arXiv subjects

Karl E. Haisch Jr.

Publications and source records attributed to Karl E. Haisch Jr..

9 recordsLinked to original sources

arXiv2006

Mid-Infrared Observations of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We have obtained new mid-infrared observations of 65 Class I/Flat-Spectrum (F.S.) objects in the Perseus, Taurus, Chamaeleon I/II, Rho Ophiuchi, and Serpens dark clouds. We detected 45/48 (94%) of the single sources, 16/16 (100%) of the primary components, and 12/16 (75%) of the secondary/triple components of the binary/multiple objects surveyed. The composite spectral energy distributions (SEDs) for all of our sample sources are either Class I or F.S., and, in 15/16 multiple systems, at least one of the individual components displays a Class I or F.S. spectral index. However, the occurrence of mixed pairings, such as F.S. with Class I, F.S. with Class II, and, in one case, F.S. with Class III, is surprisingly frequent. Such behaviour is not consistent with that of multiple systems among T Tauri stars (TTS), where the companion of a classical TTS also tends to be a classical TTS, although other mixed pairings have been previously observed among Class II objects. Based on an analysis of the spectral indices of the individual binary components, there appears to be a higher proportion of mixed Class I/F.S. systems (65-80%) than that of mixed Classical/Weak-Lined TTS (25-40%), demonstrating that the envelopes of Class I/ F.S. systems are rapidly evolving during this evolutionary phase. We report the discovery of a steep spectral index secondary companion to ISO-ChaI 97, detected for the first time via our mid-infrared observations. In our previous near- infrared imaging survey of binary/multiple Class I/F.S. sources, ISO-ChaI 97 appeared to be single. With a spectral index of Alpha >= 3.9, the secondary component of this system is a member of a rare class of very steep spectral index objects, those with Alpha > 3. Only three such objects have previously been reported, all of which are either Class 0 or Class I.

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arXiv2005

Constraints on Inner Disk Evolution Timescales: A Disk Census of the eta Chamaeleontis Young Cluster

We present new L' (3.8-micron) observations of stars in the nearby (~97 pc) young (~6 Myr) compact cluster around eta Chamaeleontis, obtained with the European Southern Observatory's Very Large Telescope in Paranal, Chile. Our data, combined with J,H, Ks photometry from the 2-Micron All Sky Survey, reveal that only two of the 12 members surveyed harbor L'-band excesses consistent with optically thick inner disks; both are also likely accretors. Intriguingly, two other stars with possible evidence for on-going accretion, albeit at very low rates, do not show significant infrared excess: this may imply substantial grain growth and/or partial clearing of the inner disk region, as expected in planet formation scenarios. Our findings suggest that eta Cha stars are in an epoch when disks are rapidly evolving, perhaps due to processes related to planet building, and provide further constraints on inner disk lifetimes.

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arXiv2003

A Near-Infrared Multiplicity Survey of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We present new near-IR observations of 76 Class I/flat-spectrum objects in the nearby (d < 320 pc) Perseus, Taurus, Chamaeleon I and II, rho Ophiuchi, and Serpens dark clouds. These observations are part of a larger systematic infrared multiplicity survey of self-embedded objects in the nearest dark clouds. When combined with the results of our previously published near-infrared multiplicity survey, we find a restricted companion star fraction of 14/79 (18% +/- 4%) of the sources surveyed to be binary or higher order multiple systems over a separation range of ~300 - 2000 AU with a magnitude difference $Δ$ K <= 4, and with no correction for background contamination or completeness. This is consistent with the fraction of binary/multiple systems found among older pre-main-sequence T Tauri stars in each of the Taurus, rho Ophiuchi, and Chamaeleon star-forming regions over a similar separation range, as well as the combined companion star fraction for these regions. However, the companion star fraction for solar-type, and lower mass M dwarf, main-sequence stars in the solar neighborhood in this separation range (11% +/- 3%) is approximately one-half that of our sample. Together with multiplicity statistics derived for previously published samples of Class 0 and Class I sources, our study suggests that a significant number of binary/multiple objects may remain to be discovered at smaller separations among our Class I/flat-spectrum sample and/or most of the evolution of binary/multiple systems occurs during the Class 0 phase of early stellar evolution.

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arXiv2003

Close Companions to Nearby Young Stars from Adaptive Optics Imaging on VLT and Keck

We report the results of VLT and Keck adaptive optics surveys of known members of the Eta Chamaeleontis, MBM 12, and TW Hydrae (TWA) associations to search for close companions. The multiplicity statistics of Eta Cha, MBM 12, and TWA are quite high compared with other clusters and associations, although our errors are large due to small number statistics. We have resolved S18 in MBM 12 and RECX 9 in Eta Cha into triples for the first time. The tight binary TWA 5Aab in the TWA offers the prospect of measuring the dynamical masses of both components as well as an independent distance to the system within a few years. The AO detection of the close companion to the nearby young star Chi^1 Orionis, previously inferred from radial velocity and astrometric observations, has already made it possible to derive the dynamical masses of that system without any astrophysical assumption.

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arXiv2001

Disk Frequencies and Lifetimes in Young Clusters

We report the results of the first sensitive L-band (3.5 micron) survey of the intermediate age (2.5 - 30 Myr) clusters NGC 2264, NGC 2362 and NGC 1960. We use JHKL colors to obtain a census of the circumstellar disk fractions in each cluster. We find disk fractions of 52% +/- 10%, 12% +/- 4% and 3% +/- 3% for the three clusters respectively. Together with our previously published JHKL investigations of the younger NGC 2024, Trapezium and IC 348 clusters, we have completed the first systematic and homogenous survey for circumstellar disks in a sample of young clusters that both span a significant range in age (0.3 - 30 Myr) and contain statistically significant numbers of stars whose masses span nearly the entire stellar mass spectrum. Analysis of the combined survey indicates that the cluster disk fraction is initially very high (> 80%) and rapidly decreases with increasing cluster age, such that half the stars within the clusters lose their disks in < ~3 Myr. Moreover, these observations yield an overall disk lifetime of ~ 6 Myr in the surveyed cluster sample. This is the timescale for essentially all the stars in a cluster to lose their disks. This should set a meaningful constraint for the planet building timescale in stellar clusters. The implications of these results for current theories of planet formation are briefly discussed.

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arXiv2001

Circumstellar Disks in the IC 348 Cluster

We report the results of the first sensitive L-band (3.4 micron) imaging survey of the young IC 348 cluster in Perseus. In conjunction with previously acquired JHK (1.25, 1.65, 2.2 micron) observations, we use L-band data to obtain a census of the circumstellar disk population to m_K=m_L<=12.0 in the central 110 square arcmin region of the cluster. An analysis of the JHKL colors of 107 sources indicates that 65% +/- 8% of the cluster membership possesses (inner) disks. This fraction is lower than those (86% +/- 8% and 80% +/- 7%) obtained from similar JHKL surveys of the younger NGC 2024 and Trapezium clusters, suggesting that the disk fraction in clusters decreases with cluster age. Sources with circumstellar disks in IC 348 have a median age of 0.9 Myr, while the diskless sources have a median age of 1.4 Myr, for a cluster distance of 320 pc. Although the difference in the median ages between the two populations is only marginally significant, our results suggest that over a timescale of 2 - 3 Myr, more than a third of the disks in the IC 348 cluster disappear. Moreover, we find that at a very high confidence level, the disk fraction is a function of spectral type. All stars earlier than G appear diskless, while stars with spectral types G and later have a disk fraction ranging between 50% - 67%, with the latest type stars having the higher disk fraction. This suggests that the disks around stars with spectral types G and earlier have evolved more rapidly than those with later spectral types. The L-band disk fraction for sources with similar ages in both IC 348 and Taurus is the same, within the errors, suggesting that, at least in clusters with no O stars, the disk lifetime is independent of environment.

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arXiv2000

A Mid-Infrared Study of the Young Stellar Population in the NGC 2024 Cluster

We present the results of the first broadband 10.8 um survey of the NGC 2024 cluster. The mid-infrared data were combined with our previously published JHKL photometry in order to construct spectral energy distributions for all detected sources. The main scientific goals were to investigate the nature of the young stellar objects (YSOs) in the cluster, and to examine the efficiency of detecting circumstellar disk sources from near-infrared JHKL color-color diagrams. Out of 59 sources surveyed having K-band magnitudes K < 10.5, we detected 35 (~59%) at 10 um. Combining these detections, and upper limits for the non-detections, with existing JHKL data, we identify 1 Class I, 6 flat spectrum, 28 Class II and 5 Class III sources. We find a circumstellar disk fraction for NGC 2024 of ~85% +/- 15%, which confirms earlier published suggestions that the majority, if not all, of the stars in NGC 2024 formed with disks, and these disks still exist at the present time. In addition, all but one of the disk sources identified in our survey lie in the infrared excess region of the JHKL color-color diagram for the cluster. This demonstrates that JHKL color-color diagrams are extremely efficient in identifying YSOs with disks. Of the 14 sources with K - L colors suggestive of protostellar objects, \~29% are protostellar in nature, while ~7% are true Class I YSOs. This may be due to extinction producing very red K - L colors in Class II YSOs, thus making them appear similar in color to protostars. This suggests caution must be applied when estimating the sizes and lifetimes of protostellar populations within star forming regions based on K - L colors alone. Finally, we calculate the luminosities of the Class II YSOs in NGC 2024, rho Oph and Taurus and discuss the results.

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arXiv2000

Infrared L Band Observations of the Trapezium Cluster: A Census of Circumstellar Disks and Candidate Protostars

We report the results of a sensitive near-infrared JHKL imaging survey of the Trapezium cluster in Orion. We use the JHKL colors to obtain a census of infrared excess stars in the cluster. Of (391) stars brighter than 12th magnitude in the K and L bands, 80 +/- 7% are found to exhibit detectable infrared excess on the J-H, K-L color-color diagram. Examination of a subsample of 285 of these stars with published spectral types yields a slightly higher infrared excess fraction of 85%. We find that 97% of the optical proplyds in the cluster exhibit excess in the JHKL color-color diagram indicating that the most likely origin of the observed infrared excesses is from circumstellar disks. We interpret these results to indicate that the fraction of stars in the cluster with circumstellar disks is between 80-85%. Moreover, we find that the probability of finding an infrared excess/protoplanetary disk around a star is independent of stellar mass over essentially the entire range of the stellar mass function down to the hydrogen burning limit. We identify 78 stars in our sample characterized by K-L colors suggestive of deeply embedded protostellar objects. If even a modest fraction fraction (i.e., ~ 50%) of these objects are protostars, then star formation could be continuing in the molecular ridge at a rate comparable to that which produced the foreground Trapezium cluster.

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arXiv2000

A Near-Infrared L Band Survey of the Young Embedded Cluster NGC 2024

We present the results of the first sensitive L band (3.4 micron) imaging study of the nearby young embedded cluster NGC 2024. Two separate surveys of the cluster were acquired in order to obtain a census of the circumstellar disk fraction in the cluster. From an analysis of the JHKL colors of all sources in our largest area, we find an infrared excess fraction of > 86%. The JHKL colors suggest that the infrared excesses arise in circumstellar disks, indicating that the majority of the sources which formed in the NGC 2024 cluster are currently surrounded by, and likely formed with circumstellar disks. The excess fractions remain very high, within the errors, even at the faintest L magnitudes from our deeper surveys suggesting that disks form around the majority of the stars in very young clusters such as NGC 2024 independent of mass. From comparison with published JHKL observations of Taurus, we find the K - L excess fraction in NGC 2024 to be consistent with a high initial incidence of circumstellar disks in both NGC 2024 and Taurus. Because NGC 2024 represents a region of much higher stellar density than Taurus, this suggests that disks may form around most of the YSOs in star forming regions independent of environment. We find a relatively constant JHKL excess fraction with increasing cluster radius, indicating that the disk fraction is independent of location in the cluster. In contrast, the JHK excess fraction increases rapidly toward the central region of the cluster, and is most likely due to contamination of the K band measurements by bright nebulosity in the central regions of the cluster. We identify 45 candidate protostellar sources in the central regions of the NGC 2024 cluster, and find a lower limit on the protostellar phase of early stellar evolution of 0.4 - 1.4 X 10^5 yr, similar to that in Taurus.

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