Nearby Young Clusters Research Articles

DISCOVERY OF A HIGHLY ENERGETIC X-RAY PULSAR POWERING HESS J1813–178 IN THE YOUNG SUPERNOVA REMNANT G12.82–0.02

We report the discovery of 44.7 ms pulsations from the X-ray source CXOU J181335.1–174957 using data obtained with the XMM-Newton Observatory. PSR J1813–1749 lies near the center of the young radio supernova remnant G12.82–0.02, which overlaps the compact TeV source HESS J1813–178. This rotation-powered pulsar is the second most energetic in the Galaxy, with a spin-down luminosity of erg s–1. In the rotating dipole model, the surface dipole magnetic field strength is Bs = (2.7 ± 0.6) × 1012 G and the spin-down age -7.5 kyr, consistent with the location in the small, shell-type radio remnant. At an assumed distance of 4.7 kpc by association with an adjacent young stellar cluster, the efficiency of PSR J1813–1749 in converting spin-down luminosity to radiation is 0.03% for its 2-10 keV flux, 0.1% for its 20-100 keV INTEGRAL flux, and 0.07% for the >200 GeV emission of HESS J1813–178, making it a likely power source for the latter. The nearby young stellar cluster is possibly the birthplace of the pulsar progenitor, as well as an additional source of seed photons for inverse Compton scattering to TeV energies.

A deep look into the cores of young clusters

Context. Nearby young clusters are privileged places to study the star formation history. Over the last decade, the σ-Orionis cluster has been a prime location for the study of young very low mass stars, substellar and isolated planetary mass objects and the determination of the initial mass function. Aims. To extend previous studies of this association to its core, we searched for ultracool members and new multiple systems within the 1. � 5 × 1. 5 central region of the cluster. Methods. We obtained deep multi-conjugate adaptive optics (MCAO) images of the core of the σ-Orionis cluster with the prototype MCAO facility MAD at the VLT using the H and Ks filters. These images allow us to detect companions fainter by ΔH ≈ 5 mag as close as 0. �� 2 on a typical source with H = 14.5 mag. These images were complemented by archival SofI K s-band images and Spitzer IRAC and MIPS mid-infrared images Results. We report the detection of 2 new visual multiple systems, one being a candidate binary proplyd and the other one a low mass companion to the massive star σ Ori E. Of the 36 sources detected in the images, 25 have a H-band luminosity lower than the expected planetary mass limit for members, and H − Ks color consistent with the latest theoretical isochrones. Nine objects have additional Spitzer photometry and spectral energy distribution consistent with them being cluster members. One of them has a spectral energy distribution from H to 3.6 μm consistent with that of a 5.5 MJup cluster member. Complementary NTT/SofI and Spitzer photometry allow us to confirm the nature and membership of two L-dwarf planetary mass candidates.

The molecular hydrogen explorer H2EX

The Molecular Hydrogen Explorer, H2EX, was proposed in response to the ESA 2015 - 2025 Cosmic Vision Call as a medium class space mission with NASA and CSA participations. The mission, conceived to understand the formation of galaxies, stars and planets from molecular hydrogen, is designed to observe the first rotational lines of the H2 molecule (28.2, 17.0, 12.3 and 9.7 μm) over a wide field, and at high spectral resolution. H2EX can provide an inventory of warm (≥ 100 K) molecular gas in a broad variety of objects, including nearby young star clusters, galactic molecular clouds, active galactic nuclei, local and distant galaxies. The rich array of molecular, atomic and ionic lines, as well as solid state features available in the 8 to 29 μm spectral range brings additional science dimensions to H2EX. We present the optical and mechanical design of the H2EX payload based on an innovative Imaging Fourier Transform Spectrometer fed by a 1.2 m telescope. The 20’×20’ field of view is imaged on two 1024×1024 Si:As detectors. The maximum resolution of 0.032 cm − 1 (full width at half maximum) means a velocity resolution of 10 km s − 1 for the 0 – 0 S(3) line at 9.7 μm. This instrument offers the large field of view necessary to survey extended emission in the Galaxy and local Universe galaxies as well as to perform unbiased extragalactic and circumstellar disks surveys. The high spectral resolution makes H2EX uniquely suited to study the dynamics of H2 in all these environments. The mission plan is made of seven wide-field spectro-imaging legacy programs, from the cosmic web to galactic young star clusters, within a nominal two years mission. The payload has been designed to re-use the Planck platform and passive cooling design.

NTT and VLT diffraction limited imaging of Trumpler 14: revealing a massive core-halo cluster

Aims. We present the deepest and highest resolution near-infrared imaging to date of cluster Trumpler 14 in Carina. Our goal is to identify and characterise the young stellar population of this massive cluster. Methods. We made use of deep and wide-field NIR images from NTT and VLT observations, that were sensitive enough to detect substellar sources at the distance to this cluster, and at high enough resolution (VLT diffraction limited) to fully resolve the core of the cluster crowded with O stars. Results. We find that Tr14 has a well-defined core-halo structure, where less than 30% of the cluster's members reside in the core. The core is well characterised by a King function with a core radius of $0\farcm17$ (0.14 pc at the adopted distance) and a constant baseline, the halo, of 125 sources/pc 2 . Despite the unusually large number of OB stars, the central number density at zero radius is ~7.3 $\times$ 10 3 pc -3 , which is loose in comparison with similar clusters. We find a normal reddening law towards the cluster and derive a global reddening of A v = 2.6 ± 0.3 mag. We find convincing evidence of a sparse foreground population (~5 sources/arcmin 2 ) reddened by about A v = 1.4 mag, which we suggest is not associated with Tr14 but is most likely an older population produced in the nearby young clusters of this complex. The colour–magnitude diagrams are compatible with ages between “zero” and ~5 Myr, although the sources from the core of the cluster appear to concentrate on the youngest isochrones, suggesting that the halo population is, on average, slightly older than the core population. Using a set of simplistic, fixed-age, mass-luminosity relations, we derive a mass of 10 4 $M_\odot$ for the cluster. From the NACO JHK s L ' data, we estimate a fraction of infrared-excess sources of 35%, although this is likely to be an underestimate given the bright completeness limits of the L' band. Finally, we argue that the formerly identified proplyd candidates that fall inside our survey are not proplyds but remnants of the disrupted molecular cloud that surround the cluster. We also find a series of interesting objects in our field that are worthy of future attention: a candidate photoionised proplyd best seen in the L' band, a compact nebula surrounding an early type star, and a tentative proplyd/small shock associated with a faint source.

Unraveling the Origins of Nearby Young Stars

A systematic search for close conjunctions and clusterings in the past of nearby stars younger than the Pleiades is undertaken, which may reveal the time, location, and mechanism of formation of these often isolated, disconnected from clusters and star-forming regions, objects. The sample under investigation includes 101 T Tauri, post-TT, and main-sequence stars and stellar systems with signs of youth, culled from the literature. Their Galactic orbits are traced back in time and near approaches are evaluated in time, distance, and relative velocity. Numerous clustering events are detected, providing clues to the origin of very young, isolated stars. Each star's orbit is also matched with those of nearby young open clusters, OB and TT associations and star-forming molecular clouds, including the Ophiuchus, Lupus, Corona Australis, and Chamaeleon regions. Ejection of young stars from open clusters is ruled out for nearly all investigated objects, but the nearest OB associations in Scorpius-Centaurus, and especially, the dense clouds in Ophiuchus and Corona Australis have likely played a major role in the generation of the local streams (TWA, Beta Pic, and Tucana-Horologium) that happen to be close to the Sun today. The core of the Tucana-Horologium association probably originated from the vicinity of the Upper Scorpius association 28 Myr ago. A few proposed members of the AB Dor moving group were in conjunction with the coeval Cepheus OB6 association 38 Myr ago.

Kinematics of the Gould Belt based on open clusters

We have redetermined the kinematic parameters of the Gould Belt using currently available data on the motion of nearby young (log t < 7.91) open clusters, OB associations, and moving stellar groups. Our modeling shows that the residual velocities reach their maximum values of −4 km s−1 for rotation (in the direction of Galactic rotation) and +4 km s−1 for expansion at a distance from the kinematic center of ≈300 pc. We have taken the following parameters of the Gould Belt center: R0 = 150 pc and l0 = 128°. The whole structure is shown to move relative to the local standard of rest at a velocity of 10.7 ± 0.7 km s−1 in the direction l = 274° ± 4° and b = −1° ± 3°. Using the derived rotation velocity, we have estimated the virial mass of the Gould Belt to be 1.5 × 106M⊙.

New nearby young star cluster candidates within 200 pc

AbstractI briefly describe two new young star cluster candidates found within 200 pc of the Sun, associated with the 4th-magnitude stars μ Oph and 32 Ori. The μ Oph group (d ≃ 170pc) has a space motion and age (~120± 25 Myr) suspiciously similar to the Pleiades, but lies in the opposite side of the sky behind ~0.9 mag of visual extinction in Ophiuchus. The 32 Ori group is a nearby (d ≃ 90 pc) loose aggregate of ~25-Myr-old post-T Tauri stars co-moving with the massive binary 32 Ori (B5V+B7V) in northern Orion. The 32 Ori group accounts for part of the population of “isolated” Li-rich RASS pre-MS stars in northern Orion.

Gould Belt kinematics on the basis of the open clusters and OB associations

The kinematic parameters of the Gould Belt are redetermined using modern data on the motion of nearby young (log t<7.91) open clusters, OB associations and moving stellar groups. The modelling carried out shows that the residual velocities achieve the maximal values equal to−4 km s−1 for rotation (in the direction of the Galactic rotation) and+4 km s−1 for expansion, when the distance from the kinematic centre is about 300 pc. We assumed the following parameters of the Gould Belt centre: l 0=128° and R 0=150 pc. It is shown that the whole structure of the Gould Belt is moving relative to the local standard of rest with a velocity of 10.7±0.7 km s−1 in the direction towards l=274±4°, b=−1±3°. Using the rotation velocity, we obtained the virial estimation of the Gould Belt mass as 1.5×106 M ⊙.

Rotational Evolution of Solar‐like Stars in Clusters from Pre–Main Sequence to Main Sequence: Empirical Results

Rotation periods are now available for ~500 pre-main-sequence (PMS) and recently arrived main-sequence stars of solar-like mass (0.4-1.2 M☉) in five nearby young clusters: the Orion Nebula cluster (ONC), NGC 2264, α Per, IC 2602, and the Pleiades. In combination with estimates of stellar radii these data allow us to construct distributions of surface angular momentum per unit mass at three different epochs: nominally, 1, 2, and 50 Myr. There are sufficient data that rotational evolution can now be discussed statistically on the basis of the evolution of these distributions, not just on the evolution of means or ranges, as has been necessary in the past. Our main result is illustrated in Figure 18 and may be summarized as follows: (1) 50%-60% of the stars on convective tracks in this mass range are released from any locking mechanism very early on and are free to conserve angular momentum throughout most of their PMS evolution, i.e., to spin up and account for the rapidly rotating young main-sequence stars; (2) the other 40%-50% lose substantial amounts of angular momentum during the first few million years and end up as slowly rotating main-sequence stars. The duration of the rapid angular momentum loss phase is ~5-6 Myr, which is roughly consistent with the lifetimes of disks estimated from infrared surveys of young clusters. The rapid rotators of Orion age lose less than 10% of their (surface) specific angular momentum during the next 50 Myr, while the slow rotators lose about two-thirds of theirs. A detectable part of this loss occurs even during the ~1 Myr interval between the ONC and NGC 2264. The data support the view that interaction between an accretion disk and star is the primary mechanism for evolving the broad, bimodal distribution of rotation rates seen for solar-like stars in the ONC into the even broader distributions seen in the young MS clusters.

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

We present new L'-band (3.8 ?m) observations of stars in the nearby (~97 pc) young (~6 Myr) compact cluster around ? Chamaeleontis, obtained with the European Southern Observatory's Very Large Telescope in Paranal, Chile. Our data, combined with J, H, Ks photometry from the Two 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 of ongoing 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 ? 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.

An X‐Ray Census of Young Stars in the Chamaeleon I North Cloud

Sensitive X-ray imaging surveys provide a new and effective tool to establish the census of pre-main-sequence (PMS) stars in nearby young stellar clusters. We report here a deep Chandra X-Ray Observatory observation of PMS stars in the Chamaeleon I North cloud, achieving a limiting luminosity of log Lt 27 ergs s-1 (0.5-8 keV band) in a 0.8 × 0.8 pc2 region. Of the 107 X-ray sources, 37 are associated with Galactic stars of which 27 are previously recognized cloud members. These include three PMS brown dwarfs; the protostellar brown dwarf ISO 192 has a particularly high level of magnetic activity. Follow-up optical photometry and spectroscopy establish that 9-10 of the Chandra sources are probably magnetically active background stars. Several previously proposed cloud members are also inferred to be interlopers because of the absence of X-ray emission at the level expected from the log Lt-K correlation. No new X-ray-discovered stars were confidently found despite the high sensitivity. From these findings, we argue that the sample of 27 PMS cloud members in the Chandra field is uncontaminated and complete down to K = 12 or M 0.1 M☉. The initial mass function (IMF) derived from our sample is deficient in 0.1-0.3 M☉ stars compared with the IMF of the rich Orion Nebula cluster and other Galactic populations. We cannot discriminate whether this is due to different star formation processes, mass segregation, or dynamical ejection of lower mass stars.

The Secrets of the Nearest Starburst Cluster. I. Very Large Telescope/ISAAC Photometry of NGC 3603

VLT/ISAAC JHKL photometry with subarcsecond resolution of the dense, massive starburst cluster NGC 3603 YC forming the core of the NGC 3603 giant molecular cloud is analyzed to reveal characteristics of the stellar population in unprecedented detail. The color-magnitude plane features a strong pre–main-sequence/main-sequence (PMS/MS) transition region, including the PMS/MS transition point, and reveals a secondary sequence for the first time in a nearby young starburst cluster. Arguments for a possible binary nature of this sequence are given. The resolved PMS/MS transition region allows isochrone fitting below the hydrogen-burning turn-on in NGC 3603 YC, yielding an independent estimate of global cluster parameters. A distance modulus of 13.9 mag, equivalent to d = 6.0 ± 0.3 kpc, is derived, as well as a line-of-sight extinction of AV = 4.5 ± 0.6 toward PMS stars in the cluster center. The interpretation of a binary candidate sequence suggests a single age of 1 Myr for NGC 3603 YC, providing evidence for a single burst of star formation without the need to employ an age spread in the PMS population, as argued for in earlier studies. Disk fractions are derived from L-band excesses, indicating a radial increase in the disk frequency from 20% to 40% from the core to the cluster outskirts. The low disk fraction in the cluster core, as compared to the 42% L-band excess fraction found for massive stars in the Trapezium cluster of a comparably young age, indicates strong photoevaporation in the cluster center. The estimated binary fraction of 30%, as well as the low disk fraction, suggest strong impacts on low-mass star formation due to stellar interactions in the dense starburst. The significant differences between NGC 3603 YC and less dense and massive young star clusters in the Milky Way reveal the importance of using local starbursts as templates for massive extragalactic star formation.

Spectroscopy of Very Low Mass Stars and Brown Dwarfs in IC 2391: Lithium Depletion and Hα Emission

We have obtained intermediate-resolution optical spectroscopy of 44 candidate very low mass members of the nearby young open cluster IC2391. Of these, 26 spectra are totally new, 14 were already analyzed in a previous paper and another four are in common. These spectra, taken at the Cerro Tololo 4-meter and Magellan I and II telescopes, allow us to confirm 33 of them as likely cluster members, based on their spectral types, presence of Li, and Halpha emission. Among these new cluster members is CTIO-160 (M7), the first IC2391 candidate to satisfy all criteria for being a substellar member of the cluster, including detection of the Li 6708A doublet. With the enlarged membership, we are able to locate the lithium depletion boundary of the cluster more reliably than in the past. Based on comparison to several theoretical models, we derive an age of 50+-5 Myr for IC2391. We also estimate new ages for the Alpha Per and Pleiades clusters; our ages are 85+-10 Myr and 130+-20 Myr, respectively. We derive an estimate of the initial mass function of IC2391 that extends to below the substellar limit, and compare it to those of other well-studied young open clusters. The index of the power law mass function for IC2391 is alpha=0.96+-0.12, valid in the range 0.5 to 0.072 M(sun).

X‐Ray Activity in the Open Cluster IC 4665

We present the results of a joint ROSAT High Resolution Imager (HRI) and optical investigation of the open cluster IC 4665. The ROSAT data contains detections for 28 stellar sources in the field, including 22 cluster members and candidate members spanning the color range -0.18 less than or equal to (B - V(sub o)) less than or equal to +1.63 (approx. B3 - M3). Upper limits are given for the remaining members (or candidate members) in the HRI field. Keck HIRES spectra have been obtained that yield radial and rotational velocity measures, respectively, for faint, low mass candidate members located within the field of the ROSAT HRI observation. In addition, photometry of possible optical counterparts to previously uncatalogued X-ray sources in the HRI field is presented. The trends in X-ray properties with (B - V) color in IC 4665 are found to be quite similar to that for other, more nearby young clusters such as the Pleiades and alpha Persei. In particular, a maximum in normalized X-ray luminosity of log (L(sub x)/L(sub bol)) approx. equal 3 is observed, beginning in the color range of (B - V)(sub o) = 0.7 - 0.8. This is similar to the corresponding color range among Pleiades members, in agreement with the earlier estimate, that the age of IC 4665 is similar to the age of the Pleiades. The correlation of rotation and X-ray emission levels is consistent with that in other young clusters. Among the high mass stars in IC 4665, five B stars are detected as X-ray sources. Of these, one is a spectroscopic binary while the remaining objects are apparently single staxs. The level of intrinsic X-ray emission observed in the rapidly rotating (v sini greater than 200 km/ s), single B stars is consistent with an origin due to shock heating of the ambient medium by radiatively driven, rotationally enhanced winds. On the basis of these observations and the results for other clusters, we argue that observed levels of X-ray emission in high mass stars of log (L(sub x)/L(sub bol)) greater than -6.0 are likely due to intrinsic emission associated with the B stax itself rather than emission from a late-type secondary. Finally, our results suggest that the initial angular momentum distributions and the subsequent evolution of rotation, L(sub x), and (L(sub x)/L(sub bol)) occur in practically identical fashions in clusters with ages approx. less than 100 Myr.