A CENSUS OF YOUNG STARS AND BROWN DWARFS IN IC 348 AND NGC 1333*

The selection of known members of star clusters as BRITE targets is highly preferable. The given apparent visual magnitude limit for the targets, can be immediately transformed into a distance limit of about 750 pc around the Sun depending on the absolute magnitude. For a spectral type of A0, we are, for example, limited to about 65 pc only. With the help of WEBDA (http://www.univie.ac.at/webda), we have found 60 possible candidates which are true members of star clusters (including associations). The membership offers the opportunity to use the results from a detailed cluster analysis “for free”. This includes the determination of the age, the distance, the reddening, the overall metallicity and thus the mass with a high accuracy. These are very valuable starting parameters for any detailed astrophysical study of an individual object. Star clusters Why are they unique? Open clusters and associations (denoted as “star clusters” in the following) are physically related groups of stars held together by mutual gravitational attraction. Therefore, they populate a limited region of space, which is typically much smaller than their distance from the Sun, so that the members are all approximately at the same distance. They are believed to originate from large cosmic gas and dust clouds (diffuse nebulae) in the Milky Way, and to continue to orbit the galaxy through the disk. In many clouds visible as bright diffuse nebulae, star formation still takes place, so that we can observe the birth of new young star clusters. This process of formation takes only a considerably short time (a few Myrs) compared to the lifetime of the cluster, so that all member stars are of similar age. Also, as all the stars in a cluster formed from the same diffuse nebula, they all have similar initial chemical composition. Hence, star clusters are of great interest for scientists: • The cluster members are all at about the same distance from the Sun. 176 Cluster and Association Members • They have the same age within approximately a few million years compared to the cluster age (up to a few billion years). • The chemical composition of cluster members is quite homogeneous. Open cluster metallicities range from about −1.0 to +0.6 dex compared to the Sun. The determination of distance, age and metallicity is, in general, not straightforward for galactic field stars. Star clusters on the other hand represent samples of stars of constant age and homogeneous chemical composition, suited for the study of processes linked to stellar structure and evolution. They allow to fix lines or loci in several important astrophysical diagrams such as the color-magnitude diagram (CMD), or the Hertzsprung-Russell diagram (HRD). Comparing the “standard” HRD, derived from nearby stars with sufficiently well known distances, or the theory of stellar evolution with the measured CMD of star clusters, provides a considerably good method to determine the distance of star clusters. Comparing their HRDs with stellar theory provides a reasonable way to estimate the age of star clusters. BRITE target stars as members of star clusters For a given apparent magnitude limit of targets stars (V =4mag) we can immediately calculate the distance limit for different absolute magnitudes. At maximum (not taking reddening into account) MV =−6mag (early O) we can reach 1000pc whereas the distance significantly decreases to 65 pc for a star with MV =+0mag (A0). With the help of WEBDA (http://www.univie.ac.at/webda), a highly efficient and successful database for the study of star clusters in the Milky Way and the Small Magellanic Cloud, it is possible to extract and retrieve all needed information. The database includes about 3.4 million individual measurements in most photometric systems in which cluster stars have been observed, spectroscopic observations, astrometric data, various kinds of supplementary information, and an extensive bibliography. Taking a distance limit of 750 pc, we find in WEBDA about 220 star clusters which are closer than this value. Including the members of associations, we find about 60 potential BRITE target stars which are members of a star cluster. Why to observe members of star clusters? From kinematic (proper motions as well as radial velocities) and photometric studies we are able to establish the membership of a star to a corresponding

Results from a combination of new spectroscopic and photometric observations in the lower main-sequence and pre-main sequence of the open cluster alpha Persei are presented. New echelle spectroscopy has provided radial and rotational velocity information for thirteen candidate members, three of which are nonmembers based on radial velocity, absence of a Li 6707A feature, and absence of H-alpha emission. A set of revised rotational velocity estimates for several slowly rotating candidates identified earlier is given, yielding rotational velocities as low as 7 km/s for two apparent cluster members. VRI photometry for several pre-main sequence members is given; the new (V,V-I(sub K)) photometry yields a more clearly defined pre-main sequence. A list of approximately 43 new faint candidate members based on the (V,V-I(sub K)) CCD photometry is presented in an effort to identify additional cluster members at very low masses. Low-dispersion spectra obtained for several of these candidates provide in some cases supporting evidence for cluster membership. The single brown dwarf candidate in this cluster is for the first time placed in a color-magnitude diagram with other cluster members, providing a better means for establishing its true status. Stars from among the list of new photometric candidates may provide the means for establishing a sequence of cluster members down to very faint magnitudes (V approximately 21) and consequently very low masses. New coordinate determinations for previous candidate members and finding charts for the new photometric candidates are provided in appendices.

view Abstract Citations (308) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectroscopy of Galaxies in Distant Clusters. IV. A Catalog of Photometry and Spectroscopy for Galaxies in Seven Clusters with 0.35 < Z < 0.55 Dressler, Alan ; Gunn, James E. Abstract We present photometry and spectroscopy for seven deep fields containing distant clusters of galaxies with 0.35 <z <0.55. Positions and photometric parameters, including r-magnitudes, g - r and r - i colors, surface brightnesses, and photometric profile types are given for ~ 2000 galaxies. We have obtained low-resolution spectroscopy from which redshifts have been determined for 289 objects, of which 190 are cluster members. We classify these according to dominant spectral features and plot examples in each cluster. Color-magnitude and color-color diagrams are formed which show trends in the cluster populations, and maps are made of the cluster field using the color-color relations to increase the contrast of cluster over field. Galaxies with spectra typical of old stellar populations cluster most strongly, with active galaxies, those with recent or ongoing star formation, or an active nucleus, distributed more diffusely. The g - r color is well correlated with active star formation as judged from spectral features. Some other weaker correlations with surface brightness and profile types are briefly discussed. Publication: The Astrophysical Journal Supplement Series Pub Date: January 1992 DOI: 10.1086/191620 Bibcode: 1992ApJS...78....1D Keywords: Astronomical Catalogs; Astronomical Photometry; Astronomical Spectroscopy; Color-Color Diagram; Galactic Clusters; Red Shift; Active Galaxies; Astrometry; Color-Magnitude Diagram; Galactic Evolution; Star Formation; Astronomy; GALAXIES: CLUSTERING; GALAXIES: DISTANCES AND REDSHIFTS; GALAXIES: PHOTOMETRY full text sources ADS | data products SIMBAD (1977) NED (1942) CDS (2) Related Materials (4) Catalog: 1993yCat..20780001D Part 1: 1982ApJ...263..533D Part 2: 1983ApJ...270....7D Part 3: 1985ApJ...294...70D

The very young open cluster (OC) NGC2244 in the Rosette Nebula was studied with fieldstar- decontaminated Two-Micron All-Sky Survey (2MASS) photometry, which shows the main-sequence (MS) stars and an abundant pre-MS (PMS) population. Fundamental and structural parameters were derived with colour–magnitude diagrams (CMDs), stellar radial density profiles (RDPs) and mass functions (MFs). Most previous studies centred NGC2244 close to the bright K0V star 12Monocerotis, which is not a cluster member. Instead, the nearinfrared RDP indicates a pronounced core near the O5 star HD46150.We derive an age within 1–6 Myr, an absorption AV =1.7±0.2, a distance from the Sun dʘ =1.6±0.2 kpc (≈1.5 kpc outside the solar circle), an MF slope χ = 0.91 ± 0.13 and a total (MS+PMS) stellar mass of ~625Mʘ. Its RDP is characterized by the core and cluster radii Rc ≈5.6 arcmin (≈2.6 pc) and RRDP ≈ 10 arcmin (≈4.7 pc), respectively. Departure from dynamical equilibrium is suggested by the abnormally large core radius and the marked central stellar excess. We also investigate the elusive neighbouring OC NGC2239, which is low mass (mMS+PMS ≈ 301Mʘ), young (5 ± 4Myr) rather absorbed (AV = 3.4 ± 0.2), and located in the background of NGC2244 at dʘ = 3.9 ± 0.4 kpc. Its RDP follows a King-like function of Rc ≈ 0.5 arcmin ≈ 0.5 pc and RRDP ≈ 5.0 arcmin ≈ 5.6 pc. The MF slope, χ = 1.24 ± 0.06, is essentially Salpeter’s initial mass function. NGC2244 is probably doomed to dissolution in a few 10 7 yr. Widefield extractions and field-star decontamination increase the stellar statistics and enhance both CMDs and RDPs, which is essential for faint and bright star clusters.

We discuss the star formation history of the SMC region NGC 346 based on Hubble Space Telescope images. The region contains both field stars and cluster members. Using a classical synthetic CMD procedure applied to the field around NGC 346 we find that there the star formation pace has been rising from a quite low rate 13 Gyr ago to \approx 1.4 \times 10^{-8} Mo yr^{-1}pc^{-2} in the last 100 Myr. This value is significantly higher than in other star forming regions of the SMC. For NGC 346 itself, we compare theoretical and observed Color-Magnitude Diagrams (CMDs) of several stellar sub-clusters identified in the region, and we derive their basic evolution parameters. We find that NGC 346 experienced different star formation regimes, including a dominant and focused "high density mode", with the sub-clusters hosting both pre-main sequence (PMS) and upper main sequence (UMS) stars, and a diffuse "low density mode", as indicated by the presence of low-mass PMS sub-clusters. Quantitatively, the star formation in the oldest sub-clusters started about 6 Myr ago with remarkable synchronization, it continued at high rate (up to 2 \times 10^{-5} Mo yr^{-1} pc^{-2}) for about 3 Myr and is now progressing at a lower rate. Interestingly, sub-clusters mainly composed by low mass PMS stars seem to experience now the first episode of star formation, following multi-seeded spatial patterns instead of resulting from a coherent trigger. Two speculative scenarios are put forth to explain the deficiency of UMS stars: the first invokes under-threshold conditions of the parent gas; the second speculates that the initial mass function (IMF) is a function of time, with the youngest sub-clusters not having had sufficient time to form more massive stars.

At a redshift of 0.18, Abell 1689 is so far the most distant cluster of galaxies for which substantial mid–infrared (MIR) data have been published. Its mapping with the ISOCAM camera onboard the ISO satellite allowed the detection of 30 cluster members at 6.75 (LW 2 filter) and 16 cluster members at 15 (LW 3 filter) within a clustercentric radius of 0.5 Mpc [CITE][ Paper I]Fadda00b. We present here the follow–up optical photometric and spectroscopic observations which were used to study the individual properties of the galaxy members of A1689. We confirm the high fraction of blue galaxies initially reported in this rich cluster by [CITE], that was challenged by some subsequent studies. We discuss the spectral and morphological properties of all cluster members in our spectroscopic sample, and of the MIR–detected galaxies in particular. Sources with a low [ 15 ] /[ 6.75 ] flux ratio typically consist of luminous passive early–type galaxies while those with a high MIR color index are mainly luminous, blue, emission–line, morphologically disturbed spirals, i.e. the star–forming galaxies usually associated with the “Butcher–Oemler” effect. On the other hand, at least 30% of the 15 sources have optical counterparts showing no evidence for current star–formation activity, while their 15 emission is most likely due to obscured star formation. We argue that the LW 3 luminosity measured in the cluster members is a reliable tracer of the total infrared luminosity which in A1689 galaxies peaks at . We derive from a star–formation rate free of dust extinction, SFR (IR), which we compare with that determined in the optical from the flux of the emission line, SFR (opt). The highest total star formation rates (11 ) and dust extinction are measured in those galaxies exhibiting in their optical spectrum a signature of a dusty starburst. In contrast, none of the galaxies with post-starburst optical spectra has been detected by ISOCAM down to a 15 flux limit corresponding to 1.4 . We find a median SFR (IR) of the LW 3–detected galaxies of 2 that is ten times higher than the median SFR (opt) of the – detected galaxies. The ratio SFR (IR)/SFR (opt) is in fact very high, ranging between 10 and 100 for LW 3–detected galaxies with emission. We conclude that a major part, at least 90%, of the star formation activity taking place in Abell 1689 is hidden. Whether the high extinction measured in the star-forming cluster members results from the cluster environment itself or reflects a comparable extinction in the coeval field is still unclear.

Young star clusters consisting of massive stars are the ideal sites to study the star formation processes and influence of massive stars on the subsequent star formation. NGC 1893 is a young star cluster associated with the Hii region Sh2-236. It contains about five ‘O’-type stars and several early ‘B’-type stars. It is located at a moderate distance of $$\sim $$ 3.25 kpc and has a reddening, $$E(B-V) \sim 0.4$$ mag. To characterize the young low-mass stellar population in the central portion of the cluster, we carried out deep VI band observations of the region using the $$4\mathrm{K} \times 4$$ K CCD IMAGER mounted on the 3.6-m Devasthal Optical Telescope. Our analysis shows that the present data are deep enough to detect stars below $$V\sim 24$$ mag. We found optical counterparts of $$\sim $$ 220 candidates, including young stars and unclassified cluster members from Caramazza et al. (2008). We estimated the membership probabilities of the Gaia sources (mostly bright stars with $$G<19$$ mag) located within the cluster radius using the Gaia EDR3. Toward the fainter end, we used the optical color-magnitude diagram (CMD) to select the cluster members from a sample of young stars. The locations of young stars on the CMD show that a majority of them are low-mass stars with age <10 Myr. The unclassified candidates and X-ray sources from Caramazza et al. (2012) are also found to be young low-mass stars. In total, we identified $$\sim $$ 425 young stars with age <10 Myr, and 110 of these are new. Most of these stars appear as kinematic members of the cluster. By examining the CMD for the stars in the cluster region, we suggest that the cluster has insignificant contamination due to field stars in the pre-main-sequence zone of the CMD. The slope of the mass function in the mass range $$0.2\le M/M_\odot \le 2.5$$ is found to be $$\Gamma = -\,1.43 \pm 0.15$$ , consistent with those of other star-forming complexes. The spatial distribution of the young stars as a function of mass suggests that toward the cluster center, most of the stars are massive.

Aims. We conducted a search for brown dwarfs (BDs) and very low mass (VLM) stars in the 625 Myr-old Hyades cluster in order to derive the cluster's mass function across the stellar-substellar boundary. Methods. We performed a deep (I=23, z=22.5) photometric survey over 16 deg^2 around the cluster center and followed up with K-band photometry to measure the proper motion of candidate members and with optical and near-IR spectroscopy of probable BD and VLM members. Results. We report the discovery of the first 2 BDs in the Hyades cluster. The 2 objects have a spectral type early-T and their optical and near-IR photometry as well as their proper motion are consistent with them being cluster members. According to models, their mass is 50 Jupiter masses at an age of 625 Myr. We also report the discovery of 3 new very low mass stellar members of the cluster and confirm the membership of 16 others. We combine these results with a list of previously known cluster members to build the present-day mass function (PDMF) of the Hyades cluster from 50 Jupiter masses to 3 Mסּ. We find the Hyades PDMF to be strongly deficient in very low mass objects and BDs compared to the IMF of younger open clusters such as the Pleiades. We interpret this deficiency as the result of dynamical evolution over the past few 100 Myr, i.e., the preferential evaporation of low mass cluster members due to weak gravitational encounters. Conclusions. We thus estimate that the Hyades cluster currently hosts about 10-15 BDs, while its initial substellar population may have amounted to up to 150-200 members.

A method for studying the full star formation history (SFH) of nearby galaxies,as a function of time, has been developped and implemented. For the first time, the full SFH of a nearby dwarf irregular galaxy (DIG) has been retrieved from the photometry of its resolved stars. NGC~6822 has been observed in UBVRI at the 2.5m INT telescope, at the Roque de los Muchachos Observatory and photometry of the resolved stellar content has been obtained [1]. The VRI frames are very deep and contain about 18000 well measured stars. Most of them are old stars populating the red-giant branch (RGB) and the asymptotic giant branch (AGB) phase, which has been well sampled. Using the new BVRI magnitudes for the Cepheids, the first self-consistent BVRI multi-wavelength Cepheid distance and reddening has been derived for NGC~6822 [1]. The values obtained are (m-M)0=23.49± 0.08 and E(B-V)=0.24±0.03, respectively. The comparison of the [(V-I),I] and [(V-R),V] colour-magnitude (CM) diagrams with theoretical isochrones allowed the qualitative understanding of the stellar populations present in NGC~6822 ([1] and [4]). Two structures, the red-tangle and the red-tail ({tocho} and {colita}, as we originally called them) have been identified as the locus of the old and intermediate-age stars (age >1 Gyr), and therefore, as the structures to be studied to retrieve information of the old and intermediate-age SFH. Since stars older and younger than 1 Gyr are situated in clearly distinct areas of the CM diagram, and because of the different time-resolution on which these populations can be studied, the old and intermediate-age SFH and the young SFH have been derived separately ([2] and [3]). Nevertheless, the tool has been in both cases the same: the comparison of the observed [(V-I),I] CM diagram with a set of model CM diagrams, computed assuming different evolutionary scenarios. This is the main core of this thesis. The comparison has been performed through a number of indicators relative to the stars' distribution over the CM diagram. The model CM diagrams have been computed in collaboration with the Padova Group of stellar evolution. The reliability of the model CM diagrams computed is a result of three main components : i) a reliable set of stellar evolutionary models covering the needed range of ages and metallicities, ii) the interpolation of the stellar evolutionary tracks of fixed mass and metallicity to determine the precise, {smooth} distribution of stars of any age, mass, and metallicity on the CM diagram, and iii) a realistic simulation of the observational effects on the synthetic CM diagram. Significant constraints have been put on the SFH of NGC~6822. The main conclusions reached are the following: --Star formation beginning at Ti≤ 6 Gyr can be ruled out. Star formation beginning at Ti≃ 6 Gyr would only be possible if the galaxy started its early star formation from gas already enriched, or if a prompt metal enrichment occurred in the galaxy. --NGC~6822 has most likely begun forming stars at a very early epoch (about 15-12 Gyr ago), from low metallicity gas. --An SFR close to constant or declining in the last few Gyr of the galaxy's lifetime seems best to reproduce the observations. Short-time and small-amplitude fluctuations of the SFR may have occurred. -- An overall enhancement of the star formation activity has occurred in the last 100-200 Myr. The strength of this enhancement has been somewhat different from one region of the galaxy to another. Although applied to the case of a DIG, the fundamentals of the method developed in this thesis can be applied to galaxies of any morphological type, provided that deep enough photometry is available. This can be achieved, from the ground, for all Local Group galaxies, and using Hubble Space Telescope, for galaxies within 4--5 Mp.

view Abstract Citations (49) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Photometric and Polarimetric Observations of the Nearby Strongly Reddened Open Cluster Stock 2 Krzeminski, W. ; Serkowski, K. Abstract Photoelectric UB V observations are given for 172 stars in the region of the cluster Stock 2 (iTi = 133 , = - 1?7) along with polarimetric observations for 127 stars. The distance modulus corrected for ab- sorption is 7 50 (r = 316 pc), and the mean reddening = 0 375 mag. The bluest cluster members are of spectral type B9; the color-magnitude diagram is similar to that of M11. The cluster membership of four red giants with -1 <Mv < +1 is confirmed by polarimetric and proper-motion data. The slope of the reddening path ! varies from 0 74 to 0 82 for the main-sequence stars with (B - V)8 between +004 and +032. The UB VRI observations of 10 cluster members indicate that in this spectral region the wavelength dependence of interstellar extinction is similar to that observed in Cygnus and Cepheus. The variable-extinction method applied to the cluster members with +020< (B - V)8 < +040 gives the ratio of total to selective interstellar extinction R = 3 4 + 0 1 only slightly larger than that for h and x Persei; the latter clusters are situated only 2?4 south of Stock 2 but are several times farther away The mean amount of polarization for the cluster members is 0.050 mag; the polarization is not correlated with reddening. The microscale 0 3 pc is found for the fluctuations in the polarizing medium. Publication: The Astrophysical Journal Pub Date: March 1967 DOI: 10.1086/149089 Bibcode: 1967ApJ...147..988K full text sources ADS | data products SIMBAD (173) GCPD (1)

In this work, we studied the variable stars in the open cluster NGC 1912 based on the photometric observations and Gaia DR2 data. More than 3600 CCD frames in B, V, R filters were reduced, and we obtained the light curves that span about 63 hours. By analyzing these light curves, we detected 24 variable stars, including 16 periodic variable stars, seven eclipsing binaries and one star whose type is unclear. Among these 24 variable stars, 11 are newly discovered, which are classified as six γ Doradus stars, one δ Scuti star, three detached binaries and one contact binary. We also confirmed 13 previously known variable stars. Based on cluster members identified by Cantat-Gaudin et al. (2018), we inferred cluster memberships for these detected variable stars. Using Gaia DR2 data, we plotted a new color-magnitude diagram for NGC 1912, and showed the nature of variable cluster members in kinematical properties and heliocentric distance. Among the 24 variable stars, seven variables are probable cluster members, which show homogeneity in kinematic characters and space position with the established cluster members. Four of the seven variable cluster members are the previously discovered stars, consisting of two γ Dor stars and two δ Sct stars. The remaining three variable cluster members, which are all γ Dor stars, are firstly detected in this work. The main physical parameters of these variable cluster members estimated from the color-magnitude diagram are log(age/yr) = 8.75, [Fe/H] = –0.1, m – M = 10.03 mag, and E(B – V) = 0.307.

In this work, we studied the variable stars in the open cluster NGC 1912\nbased on the photometric observations and $Gaia$ DR2 data. More than 3600 CCD\nframes in B, V, R filters were reduced, and we obtained the light curves that\nspan about 63 hours. By analyzing these light curves, we detected 24 variable\nstars, including 16 periodic variable stars, seven eclipsing binaries and one\nstar whose type is unclear. Among these 24 variable stars, 11 are\nnewly-discovered, which are classified as six gamma Doradus stars, one delta\nScuti star, three detached binaries and one contact binaries. We also confirmed\n13 previously known variable stars. Based on cluster members identified by\nCantat-Gaudin et al. (2018), we inferred cluster memberships for these detected\nvariable stars. Using Gaia DR2 data, we plotted a new color-magnitude diagram\nfor NGC 1912, and showed the nature of variable cluster members in kinematical\nproperties and heliocentric distance. Among the 24 variable stars, seven\nvariables are probable cluster members, which show homogeneity in kinematic\ncharacters and space position with the established cluster members. Four of the\nseven variable cluster members are the previously discovered stars, consisting\nof two gamma Dor stars and two delta Sct stars. The remaining three variable\ncluster members, which are all gamma Dor stars, are firstly detected in this\nwork. The main physical parameters of these variable cluster members estimated\nfrom the color-magnitude diagram are log(age/yr)=8.75, [Fe/H]=-0.1, m-M=10.03\nmag, and E(B-V)=0.307.\n

We report on the results of a V- and i-band time-series photometric survey of M34 (NGC 1039) using the Wide Field Camera (WFC) on the Isaac Newton Telescope (INT), achieving better than 1 per cent precision per data point for 13 ≲i≲ 17. Candidate cluster members were selected from a V versus V−I colour–magnitude diagram over 14 < V < 24 (0.12 ≲M/M⊙≲ 1.0), finding 714 candidates, of which we expect ∼400 to be real cluster members (taking into account contamination from the field). The mass function was computed, and found to be consistent with a lognormal distribution in dN/d log M. Searching for periodic variable objects in the candidate members gave 105 detections over the mass range 0.25 < M/M⊙ < 1.0. The distribution of rotation periods for 0.4 < M/M⊙ < 1.0 was found to peak at ∼7 d, with a tail of fast rotators down to periods of ∼0.8 d. For 0.25 < M/M⊙ < 0.4 we found a peak at short periods, with a lack of slow rotators (e.g. P≳ 5 d), consistent with the work of other authors at very low masses. Our results are interpreted in the context of previous work, finding that we reproduce the same general features in the rotational period distributions. A number of rapid rotators were found with velocities ∼ a factor of 2 lower than in the Pleiades, consistent with models of angular momentum evolution assuming solid body rotation without needing to invoke core–envelope decoupling.

Context. We study the stellar population of the very young cluster NGC 2362, using a deep Chandra ACIS-I X-ray observation. This cluster, only 5 Myr old, has already cleared most of its inter- and circumstellar dust, and with its small and uniform reddening offers a unique opportunity of studying its pre-main-sequence stellar population with minimal disturbance from a dense interstellar medium. Aims. Our main purposes are to select cluster members down to low masses and to study their properties as a population (spatial properties, initial mass function, and coronal properties). Methods. We compare existing deep optical photometry and H α data with new X-ray data. We use combined optical and X-ray criteria to select cluster members. Results. We detect 387 X-ray sources down to $\log L_{\rm X} = 29.0$ (erg/s), and identify most of them (308) with star-like objects. The majority (88%) of optically identified X-ray sources are found to be very good candidate low-mass pre-main-sequence stars, with minimal field-object contamination. This increases the known cluster census by a substantial amount at low masses, with respect to previous optical/IR studies. The fraction of stars with active accretion is found to be in the range 5–9%. We find a significantly wider spatial distribution for low-mass stars than for massive stars (mass segregation). We find only a small spread around the low-mass cluster sequence in the HR diagram, indicating that star formation lasted only about 1–2 Myr. We have derived the cluster initial mass function, which appears to flatten (on the low-mass side) at higher masses with respect to other very young clusters. The quiescent X-ray emission of low-mass cluster stars is found to be rather strictly correlated with the stellar bolometric luminosity: the small spread in this correlation puts an upper bound on the amplitude of X-ray variability on time scales longer than one day (e.g., activity cycles) in such young coronal sources. We find significant X-ray spectral differences between low-mass stars brighter and fainter than $\log L_{\rm X} \sim 30.3$ (erg/s), respectively, with X-ray brighter stars showing hotter components ($kT \sim 2$ keV), absent in fainter stars.

Context. We present the results of a near-infrared survey of the young stellar cluster associated with the IRAS 16571-4029 source. Aims. The main purpose of this survey is to study the cluster members and find the ionizing sources of the associated HII region. Methods. The stellar population was studied by using color–color and color–magnitude diagrams, as well as by analysing the spectral energy distributions in the near- and mid-infrared wavelengths. The extended emission was studied by the construction of contour diagrams, which were compared with near- and mid-infrared images. We computed the corresponding number of Lyman continuum photons (using the integrated Br γ flux density) and compared it with that obtained from the 5 GHz flux density to derive a mean visual extinction. Results. NIR observations in the direction of RCW116B reveal the presence of a young cluster of massive stars coincident with the IRAS 16571-4029 source. These observations, together with published radio data, MSX, and Spitzer images were used to determine some of the physical parameters of the region. We found 102 cluster member candidates in an area of about 3 $\times$ 3 square arcmin, the majority of them showing excess emission in the NIR. We found that IRAS 16571-4029 is formed by multiple infrared sources, all but one are associated with small groups of stars. This suggests that the fragmentation of massive molecular clouds generates the massive sub-clusters. We derived a mean visual extinction of $A_{\rm V}=12.8\pm ^{4.7}_{3.2}$. This result is independent of the assumed distance and agrees with the mean visual extinction $A_{\rm V}=14.4$, as obtained by previous spectroscopic observations of two NIR sources in the direction of the IRAS 16571-4029 source. We also compare the results obtained in this study with those obtained in previous papers in this series finding a very good correlation between the number of cluster members N s and the cluster radius r c . The cluster radius varies from 0.2-0.3 pc (IRAS 15411-5352 and IRAS 16132-5039) until about 1 pc (IRAS 15408-5356). The youngest clusters are those associated with the RCW95 complex (IRAS 15408-5356 and IRAS 15411-5353) with ages in the range 1.5-2 $\times$ 10 6 years, while the sources associated with the RCW106 (IRAS 16132-5039, IRAS 16177-5018) and RCW116B (IRAS 16571-4029) complexes have ages in the range 2.5-3 $\times$ 10 6 years. The oldest of them is the cluster associated with the RCW121 region (IRAS 17149-4029), which has an estimated age of 4.2 $\times$ 10 6 years.