Structure and mass function of three young open clusters

Using UBVRI CCD data taken from 104-cm Sampurnanand Telescope, ARIES, Nainital, we present the structure, initial mass function and mass segregation of three young age (∼10 Myr) open star clusters: NGC 2129, NGC 1502 and King 12. Based on photometric as well as astrometric criteria, the cluster member stars as well as field stars have been identified. We construct luminosity function which is further used to estimate the mass functions by employing theoretical stellar evolutionary isochrones. The entire cluster region mass function (MF) slopes for NGC 2129, NGC 1502 and King 12 are obtained as −2.55 ± 0.14, −2.73 ± 0.36 and −1.94 ± 0.12 respectively. It is found that changes in the MF slope of King 12 are significantly different compared to NGC 2129 and NGC 1502 from inner region to outer region. The MF slope for King 12 is steeper at larger radii. The dynamical relaxation times for all three clusters are found to be less than age of the clusters. This indicates that all these clusters are dynamically relaxed. We show that for NGC 1502 and King 12, passing off of low mass stars from the inner region of the clusters to the halo occurs during the course of evolution.     

The abundance of brown dwarfs

The amount of mass contained in low-mass objects is investigated anew. Instead of using a mass–luminosity relation to convert a luminosity function to a mass function, I predict the mass–luminosity relation from assumed mass functions and the luminosity functions of Jahreiss & Wielen and Gould, Bahcall & Flynn. Comparison of the resulting mass–luminosity relations with data for binary stars constrains the permissible mass functions. If the mass function is assumed to be a power law, the best-fitting slope lies either side of the critical slope, α =−2, below which the mass in low-mass objects is divergent, depending on the luminosity function adopted. If these power-law mass functions are truncated at 0.001 M_⊙, the contribution to the local density from stars lies between 0.013 and 0.10 M_⊙ pc⁻³ depending on the mass at which the mass function is normalized and the adopted value of α . Recent dynamical estimates of the local mass density rule out stellar mass densities above ∼0.05 M_⊙ pc⁻³. Hence, power laws steeper than α =−2 that extend down to 0.001 M_⊙ are allowed only if one adopts an implausible normalization of the mass function. If the mass function is generalized from a power law to a low-order polynomial in log( M ), the mass in stars with M <0.1 M_⊙ is either negligible or strongly divergent, depending on the order of the polynomial adopted.     

The period–luminosity relation for type II Cepheids in globular clusters

We report the result of our near-infrared observations ( JHK _s) for type II Cepheids (including possible RV Tau stars) in galactic globular clusters. We detected variations of 46 variables in 26 clusters (10 new discoveries in seven clusters) and present their light curves. Their periods range from 1.2 d to over 80 d. They show a well-defined period–luminosity relation at each wavelength. Two type II Cepheids in NGC 6441 also obey the relation if we assume the horizontal branch stars in NGC 6441 are as bright as those in metal-poor globular clusters in spite of the high metallicity of the cluster. This result supports the high luminosity which has been suggested for the RR Lyr variables in this cluster. The period–luminosity relation can be reproduced using the pulsation equation     assuming that all the stars have the same mass. Cluster RR Lyr variables were found to lie on an extrapolation of the period–luminosity relation. These results provide important constraints on the parameters of the variable stars.
Using Two Micron All-Sky Survey (2MASS) data, we show that the type II Cepheids in the Large Magellanic Cloud (LMC) fit our period–luminosity relation within the expected scatter at the shorter periods. However, at long periods (   P > 40  d, i.e. in the RV Tau star range) the LMC field variables are brighter by about one magnitude than those of similar periods in galactic globular clusters. The long-period cluster stars also differ from both these LMC stars and galactic field RV Tau stars in a colour–colour diagram. The reasons for these differences are discussed.     

The astrophysical parameters of Majaess-95 and Majaess-225

Most astrophysical parameters of the two stellar clusters Majaess 95 and Majaess 225 have been investigated here for the first time using the third data release of the Gaia space mission (Gaia-DR3). The membership probabilities of stars were assigned using the pyUPMASK algorithm. The parallaxes offset has been corrected using the zero-point correction code of Lindegren et al. (Lindegren et al. 2021, A&A, 649, A4). Using the clusters' members with membership probabilities >0.50, we calculated the distance to the clusters of 3311 and 3020 (±130) pc, respectively, which agree with the results from our isochrone fitting on the color-magnitude diagram. The ages are found to be 282 (±20) and 12.6 (±2) Ma. Also, the reddening, heliocentric distances, luminosity function, mass function, and total mass were estimated. On studying the dynamic state (relaxation time), we found that Majaess-95 is a relaxed cluster, while Majaess-225 is not relaxed yet.     

Stellar contents and star formation in the young star cluster Be 59

We present   UBV  I _c   CCD photometry of the young open cluster Be 59 with the aim to study the star formation scenario in the cluster. The radial extent of the cluster is found to be ∼10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between   E ( B − V ) ≃ 1.4  to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as  3.7 ± 0.3  . The distance of the cluster is found to be  1.00 ± 0.05 kpc  . Using near-infrared (NIR) colours and slitless spectroscopy, we have identified young stellar objects (YSOs) in the open cluster Be 59 region. The ages of these YSOs range between <1 and ∼2 Myr, whereas the mean age of the massive stars in the cluster region is found to be ∼2 Myr. There is evidence for second-generation star formation outside the boundary of the cluster, which may be triggered by massive stars in the cluster. The slope of the initial mass function, Γ, in the mass range  2.5 < M /M_⊙≤ 28  is found to be  −1.01 ± 0.11  which is shallower than the Salpeter value (−1.35), whereas in the mass range  1.5 < M /M_⊙≤ 2.5  the slope is almost flat. The slope of the K -band luminosity function is estimated as  0.27 ± 0.02  , which is smaller than the average value (∼0.4) reported for young embedded clusters. Approximately 32 per cent of Hα emission stars of Be 59 exhibit NIR excess indicating that inner discs of the T Tauri star (TTS) population have not dissipated. The Midcourse Space Experiment (MSX) and IRAS-HIRES images around the cluster region are also used to study the emission from unidentified infrared bands and to estimate the spatial distribution of optical depth of warm and cold interstellar dust.     

Faint stars in the Ursa Minor dwarf spheroidal galaxy: implications for the low-mass stellar initial mass function at high redshift

The stellar initial mass function at high redshift is an important defining property of the first stellar systems to form and may also play a role in various dark matter problems. We here determine the faint stellar luminosity function in an apparently dark-matter-dominated external galaxy in which the stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a system with a particularly simple stellar population—all of the stars being old and metal-poor—similar to that of a classical halo globular cluster. A direct comparison of the faint luminosity functions of the UMi dSph and of similar metallicity, old globular clusters is equivalent to a comparison of the initial mass functions and is presented here, based on deep HST WFPC2 and STIS imaging data. We find that these luminosity functions are indistinguishable, down to a luminosity corresponding to ∼0.3 M_⊙. Our results show that the low-mass stellar IMF for stars that formed at very high redshift is apparently invariant across environments as diverse as those of an extremely low-surface-brightness, dark-matter-dominated dwarf galaxy and a dark-matter-free, high-density globular cluster within the Milky Way.     

On the initial cluster mass distribution inferred from synthesis models

In this contribution we examine the problem of inferring ages and initial cluster masses from synthesis models at the limit of low-mass clusters (M≤ a few ×10⁴ M_⊙). We show that it is not possible to apply directly synthesis models using standard methods to such clusters, since the basic hypothesis implicit in the models (a fixed proportionality between the number of stars in different evolutionary phases) is not fulfilled due to an insufficient number of stars for a reliable sampling of the stellar initial mass function. The consequence of this incomplete sampling is a non-Gaussian distribution of the mass–luminosity relation for clusters that share the same evolutionary conditions (age, metallicity and stellar initial mass distribution function). We review some tests, that can be performed before the start of the analysis, to estimate if the observed cluster can be analyzed with synthesis models following traditional procedures (like χ ² minimization) or if it is necessary make use of synthesis models in a probabilistic framework. Finally, we show the implications of these results for estimating the low-mass tail in the initial cluster mass distribution function.     

Evolution of Population II stars

The current knowledge of the evolution of Population II stars, as observed in galactic halos and globular clusters, is outlined. The recent theoretical results provided by an improved physical understanding of the stars are reported, with a particular emphasis upon those stellar evolutionary phases which are the keystones of the interpretation of globular cluster stars. Within the up–to–date theoretical scenario, the luminosity of the turn–off stars and horizontal branch stars, which are of fundamental importance for distance and age determinations, turns out to be fainter and brighter, respectively, in comparison with previous theoretical computations. The predicted absolute magnitude of RR Lyrae stars is consistent with the bright values suggested from the long distance scale (Sandage 1993), but the slope of the relation between and metallicity ([Fe/H]) agrees with that suggested from the short distance scale (see, e.g., Carney et al. 1992, Clementini et al. 1995), at least with [Fe/H]-1.3. As to the globular cluster ages, the new computations provide younger ages ( 10 Gyr–13 Gyr), weakening the conflict with current cosmological estimates. The recent results derived from the fitting of HIPPARCOS parallaxes of field subdwarfs to the fiducial main sequence of globular clusters are consistent with the up–to–date theoretical models. However, the direct determinations of , which are based on the HIPPARCOS proper motions and trigonometric parallaxes of field RR Lyrae stars, give fainter absolute magnitudes than those obtained from the improved evolutionary models. This seems to suggest that the problem of the globular cluster distance and age is still open. Received 6 April 1998     

An SZ temperature decrement–X-ray luminosity relation for galaxy clusters

We present the observed relation between Δ T _SZ, the cosmic microwave background (CMB) temperature decrement due to the Sunyaev–Zeldovich (SZ) effect, and L , the X-ray luminosity of galaxy clusters. We discuss this relation in terms of the cluster properties, and show that the slope of the observed Δ T _SZ– L relation is in agreement with both the L – T _e relation based on numerical simulations and X-ray emission observations, and the M _gas– L relation based on observation. The slope of the Δ T _SZ– L relation is also consistent with the M _tot– L relation, where M _tot is the cluster total mass based on gravitational lensing observations. This agreement may be taken to imply a constant gas mass fraction within galaxy clusters, however, there are large uncertainties, dominated by observational errors, associated with these relations. Using the Δ T _SZ– L relation and the cluster X-ray luminosity function, we evaluate the local cluster contribution to arcmin-scale cosmic microwave background anisotropies. The Compton distortion y -parameter produced by galaxy clusters through the SZ effect is roughly two orders of magnitude lower than the current upper limit based on FIRAS observations.     

疏散星团的质量分层

对疏散星团质量分层的有关问题做了简要的评述,包括空间质量分层和速度质量分层的表现形式和探测途径,质量分层形成机制的研究现状.最后概要介绍了2MASS测光资料对探讨疏散星团质量分层效应的作用.     

Untersuchungen über den Aufbau offener Sternhaufen

From strip counts in 20 open star clusters the characteristic parameters of their structure are derived. At the boundaries of the clusters the relative potential amounts to 0.32 on the average. The effective mean distance of the stars from the centre of the cluster introduced by SCHWARZSCHILD corresponds nearly to the radius of the cluster. The counts in two clusters allow to continue the cumulative mass functions up to Mv ≈ 8, and their exponential convergence is used for the determination of the cluster masses. The same has been done for the Pleiades and seven clusters according to the counts of VAN DEN BERGH and SHER . The mean velocities and the mean periods of revolution T of the cluster stars are of the order 0.90 km/s and 12 · 10⁶ years respectively. Between the latter and the mean (mass) density the relation T ≈≈ 20 · 10⁶ is valid. During 1 · 10⁹ years the mean cluster loses 50% of its stars by relaxation.     

Effects of rotation on the colours and line indices of stars 5. The ZRMS and the ZRZAMS

The observeduvby and H_α indices of member stars of the Hyades and Praesepe clusters have been analysed in detail for rotation effects. The Alpha Persei, Pleiades and the Centaurus subgroup of the Scorpio—Centaurus association have been reanalysed using the observed indices instead of the extinction-corrected indices used earlier. The observed rotation effects from the analysis of these cluster data are found to be in excellent agreement with the theoretical predictions of Collins & Sonneborn (1977). We have also analysed the α,c and (u − b) values of the member stars of NGC 1976, 2264, 2287, 2422, 4755,1C 2391, IC 2602 and IC 4665 for rotation effects. The results are found to be consistent with the theoretical predictions. The observed slopes of the rotation effects were used to determine the zero rotation main sequence values of the intermediate band photometric indices for selected clusters. We also corrected the observed indices for each star in each cluster using the theoretical predictions of Collins and Sonneborn and derived the ZRMS values for each cluster. The agreement between the two determinations is found to be good. The various ZRMS curves were utilised to derive the ZRZAMS values. A preliminary calibration of the absolute visual magnitudes as a function of α valid for ZRZAMS has also been derived. The ZRMS values of the intermediate band photometric indices for different clusters and the ZRZAMS values are listed as a function of α. on leave from Assumption College, Changanacherry, Kerala.     

Neutron stars in globular clusters: Formation and observational manifestations

Population synthesis is used to model the number of neutron stars in globular clusters that are observed as low-mass X-ray sources and millisecond radio pulsars. The dynamical interactions between binary and single stars in a cluster are assumed to take place only with a continuously replenished “background” of single stars whose properties keep track of the variations in parameters of the cluster as a whole and the evolution of single stars. We use the hypothesis that the neutron stars forming in binary systems from components with initial masses of ～8–12 M _⊙ during the collapse of degenerate O-Ne-Mg cores through electron captures do not acquire a high space velocity. The remaining neutron stars (from single stars with masses >8 M _⊙ or from binary components with masses >12 M _⊙) are assumed to be born with high space velocities. According to this hypothesis, a sizeable fraction of the forming neutron stars remain in globular clusters (about 1000 stars in a cluster with a mass of 5 × 10⁵ M _⊙). The number of millisecond radio pulsars forming in such a cluster in the case of accretion-driven spinup in binary systems is found to be ～10, in agreement with observations. Our modeling also reproduces the observed shape of the X-ray luminosity function for accreting neutron stars in binary systems with normal and degenerate components and the distribution of spin periods for millisecond pulsars.     

疏散星团中的耀星及其演化学意义

耀星是红矮星早期演化的一个必经阶段，这个阶段的持续时间取决于恒星的质量。在年轻和比较年轻的疏散星闭中，都含有大量耀星，它们的光度分布是疏散星闭年龄的标志。在昴生闭中已发现大量耀星，对它们的自行，测光和分光资料进行深入的统计研究，对于解决疏散星闭及矮星起源和演化问题，有着极其重要的价值。     

Pleiades luminosity function: fine structure and new Pre‐MS models

In order to study a model‐dependence of the results achieved in the previous investigations of the Pleiades luminosity function using D'Antona and Mazzitelli (1994) evolutionary tracks, we repeated the computations with the new track system by D'Antona and Mazzitelli (1997). The following main conclusions can be drawn: the new models agree better with observations; the helium abundance needed to fit the Hipparcos distance modulus is reduced to a more reasonable value of Y = 0.31; the cluster age becomes slightly higher and the slope of the initial mass function somewhat lower. The conclusions on the fine structure of the luminosity function do not change significantly due to the application of the new models.     

Period–luminosity relations for type II Cepheids and their application

JHK _s magnitudes corrected to mean intensity are estimated for Large Magellanic Cloud (LMC) type II Cepheids in the OGLE-III survey the third phase of the Optical Gravitational Lensing Experiment (OGLE). Period–luminosity (PL) relations are derived in JHK _s as well as in a reddening-free VI parameter. Within the uncertainties, the BL Her stars  ( P < 4 d)  and the W Vir stars (   P = 4  to 20 d) are colinear in these PL relations. The slopes of the infrared relations agree with those found previously for type II Cepheids in globular clusters within the uncertainties. Using the pulsation parallaxes of V553 Cen and SW Tau, the data lead to an LMC modulus uncorrected for any metallicity effects of  18.46 ± 0.10  mag. The type II Cepheids in the second-parameter globular cluster, NGC 6441, show a PL( VI ) relation of the same slope as that in the LMC, and this leads to a cluster distance modulus of  15.46 ± 0.11  mag, confirming the hypothesis that the RR Lyrae variables in this cluster are overluminous for their metallicity. It is suggested that the Galactic variable κ Pavonis is a member of the peculiar W Vir class found by the OGLE-III group in the LMC. Low-resolution spectra of OGLE-III type II Cepheids with   P > 20  d (RV Tau stars) show that a high proportion have TiO bands; only one has been found showing C₂. The LMC RV Tau stars, as a group, are not colinear with the shorter period type II Cepheids in the infrared PL relations in marked contrast to such stars in globular clusters. Other differences between LMC, globular cluster and Galactic field type II Cepheids are noted in period distribution and infrared colours.     

星系团的分层效应

对星系团各类分层效应的有关问题做了概要的评述,包括成员星系在位置空间和(或)速度空间中的形态分层、光度(质量)分层和元素丰度分层的表现形式和探测途径,分层效应可能的形成机制及其对星系和星系团的结构和演化的影响。     

The SFR and IMF of the galactic disk

There is a long term dynamical heating of stellar populations with age observed in the age – velocity dispersion – relation (AVR). This effect allows a determination of the star formation history SFR(t) from local kinematical data of main sequence stars. Using a self-consistent disk model for the vertical structure of the disk, we find from the kinematics of the stars in the solar neighbourhood that the SFR shows a moderate star burst about 10 Gyr ago followed by a continuous decline to the present day value consistent with the observed number of OB stars. The gravitational potential of the gas component and of the Dark Matter Halo is included and the effect of chemical enrichment, finite lifetime of the stars and mass loss of the stellar component are taken into account. The scale heights for main sequence stars together with the SFR is then used to determine constistently the IMF from the observed local luminosity function. The main new result is that the power law break in the present day mass function (PDMF) around 1 M _⊙ is entirely due to evolutionary effects of the disk and does not appear in the IMF. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dwarf LSB galaxies and their environment: The Virgo Cluster, the Ursa Major Cluster, isolated galaxies and voids

The widely varying dwarf galaxy counts in different environments provide a strong challenge to standard hierarchical clustering models. The luminosity function is not universal, but seems to be strongly dependent upon environment. In this paper we describe an automated procedure for detecting and measuring very Low Surface Brightness (LSB) features in deep CCD data. We apply this procedure to large area CCD survey fields of the Virgo cluster and find 105 dwarf LSB galaxies previously uncatalogued over an area of ∼14 sq deg. We show that there are many more faint (14≤M_B≤–10) LSB galaxies than would be predicted from extrapolation of the Virgo cluster catalogue luminosity function. Over our limited range of measurement the faint end slope of the luminosity function becomesα=–1.6. Although these galaxies contribute a small fraction of the total stellar light of the cluster, they may contribute significantly to the mass in galaxies if they have large mass-to-light ratios similar to those recently measured for Local Group dwarf galaxies. By a comparison with different environments, the Virgo Cluster seems to be highly `dwarf rich' if considering the dwarf to giant ratio or it's luminosity function faint end slope. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Global, Local and Cluster Galaxy Luminosity Function

We review selected measurements of the galaxy luminosity function including the field, the local group, the local sphere, nearby clusters (Virgo, Coma and Fornax) and clusters in general. We conclude that, excluding the super-luminous cD and D galaxies, the overall cluster luminosity function is fully consistent with the field luminosity function over the magnitude range in common (–22 ≤ M _B –5log _{h 0.68} ≤ –17). We find that only in the core regions of clusters (r ≤ 300 kpc) does the overall form of the luminosity function show significant variation. However when the luminosity function is subdivided by spectral type some further variations are seen. We argue that these results imply: substantial late infall, efficient star-formation suppression, and the confinement of mass-changing evolutionary processes to the core regions only. This revised version was published online in July 2006 with corrections to the Cover Date.