Cluster Main Sequence Research Articles

Luminosity and Mass Function of the Galactic open cluster NGC 2422

We present UBVRI photometry of the open cluster NGC 2422 (age yr) down to a limiting magnitude . These data are used to derive the Luminosity and Mass Functions and to study the cluster spatial distribution. By considering the color–magnitude diagram data and adopting a representative cluster main sequence, we obtained a list of candidate cluster members based on a photometric criterion. Using a reference field region and an iterative procedure, a correction for contaminating field stars has been derived in order to obtain the Luminosity and the Mass Functions in the range. By fitting the spatial distribution, we infer that a non-negligible number of cluster stars lies outside our investigated region. We have estimated a correction to the Mass Function of the cluster in order to take into account the “missing” cluster stars. The Present Day Mass Function of NGC 2422 can be represented by a power-law of index (rms) – the Salpeter Mass Function in this notation has index – in the mass range . The index α and the total mass of the cluster are very similar to those of the Pleiades.

The Evolutionary State of Stars in the NGC 1333S Star Formation Region

We present 2 μm near-IR spectroscopic observations of a sample of 33 objects in the NGC 1333S active star-forming cluster centered on the pre–main-sequence star SSV 13. We have previously studied this region photometrically in the optical and near-IR, and with the addition of these near-IR spectra, we further probe the pre–main-sequence cluster membership and evolutionary state. From the atomic and molecular absorption features observed, together with the earlier photometry, we derive spectral types, effective temperatures, masses, and ages of the stars and conclude that almost all (90%) the stars observed in this sample are pre–main-sequence objects. This result significantly refines the evolutionary information obtained from photometric evidence alone. Comparison with theoretical evolutionary tracks and isochrones suggests that our survey has sampled sources with masses in the range 0.2–2 M⊙ and stellar ages between 7 × 104 and 1 × 108 yr with a preponderance of sources around 3 × 106 yr. This implies the presence of low- to intermediate-mass T Tauri stars of evolutionary designation Class I to Class III. We conclude that star formation seems to have occurred in likely several bursts rather than occurring coevally. Star formation in such a region as NGC 1333S is likely significantly affected by the large number of active molecular outflows in the region, which could provide a mechanism for cloud turbulence and the onset of subsequent star formation.

The Star Population of Young Open Clusters: A Photometric and Spectroscopic Study

I have determined physical parameters (interstellar reddening, distance, and age) for three young open clusters: h Persei, x Persei, and NGC 1893. I have studied their massive star populations and their formation epochs. I present photometric data for 350 stars in the double open cluster h and x Persei. For both clusters, the distance moduli and reddening are determined to be the same: V0 M p V and E(b y) p . However, the ages for 11.6 0.2 0.42 0.2 the clusters are different. Most stars in h Persei follow the isochrone, although some of the massive stars seem log t p 7.3 younger. By contrast, the stars in x Persei better fit isochrones for (A. Marco & G. Bernabeu 2001, A&A, log t p 7.10–7.15 372, 477). I give photometric data for 114 stars in NGC 1893. The distance modulus is found to be V0 . This M p 13.9 0.2 V distance is much larger than the traditional value for Aur OB2. I have also obtained spectroscopy for photometrically peculiar stars in NGC 1893. Among them, I have found six emissionline stars, identified as pre–main-sequence (PMS) stars with spectral types between B0 and late F. For two of these stars, the derived spectral types indicate that they are very massive Herbig Be stars. However, one other B-type emission-line star looks like a typical Be star. The presence of massive PMS stars in NGC 1893 suggests that many other fainter PMS stars could be present. A spectroscopic study of objects not falling on the cluster main sequence found several objects whose spectral types and colors suggested that they could be absorption-line PMS stars. While many early and intermediate B-type stars are settled onto the main sequence, the detection of early-B Herbig Be stars indicates that star formation has been taking place for some time in NGC 1893. This work has produced a list of new b standard stars uvby for the study of moderately reddened clusters and has shown that the combination of Stromgren photometry and low-resolution spectroscopy is a powerful tool for the detection of a PMS population in young open clusters (A. Marco, G. Bernabeu, & I. Negueruela 2001, AJ, 121, 2075).

Wide-Field CCD Photometry of the Globular Cluster M92

We present wide field CCD photometry of a galactic globular cluster M92 obtained in the V and I bands with the CFH12K mosaic CCD at the Canada-France-Hawaii Telescope. A well-defined color-magnitude diagram is derived down to 5 magnitudes fainter than the cluster main sequence turn-off. After removing the background contribution, we obtain luminosity and mass functions, surface density profiles, and the surface number density maps of the stars belonging to the cluster. The surface density profile of all stars shows that the cluster's halo extends at least out to ~30' from the cluster center in agreement with previous study, but the profile of faint stars at the very outer region of the cluster shows a different gradient compared with that of bright stars. For a mass function of the cluster, we find that the inner region of the cluster has x = 1.2+-0.2, whereas the outer region has x = 1.8 +-0.3, clearly indicating a mass segregation of the cluster. An estimate of the photometric mass of the cluster implies that the remnant populations (white dwarfs and neutron stars) contribute at least 25% of the total cluster mass. The surface density map of M92 shows some evidence that the tidal tail of M92 may be oriented perpendicular to the direction toward the Galactic center.

A Far‐Ultraviolet Survey of 47 Tucanae. I. Imaging

We present results from the imaging portion of a far-ultraviolet (FUV) survey of the core of 47 Tucanae. We have detected 767 FUV sources, 527 of which have optical counterparts in archival HST/WFPC2 images of the same field. Most of our FUV sources are main-sequence (MS) turn-off stars near the detection limit of our survey. However, the FUV/optical color-magnitude diagram (CMD) also reveals 19 blue stragglers (BSs), 17 white dwarfs (WDs) and 16 cataclysmic variable (CV) candidates. The BSs lie on the extended cluster MS, and four of them are variable in the FUV data. The WDs occupy the top of the cluster cooling sequence, down to an effective temperature of T_{eff} \simeq 20,000 K. Our FUV source catalog probably contains many additional, cooler WDs without optical counterparts. Finally, the CV candidates are objects between the WD cooling track and the extended cluster MS. Four of the CV candidates are previously known or suspected cataclysmics. All of these are bright and variable in the FUV. Another CV candidate is associated with the semi-detached binary system V36 that was recently found by Albrow et al. (2001). V36 has an orbital period of 0.4 or 0.8 days, blue optical colors and is located within 1 arcsec of a Chandra x-ray source. A few of the remaining CV candidates may represent chance superpositions or SMC interlopers, but at least half are expected to be real cluster members with peculiar colors. However, only a few of these CV candidates are possible counterparts to Chandra x-ray sources. Thus it is not yet clear which, if any, of them are true CVs, rather than non-interacting MS/WD binaries or Helium WDs.

A comprehensive study of the rich open star cluster NGC 2099 based on deep $\vec{BVI}$ CCD observations

The CCD observations of the rich open star cluster NGC 2099 and its surrounding eld region have been carried out up to a limiting magnitude of V 22 mag in B, V and I passbands for the rst time. A total of12 000 stars have been observed in the area of about 24 0 34 0 in the cluster region, as well as2180 stars in the 12 0 12 0 area of the eld region located 45 0 away from the cluster center. The cluster parameters determined by tting the convective core overshoot isochrones in the V;(B V )a ndV;(V I) diagrams are E(B V )=0 :30 0:04 mag, distance = 1360 100 pc, age = 400 Myr and metallicity Z =0 :008. A well- dened cluster main sequence spread over about 8 mag in range is observed for the rst time. Its intrinsic spread amounting to0.06 mag in colour is almost the same over the entire brightness and can be understood in terms of the presence of physical/optical binaries. The core and cluster radii determined from the radial stellar density proles are 185 00 and 1000 00 respectively. Only about 22% of cluster members are present in the core region. The eects of mass segregation, most probably due to dynamical evolution, have been observed in the cluster. The mass function slope of the entire cluster is 0:67 0:12. It becomes closer to the Salpeter value of 1:35, if flattening in the cluster mass function due to presence of both binaries and a much more extended corona is considered.

Photometric Calibrations after HIPPARCOS or: The Fine Structure of the HR-Diagram Revealed

AbstractThe Hipparcos parallaxes allow us to revisit photometric calibrations in terms of Mv and to evaluate the effects of the overall metallicity across the HR-diagram. For evolved stars the spread in mass remains the major and irreducible source of errors in the photometric Mv. If the relative locations of open cluster main-sequences are fully explained by the ∆Mv versus ∆[M/H] relation, the residual scatter in the lower main sequence appears due to additional parameters such as rotation or gravitational settling of the heavy elements. These cannot readily be studied by a purely photometric approach.

The Distance of the Large Magellanic Cloud Cluster NGC 1866

Hubble Space Telescope V,I photometry of stars in the Large Magellanic Cloud Cluster NGC 1866 shows a well defined cluster main sequence down to V=25 mag, with little contamination from field or foreground stars. We use the main sequence fitting procedure to link the distance of NGC 1866 to the Hipparcos determination of the distance for the Hyades MS stars, making use of evolutionary prescriptions to allow for differences in the chemical composition. On this basis we find a true distance modulus for NGC 1866 of 18.35 +/- 0.05 mag. If the cluster is assumed to lie in the LMC plane then the LMC modulus is 0.02 mag less.

On the stellar content of the open clusters Melotte 105, Hogg 15, Pismis 21 and Ruprecht 140

CCD observations in the B, V and I passbands have been used to generate colour-magnitude diagrams reaching down to V 19 mag for two slightly characterized (Melotte 105 and Hogg 15) and two almost unstudied (Pismis 21 and Ruprecht 140) open clusters. The sample consists of about 1300 stars observed in elds of about 4 0 4 0 . Our analysis shows that neither Pismis 21 nor Ruprecht 140 are genuine open clusters since no clear main sequences or other meaningful features can be seen in their colour-magnitude diagrams. Melotte 105 and Hogg 15 are open clusters aected by E(B V )=0 :42 0:03 and 0.95 0.05, respectively. Their distances to the Sun have been estimated as 2.2 0.3 and 2.6 0.08 kpc, respectively, while the corresponding ages estimated from empirical isochrones tted to the Main Sequence cluster members are350 Myr and 300 Myr, respectively. The present data are not consistent with the membership of the WN6 star HDE 311884 to Hogg 15.

71 Tauri: Hyades Enigma Resolved?

71 Tauri (HD 28052; F0 IV-V) is an enigmatic object for two reasons: (1) it is the second brightest X-ray source in the Hyades, yet early F stars as a rule are not strong coronal emitters; and (2) it lies a magnitude above the cluster main sequence, but radial velocity studies and speckle imaging suggest that it is single. Recently, long-slit ultraviolet spectra of the star, obtained with the Space Telescope Imaging Spectrograph (STIS), serendipitously have revealed the presence of a stellar companion at a distance of 01 directly south of the primary. The companion is seen only in its far-UV chromospheric emission lines. The nature of this object cannot be determined from our STIS spectra alone, but its high emission levels are most readily explained if it is a close binary of coronally active dG/dK stars. The presence of the secondary can account for the striking X-ray properties of 71 Tau but not its unusual location in the cluster color-magnitude diagram. It is conceivable that the primary itself is a close double of nearly equal stars, making 71 Tau a possible quadruple system. The alternative—that 71 Tau is ~150 Myr older than other members of the Hyades, approaching the end of core hydrogen burning for a 2 M☉ star—would challenge the presumed synchrony of star formation in the cluster.

On the Globular Cluster Initial Mass Function below 1 \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $M_{\odot }$ \end{document}

Accurate luminosity functions (LFs) for a dozen globular clusters have now been measured at or just beyond their half-light radius using HST. They span almost the entire cluster main sequence (MS) below 0.75 M☉. All these clusters exhibit LFs that rise continuously from an absolute I magnitude MI 6 to a peak at MI 8.5-9 and then drop with increasing MI. Transformation of the LFs into mass functions (MFs) by means of mass-luminosity (ML) relations that are consistent with all presently available data on the physical properties of low-mass, low-metallicity stars shows that all the LFs observed so far can be obtained from MFs having the shape of a lognormal distribution with characteristic mass mc = 0.33 ± 0.03 M☉ and standard deviation σ = 0.34 ± 0.04. In particular, the LFs of the four clusters in the sample that extend well beyond the peak luminosity down to close to the hydrogen-burning limit (NGC 6341, NGC 6397, NGC 6752, and NGC 6809) can only be reproduced by such distributions and not by a single power law in the 0.1-0.6 M☉ range. After correction for the effects of mass segregation, the variation of the ratio of the number of higher to lower mass stars with cluster mass or any simple orbital parameter or the expected time to disruption recently computed for these clusters shows no statistically significant trend over a range of this last parameter of more than a factor of ~100. We conclude that the global MFs of these clusters have not been measurably modified by evaporation and tidal interactions with the Galaxy and, thus, should reflect the initial distribution of stellar masses. Since the lognormal function that we find is also very similar to the one obtained independently for much younger clusters and to the form expected theoretically, the implication seems to be unavoidable that it represents the true stellar initial mass function for this type of star in this mass range.

CCD [ITAL][CLC]uvby[/CLC][/ITAL]Hβ Photometry in Clusters. I. The Open Cluster Standard, IC 4651

CCD photometry of the intermediate-age open cluster IC 4651 on the uvbyHβ system is presented and analyzed. By using a combination of the information from the color-magnitude diagram (CMD) and the color-color diagrams, a sample of 98 highly probable main-sequence cluster members with high photometric accuracy is isolated. From this sample, adopting the intrinsic color relation of Olsen, E(b-y) = 0.062 ± 0.003 and [Fe/H] = +0.077 ± 0.012, where the errors quoted are the standard errors of the mean and refer to the internal errors alone. Use of the Nissen intrinsic color relation produces E(b-y) = 0.071 and [Fe/H] = +0.115. Adopting the lower reddening, a direct main-sequence fit to the Hyades with (m - M) = 3.33 leads to (m - M) = 10.15, while isochrones with convective overshoot and zeroed to the Hyades produce an age of 1.7 ± 0.1 Gyr, with an excellent match to the morphology of the turnoff. The higher reddening produces (m - M) = 10.3 and an age lower by 0.1 Gyr. Comparison with the CMD of NGC 3680 shows that the two clusters have virtually identical morphology, which in combination with their similar compositions produces identical ages. Coincidentally, the shifts in the CMD necessary to superpose the two clusters require that the apparent moduli of IC 4651 and NGC 3680 be the same, while E(b-y)4651 = E(b-y)3680 + 0.04.

Evolution Of The Lithium Abundances Of Solar-Type Stars. IX. High-Resolution Spectroscopy of Low-Mass Stars in NGC 2264

We have obtained Keck HIRES spectra of 35 objects in the pre–main-sequence cluster NGC 2264 in order to determine their radial velocities, rotation rates, activity levels, and Li abundances. Our precise radial velocities indicate that the eight objects we observed that lie below a 5 Myr isochrone are nonmembers. This means that the age spread within NGC 2264 is only about 4 Myr, a factor of 2 less than found previously, although this is a lower limit to the true age spread because star formation is still taking place in the cluster. After correction for non-LTE effects, our Li abundances are consistent with no depletion having yet occurred and a uniform value of log N(Li) = 3.2, provided that the uncertainty in Teff values is about 250 K. This is high, but this cluster appears to have varying extinction, which could lead to large temperature errors. Stars in NGC 2264 have spun up compared with the younger Orion Nebula cluster. Current models suggest disk-locking timescales of 1–2 Myr to account for the most rapidly rotating stars. By combining rotation periods and v sin i values, we show that the distribution of sin i is not in accord with a random distribution of rotation axes. The reason for this is not known, but it is probably due to a selection against equator-on objects that still have obscuring material around them.

Multicolor CCD Photometry and Stellar Evolutionary Analysis of NGC 1907, NGC 1912, NGC 2383, NGC 2384, and NGC 6709 Using Synthetic Color-Magnitude Diagrams

We present the first CCD photometric observations of NGC 2383 and NGC 2384 in B, V, R, and I, NGC 1912, NGC 6709 in B, V, and I and NGC 1907 in B and V passbands, reaching down to a limiting magnitude of V ~ 20 mag for ~3300 stars put together. The results of the spectroscopic observations of 43 bright stars in the field of NGC 1912, NGC 2383, NGC 2384, and NGC 6709 are also presented. The color-magnitude diagrams (CMDs) of the clusters in V versus B-V, V versus V-R, and V versus V-I are presented. The distances and reddening to these clusters are determined using the cluster CMDs. The distances to the clusters NGC 1907, NGC 1912, NGC 2383, NGC 2384, and NGC 6709 are 1785 ± 260, 1820 ± 265, 3340 ± 490, 2925 ± 430, and 1190 ± 175 pc, respectively. Some gaps in the cluster main sequence have been identified. We have compared the observed color-magnitude diagrams of these four open clusters with the synthetic ones derived from one classical and two overshoot stellar evolutionary models. Overshoot models estimate older ages for clusters when compared to the classical models. The age of the clusters estimated using the isochrones of Bertelli et al. are 400, 250, 400, 20, and 315 Myr for the clusters NGC 1907, NGC 1912, NGC 2383, NGC 2384, and NGC 6709, respectively. A comparison of the synthetic color-magnitude diagrams with the observed ones indicates that the overshoot models should be preferred. The comparison of integrated luminosity functions do not clearly indicate as to which model is to be preferred. The values of the mass function slopes estimated for the clusters are x = 1.7 ± 0.15 for NGC 1912 (mass range: 1.7–3.9 M⊙) and NGC 6709 (1.7–3.4 M⊙), x = 1.3 ± 0.15 for NGC 2383 (1.7–3.1 M⊙), and x = 1.0 ± 0.15 for NGC 2384 (2.0–14.0). The present age estimates show that the closely located cluster pair NGC 1912 + NGC 1907 have similar ages, indicating that they may have born together, making them a good candidate to be a binary open cluster.

The Luminosity Function of ω Centauri

Deep Hubble Space Telescope (HST) Wide Field Planetary Camera 2 observations of the stellar population in the globular cluster ω Cen (NGC 5139) have been used to measure the luminosity function of main-sequence stars at the low-mass end. Two fields have been investigated, located ~46 and ~7', respectively, away from the cluster center or near the half-light radius of this cluster (rhl 48). The color-magnitude diagrams derived from these data show the cluster main sequence extending to the detection limit at I 24. Information on both color and magnitude is used to build the luminosity functions of main-sequence stars in these fields, and the two independent determinations are found to agree very well with each other within the observational uncertainty. Both functions show a peak in the stellar distribution around MI 9, followed by a drop at fainter magnitudes well before photometric incompleteness becomes significant, as is typical of other globular clusters observed with the HST. This result is at variance with previous claims that the luminosity function of ω Cen stays flat at low masses but is in excellent agreement with recent Near Infrared Camera and Multi-Object Spectrograph observations.

WIYN Open Cluster Study. I. Deep Photometry of NGC 188

We have employed precise and carefully calibrated V- and I-band photometry of NGC 188 at WIYN Observatory to explore the cluster luminosity function (LF) and study the cluster white dwarfs. Our photometry is offset by V = 0.052 (fainter) from that of Sandage and Eggen & Sandage. All published photometry for the past three decades has been based on these two calibrations, which are in error by 0.05 ± 0.01. We employ the Pinsonneault et al. fiducial open cluster main sequence to derive a distance modulus of 11.43 ± 0.08 and E(B-V) = 0.09, with the largest single source of error caused by uncertainty in the cluster metallicity. We report observations that are ≥50% complete along the main sequence to V = 24.6. We find that the NGC 188 central-field LF peaks at MI ≈ 3 to 4. This is unlike the solar neighborhood LF and unlike the LFs of dynamically unevolved portions of open and globular clusters, all of which typically rise continuously until MI ≈ 9.5. Although we find that ≥50% of the unresolved objects in this cluster are multiple systems with mass ratios ≥0.3, their presence cannot account for the shape of the NGC 188 LF. For theoretical reasons having to do with the long-term survivability of NGC 188, we believe the cluster is highly dynamically evolved and that the low-luminosity stars missing from the central cluster LF are either in the cluster outskirts or have left the cluster altogether. We identify nine candidate white dwarfs (WDs) in NGC 188, of which we expect at least three, and perhaps six, are bona fide cluster WDs. The luminosities of the faintest likely WD indicate an age of 1.14 ± 0.09 Gyr, where the error in age includes the cluster distance uncertainty and we assume the WD has a hydrogen atmosphere. This age is a lower limit to the cluster age, and observations probing to V = 27 or 28 will be necessary to find the faintest cluster WDs and independently determine the cluster age. While our lower age limit is not surprising for this ≈6 Gyr–old cluster, our result demonstrates the value of the WD age technique with its very low internal errors.

The Red Horizontal-Branch Star HD 17072

We summarize the results of a spectroscopic analysis of HD 17072, finding it to be a metal-poor ([Fe/H] = -1.17) red horizontal-branch star with Teff = 5375 K and log g = 2.4. We also derive a radial velocity of 62.8 km s-1. It has the best determined Hipparcos trigonometric parallax among the metal-poor field horizontal-branch stars and supports the fainter luminosities for such stars found from statistical parallax and Baade-Wesselink analyses, in contradiction to the results of main-sequence fitting of metal-poor field dwarfs to globular cluster main sequences.

The Globular Cluster M54 and the Star Formation History of the Sagittarius Dwarf Galaxy

We present a deep color-magnitude diagram in the VI passbands of the globular cluster M54, a member of the Sagittarius dwarf galaxy. The data extend below the cluster's main sequence turn-off, allowing us to estimate the cluster's age. We find that M54 is 0.5--1.5 gigayears older than the Galactic globulars M68 and M5. In absolute terms, the age is comparable to the published age estimates of the other member clusters Arp 2 and Terzan 8, but is significantly older than the member cluster Terzan 7. An age estimate of the Sagittarius field population relative to M54 suggests that M54 is \gtrsim 3 Gyr older than the field. We discuss briefly the star formation history of the Sagittarius dwarf galaxy.

NTT Photometry in UBV of the Distant Globular Cluster NGC 1841

We present UBV CCD photometry of 1347 stars in the populous globular cluster NGC 1841 located approximately 15^deg^ from each of the Magellanic Clouds and probably dynamically associated with the LMC. The photometric limit was close to the cluster's main-sequence turnoff. The structure of the blue horizontal branch and possible radial dependence of the horizontal branch luminosity function are discussed. We estimate the cluster metallicity as [Fe/H]= -2.0+/-0.15, the reddening as E(B-V) = 0.17+/-0.03 and the true distance modulus as (m - M)_0_ = 18.26^m^+/- 0.11, somewhat lower than the majority of recent estimates of the LMC distance modulus.

A CCD BVI color-magnitude study of the metal-rich globular cluster NGC 5927

We present for the globular cluster NGC 5927 the rst multicolor CCD photometry that reaches below the cluster's main sequence turno. The turno is located approximately at VTO =2 0 : m 1, (B V ) TO =1 : m 18, (V I)TO =1 : m 30. There is a hint of horizontal branch morphology variations with distance from the cluster center. Using the isochrones of VandenBerg & Bell (1985), we derive the cluster age to be about 15 Gyr.