Cluster IC Research Articles

Detailed chemical composition of the open cluster IC 4651: The iron peak, $\mathsf{\alpha}$ elements, and Li

We present a detailed chemical analysis of 22 stars along the colour-magnitude sequence of the intermediate-age (1.7 Gyr) open cluster IC 4651, based on high-resolution, high S/N ratio spectra from UVES/VLT. IC 4651 thus becomes one of the few open clusters for which a detailed composition analysis exists for stars spanning 3.5 mag, from solar-type main- sequence stars to giants above the RGB clump. In a strict comparison with the Sun, we find for the cluster a well-defined Fe abundance of (Fe/H) = 0.10 ± 0.03 (internal errors), with a reddening E(b − y) = 0.091. We also derive abundances for the α elements Mg, Si, Ti, and Ca and find a moderate enhancement of the three former elements, in excellent agreement with the results for field stars of similar Fe abundance. Among the Fe group elements, Cr and Ni are slightly overabundant ((X/Fe) ∼ 0.05). As also observed in M 67, the Na abundance among the giants is more than 0.2 dex higher than in the dwarfs. We interpret this effect as real, and due to dredge-up of 23 Na in the giants. Four turnoff stars, all fairly rapid rotators (v sini ≥ 25 km s −1 ), appear to have higher (Fe/H) than the other stars; our tests show that, while a spurious enhancement of 0.1 dex can be produced by the effect of high rotation on our spectral analysis, this cannot explain the whole difference. These stars appear overabundant because we tend to overestimate their effective temperatures by forcing excitation equilibrium. Li abundances have been computed for all the stars and show a well-defined pattern: the Solar-type stars have an almost constant Li abundance, just below that of the Hyades, and the Li-dip is pronounced and well determined. Turnoff stars just above the dip have a cosmic Li abundance, but within a very small range of magnitudes (0.25 mag) higher on the turnoff ,t he Li level drops by more than a factor 10. This cannot be due just to dilution; rather some extra mixing is required. Among the giants, two probable clump stars show detectable Li, while all the other (likely) RGB stars do not - as is also observed in the similar cluster NGC 3680. None of these patterns can be explained by classical stellar evolution models. Again, some extra mixing is required. We show that rotating stellar models including the most recent developments for meridional circulation and turbulence by shear instabilities explain very well the behaviour of the abundance along the colour-magnitude diagram of IC 4651, including subgiant and giant phases. The possibility remains open that the giants exhibiting the highest abundances are actual RGB bump stars which have just been through the so-called lithium flash.

The Spectroscopically Determined Substellar Mass Function of the Orion Nebula Cluster

We present a spectroscopic study of candidate brown dwarf members of the Orion Nebula cluster (ONC). We obtained new J- and/or K-band spectra of ~100 objects within the ONC that are expected to be substellar on the basis of their K magnitudes and H-K colors. Spectral classification in the near-infrared of young low-mass objects is described, including the effects of surface gravity, veiling due to circumstellar material, and reddening. From our derived spectral types and existing near-infrared photometry, we construct an H-R diagram for the cluster. Masses are inferred for each object and used to derive the brown dwarf fraction and assess the mass function for the inner 5farcm1 × 5farcm1 of the ONC, down to ~0.02 M☉. The logarithmic mass function rises to a peak at ~0.2 M☉, similar to previous initial mass function determinations derived from purely photometric methods but falls off more sharply at the hydrogen-burning limit before leveling through the substellar regime. We compare the mass function derived here for the inner ONC with those presented in recent literature for the sparsely populated Taurus cloud members and the rich cluster IC 348. We find good agreement between the shapes and peak values of the ONC and IC 348 mass distributions but little similarity between the ONC and Taurus results.

XMM-Newtonstudy of the very young stellar cluster IC 348

We analyze a XMM-Newton X-ray imaging observation of the very young stellar cluster IC 348 with an observing time of 12 h and a corresponding total (all detector) MOS-equivalent exposure time of 207 ks. Our observation is strongly affected by a very intense solar particle flare, due to which only the first half of the exposure time can be used for scientific analysis. We compare the X-ray sources seen by XMM-Newton with those found in our previous Chandra study of IC 348 and find that XMM-Newton reveals 71 new X-ray sources, most of which are located outside the field-of-view of the Chandra observation. 20 of these new X-ray sources can be identified with known cluster members, and 19 sources are likely to be new low-mass members of the cluster. The lightcurves of the XMM-Newton sources show at most moderate levels of variability, but no large flares. We compare the spectral fitting results for 10 stars for which we have good spectra from both Chandra and XMM-Newton , and find good agreement in the fitted plasma temperatures, but a systematic difference in the fitted values of the hydrogen column density (extinction). We finally discuss the X-ray properties of the optically bright B1 star o Per and the X-ray detections among the deeply embedded young stellar objects to the south of the optical cluster center (including HH 211-mm), and find that none of the 23 spectroscopically identified brown dwarfs in IC 348 is detected in our XMM-Newton data.

Iron abundances from high-resolution spectroscopy of the open clusters NGC 2506, NGC 6134, and IC 4651

This is the first of a series of papers devoted to deriving the metallicity of old open clusters to study the time evolution of the chemical abundance gradient in the Galactic disk. We present detailed iron abundances from high resolution () spectra of several red clump and bright giant stars in the open clusters IC 4651, NGC 2506 and NGC 6134. We observed 4 stars of NGC 2506, 3 stars of NGC 6134 and 5 stars of IC 4651 with the FEROS spectrograph with the ESO 1.5 m telescope; moreover, 3 other stars of NGC 6134 were observed with the UVES spectrograph on Kueyen (VLT UT2). After excluding the cool giants near the red giant branch tip (one in IC 4651 and one in NGC 2506), we found overall [Fe/H] values of , dex (2 stars) for NGC 2506, , dex (6 stars) for NGC 6134 and , dex (4 stars) for IC 4651. The metal abundances derived from a line analysis for each star were extensively checked using spectrum synthesis of about 30 to 40 Fe I lines and 6 Fe II lines. Our spectroscopic temperatures provide reddening values in good agreement with literature data for these clusters, strengthening the reliability of the adopted temperature and metallicity scale. Also, gravities from the Fe equilibrium of ionization agree quite well with expectations based on cluster distance moduli and evolutionary masses.

Pulsating pre-Main sequence stars in young open clusters

New pulsating pre-main sequence (PMS) stars have been discovered in the young open clusters IC 4996 and NGC 6383 using CCD time series photometry in Johnson B and V filters. As the cluster ages are both smaller than 10 million years, all members later than spectral type A0 are still contracting towards the ZAMS, hence providing ideal candidates for searches of pulsation. A dozen stars in NGC 6383 and 35 stars in IC 4996 lie within the boundaries of the classical instability region in the Hertzsprung-Russell (HR) diagram, but pulsation was detected for only two of them in each cluster. For the well-studied cluster NGC 2264, the two already known PMS pulsating members, V 588 Mon and V 589 Mon, have been analysed using new data from a multi-site campaign. All data collected since their discovery in 1972 build the basis for the first measurements of period changes in PMS pulsators. To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Early-type stars in the young open cluster IC 1805

We investigate the multiplicity of three O-type stars in the very young open cluster IC 1805. All our targets were previously considered as spectroscopic binaries, but no orbital solution was available for any of them. Our results confirm the binarity of BD+60°497 and we provide the very first orbital solution for this double-lined spectroscopic binary. This is only the second O-star binary in IC 1805, and the first SB2 system, for which an orbital solution is now available. BD+60°497 has an orbital period of 3.96 days and consists of an evolved O6.5 V((f)) primary and an O8.5-9.5 V((f)) secondary with minimum masses of and . The observed primary/secondary mass ratio (1.28) appears lower than expected from a comparison with single star evolutionary models (1.60-1.74). For the other two stars, BD+60°501 and BD+60°513, we find no significant radial velocity variations, suggesting that they are most probably single. Although a fraction of binaries among the early-type stars of IC 1805 as high as 80% has been advocated in the literature, our results suggest that this number might be overestimated.

Coronal Variability in the Young Cluster NGC 2516

NGC 2516 has been observed by Chandra several times in order to correct the plate scale of the spacecraft's focal plane instruments. Because of this, Chandra has observed NGC 2516 with all four imaging arrangements available. In addition, NGC 2516 has been observed as part of the High Resolution Camera (HRC) guaranteed time program and is scheduled for return plate scale calibration visits. This makes it the best cluster to study for long-term variability. NGC 2516 is about 140 Myr old and less than 400 pc away. In our first paper, we discussed the detection of 150 X-ray sources (42% of which are identified as cluster members) in the calibration data taken during the orbital activation phase of the Chandra mission. In our second paper, we combined all the extant data sets and detected 284 sources, more than half of which are considered likely cluster members. In this our third paper, we further explore techniques of combining Advanced CCD Imaging Spectrometer (ACIS) and HRC Chandra data for timing analysis. We have been able to combine almost 70 ks of observation time, spread over five epochs, to study variability in this cluster on multiple timescales. We find that while stochastic variability rates are about the same for all objects in the sample, the timescale for detecting variability is shorter for late-type stars. Both stochastic and flare variability rates seen in NGC 2516 are similar to those seen in younger clusters IC 348, NGC 1333, and M42.

A Multiyear Photometric Study of IC 348

The extremely young cluster IC 348 has been monitored photometrically over 5 observing seasons from Dec 1998 to March 2003 in Cousins I with a 0.6 m telescope at Van Vleck Observatory. Twenty-eight periodic variables and 16 irregular variables have been identified. Among the brighter stars, 14 of the 16 known K or M-type WTTS were found to be periodic variables, while all 5 of the known CTTS were found to be irregular variables. In the full sample, which includes 150 stars with I mag as faint as 18, we find that 40% of the 63 WTTS are detected as variables, nearly all of them periodic, while 55% of the 20 CTTS are also detected as variable, with none of them periodic. Our study suggests that 80-90% of all WTTS in young clusters will be detected as periodic variables given sufficiently precise and extended monitoring, whereas CTTS will reveal themselves primarily or solely as irregular variables. This has clear consequences for PMS rotational studies based on photometric periods. We examine the stability of the periodic light curves from season to season. All periodic stars show modulations of their amplitude, mean brightness and light curve shape on time scales of less than 1 yr, presumably due to changes in spot configurations and/or physical characteristics. In no case, however, can we find definitive evidence of a change in period, indicating that differential rotation is probably much less on WTTS than it is on the Sun. Among the non-periodic stars, we report the detection of two possible UXors as well as a pre-main sequence star, HMW 15, which apparently undergoes an eclipse with a duration exceeding three years.

The distance to IC 4499 from K-band photometry of 32 RR Lyrae stars

Single epoch K-band photometry of 32 RR Lyrae stars in the young galactic globular cluster IC 4499 is presented. The mean magnitudes have been estimated using the K-band template light-curves of Jones et al. (1996). We derive an independent estimate of the distance and reddening for the cluster based on the new -log P-[Fe/H] relation from Bono et al. (2003) which has a zero-point in agreement with the canonical LMC distance of (m - M) 0 = 18.5. For an assumed metallicity of [Fe/H] = -1.65 ± 0.1 we find (m - M) 0 = 16.47 ± 0.04 ± 0.06 (random and systematic errors respectively) and E(B - V) = 0.24 ± 0.03 in good agreement with the findings of Walker & Nemec (1996) based on optical data. The distance estimate is very robust to uncertainties in the reddening estimate but naturally depends directly on the adopted zero-point of the -log P-[Fe/H] relation.

Distribution of Circumstellar Disk Masses in the Young Cluster NGC 2024

We determine the distribution of circumstellar disk masses in the young (~0.3 Myr) cluster NGC 2024 by imaging a 25 × 25 region in 3 mm continuum emission to an rms noise level of ~0.75 mJy beam-1 with the Owens Valley Millimeter Array. The mosaic encompasses 147 K-band sources, as well as the molecular ridge seen previously in dust continuum emission. We detect 10 pointlike sources in 3 mm continuum emission above the level of 5 σ within the unit gain region of the mosaic. One of these sources corresponds to the near-IR source IRS 2, an early B-type star. Two other sources are tentatively associated with low-mass near-IR cluster members, and the remaining seven sources have no K-band counterparts. Assuming the millimeter continuum point sources represent emission from circumstellar disks and/or envelopes, then ~6% of the total population (infrared and millimeter sources) in the NGC 2024 mosaic have a circumstellar mass in excess of ~0.06 M☉. We obtain further constraints on the average circumstellar disk mass by considering the mean millimeter continuum flux observed toward a sample of 140 K-band sources that likely have stellar masses 1-2 M☉. While none of these sources are detected individually above the 3 σ limit of ~0.035 M☉, the ensemble of sources are detected in the mean at the 5 σ level with a mean disk mass of ~0.005 M☉. Compared with the older (~2 Myr) cluster IC 348, NGC 2024 contains a higher frequency of massive disks or envelopes and has a higher mean disk mass by a factor of 2.5 ± 1.3 among K-band sources, suggesting that the mean circumstellar mass is decreasing with cluster age. We also compare the results for the NGC 2024 and IC 348 clusters with those for the lower density Taurus star-forming region. Finally, we compare our detection limits with the minimum mass estimate for the proto-solar nebula and discuss possible implications for planet formation.

Using Narrowband Photometry to Detect Young Brown Dwarfs in IC 348

We report the discovery of a population of young brown dwarf candidates in the open star cluster IC 348 and the development of a new spectroscopic classification technique using narrowband photometry. Observations were made using FLITECAM, the First Light Camera for SOFIA, at the 3 m Shane telescope at Lick Observatory. FLITECAM is a new 1-5 μm camera with an 8' field of view. Custom narrowband filters were developed to detect absorption features of water vapor (at 1.495 μm) and methane (at 1.66 μm) characteristic of brown dwarfs. These filters enable spectral classification of stars and brown dwarfs without spectroscopy. FLITECAM's narrow- and broadband photometry was verified by examining the color-color and color-magnitude characteristics of stars whose spectral type and reddening was known from previous surveys. Using our narrowband filter photometry method, it was possible to identify an object measured with a signal-to-noise ratio of 20 or better to within ±3 spectral class subtypes for late-type stars. With this technique, very deep images of the central region of IC 348 (H ~ 20.0) have identified 18 sources as possible L or T dwarf candidates. Out of these 18, we expect that between 3 and 6 of these objects are statistically likely to be background stars, the remainder being true low-mass members of the cluster. If confirmed as cluster members, then these are very low mass objects (~5MJup). We also describe how two additional narrowband filters can improve the contrast between M, L, and T dwarfs as well as provide a means to determine the reddening of an individual object.

Luminosity Functions of Young Clusters: Modeling the Substellar Mass Regime

We predict near-infrared luminosity functions of young (5 Myr to 1 Gyr) star clusters by combining evolutionary models of very low mass (1 MJ to 0.15 M☉) dwarfs with empirical bolometric corrections. We identify several characteristic features in our results. These can be attributed to three causes: (1) deuterium burning in the most massive substellar objects; (2) methane absorption in bodies with Teff less than 1300 K, the temperature of the L/T transition; and (3) the formation of dust clouds and the rainout of dust at roughly the same effective temperature as methane formation. Accurate reconstruction of the substellar mass function from luminosity function observations requires that these phenomena are taken into account. At present, few observational studies extend to sufficient sensitivities to allow detection of these effects. However, the luminosity function of the young open cluster IC 2391 shows a clear peak at MI ~ 14, which we attribute to the result of deuterium burning in substellar objects. The location of this feature is a strong function of age, and we estimate an age of 35 Myr for IC 2391. This is significantly younger than the 53 Myr derived from the location of the lithium depletion boundary but agrees with the main-sequence turnoff age. We consider the implications of this result and our multiband luminosity functions for future observational studies. All predicted luminosity function features are, or will be, accessible to observations using new wide-field IR imagers and the Space Infrared Telescope Facility.

Constraints on the IMF and the brown dwarf population of the young cluster IC 348

We use a deep near-infrared census of the young stellar cluster IC 348 to construct and analyze its luminosity function. Our mosaic image of IC 348 covers the full extent of the cluster with a completeness limit of and is therefore sensitive for 2 Myr old cluster members with masses as low as for the mean extinction of the known cluster members ( mag). By using information on stellar ages, extinctions, and the binary population in IC 348 from several recent studies, we can derive statistical constraints on the stellar and sub-stellar mass function of the cluster by modeling the observed luminosity function. We find that the stellar part of the mass function in IC 348 is well described by the galactic field star IMF. While several brown dwarfs have recently been identified in IC 348, our data show that the cluster harbors only a relatively small population of sub-stellar objects. We find that brown dwarfs in the mass range constitute at most ~ of the total cluster population, in contrast to recent results suggesting much larger brown dwarf populations in other young clusters and also the galactic field. Our results suggest that IC 348 has ~ fewer brown dwarfs than the Orion Trapezium cluster. A similar brown dwarf “deficit” was recently found in the Taurus star forming region. We speculate about the possible causes for this result, including the presence or absence of nearby massive stars and their influence on the formation of low-mass young stellar objects.

IC 2395 and BH 47: Only one open cluster in the Vela constellation

We report UBV photoelectric photometry for 273 stars in the fields of the open clusters IC 2395 and BH 47. Our postulate is that, rather than two different clusters in this region of Vela, there is only one, which we callIC 2395 = BH 47. The cluster is centered at about α = 8 h 42 m .5, 6 = -48°06′.8 (2000), its angular diameter being 19′. The analysis of the photometric data yields 61 probable cluster members and 16 possible members. Photometric membership probabilities show good agreement with those obtained from a proper motion study for 21 stars in common. The reddening across the cluster appears to be uniform, the mean E(B - V) value being 0.09 ′ 0.02. The true distance modulus is (V - M v ) 0 = 9.52 ′ 0.10, corresponding to a distance from the Sun of (800 ′ 40) pc and 48 pc below the Galactic plane. The cluster age, determined by fitting isochrones with core overshooting, turns out to be (6 ′ 2) Myr. There is a strong likelihood that IC 2395 = BH47 is physically connected to the Vela OB1C association.

An Unusual Eclipse of a Pre-Main-Sequence Star in IC 348

A solar-like pre-main-sequence star (TJ 108=H187=LRLL 35=HMW 15) in the extremely young cluster IC 348 has been found, which apparently experienced an eclipse lasting ~3.5 years, much longer than has ever been detected for any normal eclipsing binary. The light curve is flat-bottomed and rather symmetric, with a depth of 0.66 mag in Cousins I. During eclipse, the system reddened by ~0.17 mag in R-I. We argue that the eclipsing body is not a star because of the small probability of detecting an eclipse in what would be a very widely separated binary. Instead, it appears that the eclipse was caused by a circumstellar or circumbinary cloud or disk feature that occulted the star, or one of its components, if it is a binary system. We emphasize the importance of more detailed study of this object, which appears to be a new member of a small class of pre-main-sequence stars whose variability can be firmly linked to occultation by circumstellar (or circumbinary) matter.

A Census of the Young Cluster IC 348

We present a new census of the stellar and substellar members of the young cluster IC 348. We have obtained images at I and Z for a 42'x28' field encompassing the cluster and have combined these measurements with previous optical and near-infrared photometry. From spectroscopy of candidate cluster members appearing in these data, we have identified 122 new members, 15 of which have spectral types of M6.5-M9, corresponding to masses of 0.08-0.015 M_sun by recent evolutionary models. The latest census for IC 348 now contains a total of 288 members, 23 of which are later than M6 and thus are likely to be brown dwarfs. From an extinction-limited sample of members (A_V =0.03 (<=M8). In logarithmic units where the Salpeter slope is 1.35, the mass function for IC 348 rises from high masses down to a solar mass, rises more slowly down to a maximum at 0.1-0.2 M_sun, and then declines into the substellar regime. In comparison, the similarly-derived IMF for Taurus from Briceno et al. and Luhman et al. rises quickly to a peak near 0.8 M_sun and steadily declines to lower masses. The distinctive shapes of the IMFs in IC 348 and Taurus are reflected in the distributions of spectral types, which peak at M5 and K7, respectively. These data provide compelling, model-independent evidence for a significant variation of the IMF with star-forming conditions.

Multicolour photometry and Coravel observations of stars in the southern open cluster IC 2488

We present new UBV photoelectric observations of 119 stars in the field of the southern open cluster IC 2488, supplemented by DDO and Washington photometry and Coravel radial velocities for a sample of red giant candidates. Nearly 50% of the stars sampled - including three red giants and one blue straggler - are found to be probable cluster members. Photometric membership probabilities of the red giant candidates show good agreement with those obtained from Coravel data. A mean radial velocity of ( 2.63 0.06) km s 1 is derived for the cluster giants. The reddening across the cluster is found to be uniform, the mean value being E(B V)= 0.24 0.04. IC 2488, located at a distance of (1250 120) pc from the Sun and 96 pc below the Galactic plane, is most probably not related to the planetary nebula ESO 166-PN21. A metal abundance (Fe/H)= 0.10 0.06 relative to the Sun is determined from DDO data of the red giant members, in good agreement with the (Fe/H) values derived from five independent Washington abundance indices. An age of 180 Myr is determined from the fitting of isochrones computed with convective overshooting for Z = 0.019. The isochrone for log t= 8.25 reproduces remarkably well not only the morphology of the upper main sequence but also the observed red giant pattern.

Long-term spectral variability of the O4If+ star HD 15570 in the open cluster IC 1805

We present preliminary results concerning the O4If+ star HD 15570. Of-type supergiants are believed to represent an evolved evolutionary stage of very high mass stars (Minit &gt; 40 M⊙). Their low numbers and extreme peculiarity make each of these objects worth of continuous monitoring. HD 15570 dominates the very young open cluster IC 1805 and is thought to have had an initial mass ≥ 100 M⊙. Low-, intermediate- and high-resolution spectra were collected at the Loiano and San Pedro Mártir telescopes since 1992. The comparison of our high- and low-resolution spectra shows clear variability of a number of spectral features. It is worth noticing, that the variation of Hα seems to follow a repeated secular trend, increasing its equivalent width from ~ 3Å to more than 8 Å in a few years. At the same time, its profile is varying, from a shape quite similar to the theoretical one corresponding to the Klein &amp; Castor (1978) model C, to a much more developed P-Cygni profile, with a deep blue absorption wing. The possibility of instrumental effects is ruled out by the remarkable constancy of the nearby diffuse interstellar band at 6613 Å and of the He ii 6683 Å absorption. The Hβ profile variability is evident from the comparison of the 1996 and 1998 high-resolution spectra. The emission component, which is clearly visible at all epochs, is absent in the 1996 spectrum when the line appears in pure absorption. No relevant line-profile variation seems to be present in the He ii and N iii lines contributing to the ‘feature f’, that looks remarkably constant, as well as Hγ, while their equivalent widths seem to show a modest amount of random variability.

The Substellar Population in IC 348

I present the latest results in a continuing effort to spectroscopically identify the substellar members of the young cluster IC 348. I have combined new images at I and Z with IR photometry from previous studies and from 2MASS and selected candidate members of the cluster across a larger area and at lower masses than probed in previous surveys. After observing these candidates with spectroscopy, the latest census for IC 348 contains 234 members with spectral types, 22 of which are later than M6 and thus are probably substellar. Individual masses of cluster members are estimated by combining their positions on the Hertzsprung-Russell (H-R) diagram with theoretical evolutionary models. From an extinction-limited sample of members (AV ≤ 4), I construct an IMF for IC 348 that should be complete down to 0.02 M⊙. This IMF is compared to a similarly derived mass function for Taurus by Briceño et al. (2002); these data provide clear evidence for a variation of the IMF with star-forming conditions.

Detection of δ Scuti-like pulsation in H254, a pre-main sequence F-type star in IC 348

We present time series observations of intermediate mass PMS stars belonging to the young star cluster IC 348. The new data reveal that a young member of the cluster, H254, undergoes periodic light variations with δ Scuti-like characteristics. This occurrence provides an unambiguous evidence confirming the prediction that intermediate-mass pre-main sequence (PMS) stars should experience this transient instability during their approach to the main-sequence. On the basis of the measured frequency d-1, we are able to constrain the intrinsic stellar parameters of H254 by means of linear, non adiabatic, radial pulsation models. The range of the resulting luminosity and effective temperature permitted by the models is narrower than the observational values. In particular, the pulsation analysis allows to derive an independent estimate of the distance to IC 348 of about 320 pc. Further observations could either confirm the monoperiodic nature of H254 or reveal the presence of other frequencies.