A DEEP SPITZER SURVEY OF CIRCUMSTELLAR DISKS IN THE YOUNG DOUBLE CLUSTER, h AND χ PERSEI (original) (raw)
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Astronomy and Astrophysics, 2010
Context. The Vela Molecular Ridge hosts a number of young embedded star clusters at the same evolutionary stage. Aims. We test whether the fraction of members with a circumstellar disk in a sample of clusters in the cloud D of the Vela Molecular Ridge, is consistent with relations derived for larger samples of star clusters with an age spread. In addition, we constrain the age of the young embedded star clusters associated with cloud D. Methods. We carried out L (3.78 μm) photometry using images of six young embedded star clusters associated with cloud D of the Vela Molecular Ridge, taken with ISAAC at the VLT. These data are complemented with the available HK s photometry. The 6 clusters are of roughly the same size and appear to be at the same evolutionary stage. The fraction of stars with a circumstellar disk was measured in each cluster by counting the fraction of sources found to have a NIR excess in colour-colour (HK s L) diagrams. Results. The L photometry allowed us to identify the NIR counterparts of the IRAS sources associated with the clusters. The fraction of stars with a circumstellar disk appears to be constant within the errors for the 6 clusters. There is a hint that this is lower for the most massive stars. The age of the clusters is constrained to be ∼1-2 Myr. Conclusions. The fraction of stars with a circumstellar disk in the observed sample is consistent with the relations derived from larger samples of star clusters and with other age estimates for cloud D. The fraction may be lower for the most massive stars. Our results agree with a scenario where all intermediate and low-mass stars form with a disk, whose lifetime is shorter for higher mass stars.
The Astrophysical Journal, 2007
We describe IRAC 3.6-8 µm observations and ground-based near-IR JHK s photometry from Mimir and 2MASS of the massive double cluster h & χ Persei complete to J=15.5 (M ∼ 1.3M ⊙ ). Within 25' of the cluster centers we detect ∼ 11, 000 sources with J≤ 15.5, ∼ 7000 sources with [4.5] ≤ 15, and ∼ 5000 sources with [8]≤ 14.5. In both clusters, the surface density profiles derived from the 2MASS data decline with distance from the cluster centers as expected for a bound cluster. Within 15' of the cluster centers, ∼ 50% of the stars lie on a reddened ∼ 13 Myr isochrone; at 15'-25' from the cluster centers, ∼ 40% lie on this isochrone. Thus, the optical/2MASS color-magnitude diagrams indicate that h & χ Per are accompanied by a halo population with roughly the same age and distance as the two dense clusters. The double cluster lacks any clear IR excess sources for J≤ 13.5 (∼ 2.7M ⊙ ). Therefore, disks around high-mass stars disperse prior to ∼ 10 7 yr. At least 2−3% of the fainter cluster stars have strong IR excess at both [5.8] and [8]. About 4 − 8% of sources slightly more massive than the Sun (∼ 1.4M ⊙ ) have IR excesses at [8]. Combined with the lack of detectable excesses for brighter stars, this result suggests that disks around lower-mass stars have longer lifetimes. The IR excess population also appears to be larger at longer IRAC bands ([5.8], [8]) than at shorter IRAC/2MASS bands (K s , [4.5]), a result consistent with an inside-out clearing of disks.
Incidence and survival of remnant disks around main-sequence stars
Astronomy and Astrophysics, 2001
We present photometric ISO 60 and 170 µm measurements, complemented by some IRAS data at 60 µm, of a sample of 84 nearby main-sequence stars of spectral class A, F, G and K in order to determine the incidence of dust disks around such main-sequence stars. Fifty stars were detected at 60 µm; 36 of these emit a flux expected from their photosphere while 14 emit significantly more. The excess emission we attribute to a circumstellar disk like the ones around Vega and β Pictoris. Thirty four stars were not detected at all; the expected photospheric flux, however, is so close to the detection limit that the stars cannot have an excess stronger than the photospheric flux density at 60 µm.
What Can We Learn from Protoplanetary Disk Frequency in Young Clusters?
2004
The origin and evolution of circumstellar disks is one of the main scientific quests intimately related with planetary formation, since disks are known to be planetary nurseries. A study of statistically significant young stellar populations, in different evolutionary stages and astrophysical environments, can provide fundamental tests for theories of disk and planet formation. We are presently conducting a systematic broadband infrared wavelength study of ten young clusters of different ages, in order to compare their circumstellar disk frequency. We report our results of JHK photometry of three nearby clusters of our sample, RCW 38, NGC 2316 and NGC 2547.
The structure and evolution of young stellar clusters
2006
We examine the properties of embedded clusters within 1 kiloparsec using new data from the Spitzer Space Telescope, as well as recent results from 2MASS and other ground-based near-infrared surveys. We use surveys of entire molecular clouds to understand the range and distribution of cluster membership, size and surface density. The Spitzer data demonstrate clearly that there is a continuum of star-forming environments, from relative isolation to dense clusters. The number of members of a cluster is correlated with the cluster radius, such that the average surface density of clusters having a few to a thousand members varies by a factor of only a few. The spatial distributions of Spitzer-identified young stellar objects frequently show elongation, low density halos, and sub-clustering. The spatial distributions of protostars resemble the distribution of dense molecular gas, suggesting that their morphologies result directly from the fragmentation of the natal gas. We also examine the effects of the cluster environments on star and planet formation. Although Far-UV and Extreme-UV radiation from massive stars can truncate disks in a few million years, fewer than half of the young stars in our sample (embedded clusters within 1 kpc) are found in regions of strong FUV and EUV fields. Typical volume densities and lifetimes of the observed clusters suggest that dynamical interactions are not an important mechanism for truncating disks on solar system size scales.
A Spitzer Space Telescope Study of Disks in the Young sigma Orionis Cluster
Astrophysical Journal, 2007
We report new Spitzer Space Telescope observations from the IRAC and MIPS instruments of the young (~ 3 Myr) sigma Orionis cluster. We identify 336 stars as members of the cluster using optical and near-infrared color magnitude diagrams. Using the spectral energy distribution (SED) slopes in the IRAC spectral range, we place objects in several classes: non-excess stars, stars with optically thick disks(like classical T Tauri stars), class I (protostellar) candidates, and stars with ``evolved disks''; the last exhibit smaller IRAC excesses than optically thick disk systems. In general, this classification agrees with the location expected in IRAC-MIPS color-color diagrams for these objects. We find that the evolved disk systems are mostly a combination of objects with optically thick but non-flared disks, suggesting grain growth and/or settling, and transition disks, systems in which the inner disk is partially or fully cleared of small dust. In all, we identify 7 transition disk candidates and 3 possible debris disk systems. As in other young stellar populations, the fraction of disks depends on the stellar mass, ranging from ~10% for stars in the Herbig Ae/Be mass range (>2 msun) to ~35% in the T Tauri mass range (1-0.1 msun). We find that the disk fraction does not decrease significantly toward the brown dwarf candidates (<0.1 msun). The IRAC infrared excesses found in stellar clusters and associations with and without central high mass stars are similar, suggesting that external photoevaporation is not very important in many clusters. Finally, we find no correlation between the X-ray luminosity and the disk infrared excess, suggesting that the X-rays are not strongly affected by disk accretion.
The Astronomical Journal, 2000
We present high-spatial resolution HST and ground-based adaptive optics observations, and highsensitivity ISO (ISOCAM & ISOPHOT) observations of a sample of X-ray selected weak-line (WTTS) and post (PTTS) T Tauri stars located in the nearby Chamaeleon T and Scorpius-Centaurus OB associations. HST/NICMOS and adaptive optics observations aimed at identifying substellar companions (young brown dwarfs) at separations ≥ 30 A.U. from the primary stars. No such objects were found within 300 A.U. of any of the target stars, and a number of faint objects at larger separations can very likely be attributed to a population of field (background) stars. ISOCAM observations of 5 to 15 Myr old WTTS and PTTS in ScoCen reveal infrared excesses which are clearly above photospheric levels, and which have a spectral index intermediate between that of younger (1 to 5 Myr) T Tauri stars in Chamaeleon and that of pure stellar photospheres. The difference in the spectral index of the older PTTS in ScoCen compared to the younger classical and weak-line TTS in Cha can be attributed to a deficiency of smaller size (0.1 to 1 µm) dust grains relative to larger size (≈5 µm) dust grains in the disks of the PTTS. The lack of small dust grains is either due to the environment (effect of nearby O stars and supernova explosions) or due to disk evolution. If the latter is the case, it would hint that circumstellar disks start to get dust depleted at an age between 5 to 15 Myr. Dust depletion is very likely related to the build-up of larger particles (ultimately rocks and planetesimals) and thus an indicator for the onset of the period of planet formation.
On the lifetime of discs around late-type stars
Monthly Notices of the Royal Astronomical Society, 2011
We address the question of whether protoplanetary discs around low-mass stars (e.g. M-dwarfs) may be longer lived than their solar-type counterparts. This question is particularly relevant in assessing the planet-making potential of these lower mass discs. Given the uncertainties inherent to age-dating young stars, we propose an alternative approach that is to analyse the spatial distribution of disc-bearing low-mass stars and compare it to that of disc-bearing solar-type stars in the same cluster. A significant age difference between the two populations should be reflected in their average nearest-neighbour distance (normalized to the number of sources), where the older population should appear more spread out.
The Physical Structure of Protoplanetary Disks: The Serpens Cluster Compared with Other Regions
The Astrophysical Journal, 2013
Spectral energy distributions are presented for 94 young stars surrounded by disks in the Serpens Molecular Cloud, based on photometry and Spitzer IRS spectra. Most of the stars have spectroscopically determined spectral types. Taking a distance to the cloud of 415 pc rather than 259 pc, the distribution of ages is shifted to lower values, in the 1 -3 Myr range, with a tail up to 10 Myr. The mass distribution spans 0.2 -1.2 M ⊙ , with median mass of 0.7 M ⊙ . The distribution of fractional disk luminosities in Serpens resembles that of the young Taurus Molecular Cloud, with most disks consistent with optically thick, passively irradiated disks in a variety of disk geometries (L disk /L star ∼ 0.1). In contrast, the distributions for the older Upper Scorpius and η Chamaeleontis clusters are dominated by optically thin lower luminosity disks (L disk /L star ∼ 0.02). This evolution in fractional disk luminosities is concurrent with that of disk fractions: with time disks become fainter and the disk fractions decrease. The actively accreting and non-accreting stars (based on Hα data) in Serpens show very similar distributions in fractional disk luminosities, differing only in the brighter tail dominated by strongly accreting stars. In contrast with a sample of Herbig Ae/Be stars, the T Tauri stars in Serpens do not have a clear separation in fractional disk luminosities for different disk geometries: both flared and flat disks present wider, overlapping distributions. This result is consistent with previous suggestions of a faster evolution for disks around Herbig Ae/Be stars. Furthermore, the results for the mineralogy of the dust in the disk surface (grain sizes, temperatures and crystallinity fractions, as derived from Spitzer IRS spectra) do not show any correlation to either stellar and disk characteristics or mean cluster age in the 1 -10 Myr range probed here. A possible explanation for the lack of correlation is that the processes affecting the dust within disks have short timescales, happening repeatedly, making it difficult to distinguish long lasting evolutionary effects.
Evolution of circumstellar disks around normal stars: Placing our solar system in context
Arxiv preprint astro-ph/ …, 2006
Over the past 10 years abundant evidence has emerged that many (if not all) stars are born with circumstellar disks. Understanding the evolution of post-accretion disks can provide strong constraints on theories of planet formation and evolution. In this review, we focus on developments in understanding: a) the evolution of the gas and dust content of circumstellar disks based on observational surveys, highlighting new results from the Spitzer Space Telescope; b) the physical properties of specific systems as a means to interpret the survey results; c) theoretical models used to explain the observations; d) an evolutionary model of our own solar system for comparison to the observations of debris disks around other stars; and e) how these new results impact our assessment of whether systems like our own are common or rare compared to the ensemble of normal stars in the disk of the Milky Way.