidate pre-main-sequence objects are clearly evident in these data.We also find extended emission not previously observed around the youngB star BD +65°1638, ~22" across, showing that the region is not freeof material, as otherwise inferred by recent high angular resolutionmapping at submillimeter wavelengths. The presence of this materialcomplicates interpretation of the surrounding CO cavity and origin(s) ofthe photodissociated region, and further spectroscopic observations areneeded to characterize its nature.
| Multiple Outflows in the LkHα 234 Region
We report results of radio continuum (1.3 and 3.6 cm) and H2Omaser line observations, carried out with the Very Large Array (VLA) inits A configuration, toward the star-forming region LkHα 234. Wedetected five radio continuum sources (VLA 1, VLA 2, VLA 3A, VLA 3B, andLkHα 234) in a region of ~=5" (~=5000 AU), of which three werepreviously unknown (VLA 1, VLA 2, and VLA 3B). VLA 3A and VLA 3B seem toform a close (~=220 AU) binary system. Their elongated morphologies andpositive spectral indices suggest that both VLA 3A and VLA 3B could bethermal radio jets. In addition, we detected three clusters of watermasers, which are spatially associated with VLA 1, VLA 2, and VLA 3B.Based on the analysis of the distribution of the water masers and thecharacteristics of the continuum emission, we favor the new radiocontinuum source VLA 2 as the exciting source of the large-scale CO/[SII] outflow observed in the region. Moreover, we find that the multipleoutflows observed in the region share a similar orientation. Finally,our data confirm that there is no evidence indicating that the Herbig Bestar LkHα 234 is driving any of the outflows in the region.
| UBVRI Observations of V350 Cep in the Period 2002-2004
Results from UBVRI photometric observations of the pre-main sequencestar V350 Cep are present. The historical light curve of V350 Cepresembles the FUOR type stars but its spectrum is similar to theClassical T Tauri stars. Our observations suggest that during the period2002-2004 V350 Cep be still near to the maximum value.
| Evidence of Planetesimal Infall onto the Very Young Herbig Be Star LkHα 234
We report here the first evidence of planetesimal infall onto the veryyoung Herbig Be star LkHα 234. These results are based onobservations acquired over 31 days using spectroscopy of the sodium Dlines, the He I 5876 Å line, and hydrogen Hα lines. We findredshifted absorption components (RACs) with velocities up to 200 kms-1 and very mild blueshifted emission components (BECs) upto 100 km s-1 in the Na I lines. No correlation is observedbetween the appearance of the Na I RACs and BECs and the Hα and HeI line variability, which suggests that these (Na I RACs and BECs) areformed in a process unrelated to the circumstellar gas accretion. Weinterpret the Na I RACs as evidence for an infalling evaporating body,greater than 100 km in diameter, which is able to survive at distancesbetween 2.0 and 0.1 AU from the star. The dramatic appearance of thesodium RACs and mild BECs is readily explained by the dynamics of thisinfalling body, making LkHα 234 the youngest (age~0.1 Myr) systemwith evidence for the presence of planetesimals.Based on observations from the 9.2 m Hobby-Eberly Telescope.
| Modification of Projected Velocity Power Spectra by Density Inhomogeneities in Compressible Supersonic Turbulence
The scaling of velocity fluctuation δv as a function of spatialscale L in molecular clouds can be measured from size-line widthrelations, principal component analysis, or line centroid variation.Differing values of the power-law index of the scaling relation<δv2>1/2~Lγ3Din three dimensions are given by these different methods: the first twogive γ3D~=0.5, while line centroid analysis givesγ3D~=0. This discrepancy has previously not been fullyappreciated, as the variation of projected velocity line centroidfluctuations(<δv2lc>1/2~Lγ2D)is indeed described, in two dimensions, by γ2D~0.5.However, if projection smoothing is accounted for, this implies, inthree dimensions, that γ3D~0. We suggest that aresolution of this discrepancy can be achieved by accounting for theeffect of density inhomogeneity on the observed γ2Dobtained from velocity line centroid analysis. Numerical simulations ofcompressible turbulence are used to show that the effect of densityinhomogeneity statistically, but not identically, reverses the effect ofprojection smoothing in the case of driven turbulence, so that velocityline centroid analysis does indeed predict thatγ2D~γ3D~0.5. For decaying turbulence,the effect of density inhomogeneity on the velocity line centroidsdiminishes with time so that at late times,γ2D~γ3D+0.5 as a result of projectionsmoothing alone. Deprojecting the observed line centroid statistics thusrequires some knowledge of the state of the flow. This information canbe inferred from the spectral slope of the column density power spectrumand a measure of the standard deviation in column density relative tothe mean. Using our numerical results, we can restore consistencybetween line centroid analysis, principal component analysis, andsize-line width relations, and we derive γ3D~0.5,corresponding to shock-dominated (Burgers) turbulence, which alsodescribes the simulations of driven turbulence at scales on whichnumerical dissipation is negligible. We find that this consistencyrequires that molecular clouds are continually driven on large scales orare only recently formed.
| Parsec-scale Herbig-Haro outflows from intermediate mass stars
While there are many parsec-scale Herbig-Haro (HH) outflows known to bedriven by low-mass young stars, few are associated with theirintermediate mass counterparts. Here we present the discovery of fivesuch bipolar outflows. Of these, LkHα 198, 1548C27 IRS 1,LkHα 233 and LkHα 234 were previously known to possesssmall-scale HH flows, while no such activity was observed before nearIRAS 19395+2313. The largest of the newly discovered outflows are seenin the vicinity of LkHα 234 and 1548C27 IRS 1, and stretch (inprojection) 8 pc and 7.5 pc respectively. LkHα 233 which waspreviously known to power a spectroscopically detected small-scale(≤10 arcsec) jet is now seen to drive a 3 pc outflow and LkHα198 is shown here to power a 2 pc outflow. Two HH objects in thevicinity of IRAS 19395+2313 lead us to suggest that it may also beresponsible for a 5 pc outflow. In total, 27 new HH objects/complexeswere discovered. Examination of these parsec-scale outflows show thatthey have similar lengths, morphologies, and dynamical timescales asthose from low-mass sources. Many appear to have blown out of the parentcloud, suggesting that their total lengths are much greater thanoptically observed. The degree of collimation of these outflows issimilar to those from low-mass sources suggesting that the transition tomore poorly-collimated outflows must occur at higher masses than thesources observed here.
| A Photometric and Spectroscopic Study of the FUOR Candidates V1184 Tau and V350 Cep
We present the results from optical photometric and spectroscopicobservations of the pre-main-sequence (PMS) stars V1184 Tau and V350Cep. Both objects pass through an optical outburst and they wereconsidered as fuor candidates in previous studies. The long-term lightcurve of V350 Cep resembles fuor-type stars, but its spectrum is similarto classical T Tauri stars. In the period of observations, V1184 Tauremained at the maximum brightness. Significant changes in the profileand strength of emission lines were found.
| Neutral Hydrogen Associated with NGC 7129
Observations of the environment of the star-forming region NGC 7129obtained with an angular resolution of 1' in the 21 cm line of H I aredescribed. Two features of the image are extensively discussed: (1) aring of H I emission about 30' in extent and (2) a relatively denseconcentration of H I with unusually wide line profiles positionallycoincident with the B star BD +65°1638. The H I ring is consistentwith photodissociation of H2 by the interstellar UV radiationfield at the surface of an extended molecular cloud in which both BD+65°1638 and NGC 7129 are situated. We further show that BD+65°1638 appears to be an unusual example of a ``dissociating star''surrounded by an extensive region of photodissociated H2 andaccompanied by a small H II region. The derived spectral type (B2.5) andthe absolute magnitude for BD +65°1638 further suggest that thelatter is very close to the birthline. The very young stellar ageimplied by the parameters of the H I region, considerably less than104 yr, is discussed, and the properties of the H I regionare compared with those of the prototype for this rare class of objects.We discuss both aspects within the context of star formation in NGC7129.
| A 13CO and C18O Survey of the Molecular Gas Around Young Stellar Clusters within 1 Kiloparsec of the Sun
As the first step of a multiwavelength investigation into therelationship between young stellar clusters and their environment, wepresent fully sampled maps in the J=1-0 lines of 13CO andC18O and the J=2-1 line of C18O for a selectedgroup of 30 young stellar groups and clusters within 1 kpc of the Sun.This is the first systematic survey of these regions to date. Theclusters range in size from several stars to a few hundred stars. Thirtyfields ranging in size from 8'×8' to 30'×60' were mappedwith 47" resolution simultaneously in the two J=1-0 lines at the FiveCollege Radio Astronomy Observatory. Seventeen sources were mapped overfields ranging in size from 3'×3' to 13'×13' in the J=2-1line with 35" resolution at the Submillimeter Telescope Observatory. Wecompare the cloud properties derived from each of the three tracers inorder to better understand systematic uncertainties in determiningmasses and line widths. Cloud masses are determined independently usingthe 13CO and C18O transitions; these masses rangefrom 30 to 4000 Msolar. Finally, we present a simplemorphological classification scheme, which may serve as a roughindicator of cloud evolution.
| Three New PMS Variables in the Vicinity of NGC 7129
Tree new PMS variable stars were found in the vicinity of NGC 7129. Theyare located in the dark clouds northwest of the emission nebula. BVRIphotometric observations of the newly discovered variables werepresented in the paper.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
Kataloglar ve belirtme:
|
