We provide a new view on the Cygnus-X north complex by accessing for the first time the low mass content of young stellar populations in the region. CFHT/WIRCam camera was used to perform a deep near-IR survey of this complex, sampling stellar masses down to ~0.1 M$_\odot$. Several analysis tools, including a extinction treatment developed in this work, were employed to identify and uniformly characterise a dozen unstudied young star clusters in the area. Investigation of their mass distributions in low-mass domain revealed a relatively uniform log-normal IMF with a characteristic mass of 0.32$\pm$0.08 M$_\odot$ and mass dispersion of 0.40$\pm$0.06. In the high mass regime, their derived slopes showed that while the youngest clusters (age < 4 Myr) presented slightly shallower values with respect to the Salpeter's, our older clusters (4 Myr < age < 18 Myr) showed IMF compliant values and a slightly denser stellar population. Although possibly evidencing a deviation from an 'universal' IMF, these results also supports a scenario where these gas dominated young clusters gradually 'build up' their IMF by accreting low-mass stars formed in their vicinity during their first ~3 Myr, before the gas expulsion phase, emerging at the age of ~4 Myr with a fully fledged IMF. Finally, the derived distances to these clusters confirmed the existence of at least 3 different star forming regions throughout Cygnus-X north complex, at distances of 500-900 pc, 1.4-1.7 kpc and 3.0 kpc, and revealed evidence of a possible interaction between some of these stellar populations and the Cygnus-OB2 association.
We present a multi-wavelength study of the young stellar population in the Cygnus-X DR15 region. We studied young stars forming or recently formed at and around the tip of a prominent molecular pillar and an infrared dark cloud. Using a combination of ground based near-infrared, space based infrared and X-ray data, we constructed a point source catalog from which we identified 226 young stellar sources, which we classified into evolutionary classes. We studied their spatial distribution across the molecular gas structures and identified several groups possibly belonging to distinct young star …
Similarity in shape between the initial mass function (IMF) and the core mass functions (CMFs) in star-forming regions prompts the idea that the IMF originates from the CMF through a self-similar core-to-star mass mapping process. To accurately determine the shape of the CMF, we create a sample of 8,431 cores with the dust continuum maps of the Cygnus X giant molecular cloud complex, and design a procedure for deriving the CMF considering the mass uncertainty, binning uncertainty, sample incompleteness, and the statistical errors. The resultant CMF coincides well with the IMF for core masses f…
We investigate how different stellar initial mass functions (IMFs) can affect the mass loss and survival of star clusters. We find that IMFs with radically different low-mass cut-offs (between 0.1 and 2 Msun) do not change cluster destruction time-scales as much as might be expected. Unsurprisingly, we find that clusters with more high-mass stars lose relatively more mass through stellar evolution, but the response to this mass loss is to expand and hence significantly slow their dynamical evolution. We also argue that it is very difficult, if not impossible, to have clusters with different IM…
In order to fully understand the gravitational collapse of molecular clouds, the star formation process and the evolution of circumstellar disks, these phenomena must be studied in different Galactic environments with a range of stellar contents and positions in the Galaxy. The young massive association Cygnus OB2, in the Cygnus-X region, is an unique target to study how star formation and the evolution of circumstellar disks proceed in the presence of a large number of massive stars. We present a catalog obtained with recent optical observations in r,i,z filters with OSIRIS, mounted on the $1…
We analyze the X-ray spectra of the $\sim$8000 sources detected in the Cygnus OB2 Chandra Legacy Survey (Drake et al., this issue), with the goals of characterizing the coronal plasma of the young low-mass stars in the region and estimating their intrinsic X-ray luminosities. We adopt two different strategies for X-ray sources for which more or less than 20 photons were detected. For the brighter sample we fit the spectra with absorbed isothermal models. In order to limit uncertainties, for most of the fainter Cygnus OB2 members in this sample, we constrain the spectral parameters to character…
We applied the extinction-disk-principal vectors approach to near infrared photometric data of the Taurus-Auriga region and Orion Nebula young stellar clusters. By assuming that the cluster age is represented by the median value of the age distribution we are able to derive the distribution of stellar masses. We showed that the resulting initial mass function (IMF) for these two young stellar clusters compares remarkably well and might be a robust representation of the IMF obtained by spectroscopic or photometric methods. The method also yields extinction and disk contribution for each star. T…
The initial mass function (IMF) is an essential tool with which to study star formation processes. We have initiated the photometric survey of young open clusters in the Galaxy, from which the stellar IMFs are obtained in a homogeneous way. A total of 16 famous young open clusters have preferentially been studied up to now. These clusters have a wide range of surface densities (log sigma = -1 to 3 [stars pc^2] for stars with mass larger than 5M_sun) and cluster masses (M_cl = 165 to 50,000M_sun), and also are distributed in five different spiral arms in the Galaxy. It is possible to test the d…
The young massive OB association Cygnus OB2, in the Cygnus X complex, is the closest (1400 pc) star forming region to the Sun hosting thousands of young low mass stars and up to 1000 OB stars, among which are some of the most massive stars known in our Galaxy. This region holds great importance for several fields of modern astrophysics, such as the study of the physical properties of massive and young low-mass stars and the feedback provided by massive stars on star and planet formation process. Cygnus OB2 has been recently observed with Chandra/ACIS-I as part of the 1.08Msec Chandra Cygnus OB…
Context: The shape of the stellar initial mass function (IMF) of a star cluster near its upper mass limit is a focal topic of investigation as it determines the high mass stellar content and hence the dynamics of the cluster at its embedded phase as well as during its young gas-free phase. The massive stellar content of a young cluster, however, can be substantially modified due to the dynamical ejections of the massive stars so that the present-day high-mass stellar mass function (hereafter MF) can be different than that with which the cluster is born. Aims: In the present study, we provide a…
The stellar initial mass function (IMF) is commonly interpreted to be a scale-invariant probability density distribution function (PDF) such that many small clusters yield the same IMF as one massive cluster of the same combined number of stars. Observations of the galaxy-wide IMF challenge this as dwarf galaxies do not form as many massive stars as expected. This indicates a highly self-regulated star formation process in which stellar masses are not stochastically sampled from the IMF and are instead related to the environment of star formation. Here, the nature of star formation is studied …
Aims. In this work we aim to estimate the lowest stellar mass that MICADO at the ELT will be able to reliably detect given a stellar density and distance. We also show that instrumental effects that will play a critical role, and report the number of young clusters that will be accessible for IMF studies in the local Universe with the ELT. Methods. We used SimCADO, the instrument simulator package for the MICADO camera, to generate observations of 56 dense stellar regions with densities similar to the cores of young stellar clusters. We placed the cluster fields at distances between 8 kpc and …
We present deep near-infrared (NIR) imaging of Sh 2-209 (S209), a low-metallicity (${\rm [O/H]} = - 0.5$ dex) HII region in the Galaxy. From the NIR images, combined with astrometric data from Gaia EDR3, we estimate the distance to S209 to be 2.5 kpc. This is close enough to enable us to resolve cluster members clearly ($\simeq$1000 AU separation) down to a mass-detection limit of $\simeq$0.1 $M_\odot$, and we have identified two star-forming clusters in S209, with individual cluster scales $\sim$1 pc. We employ a set of model luminosity functions to derive the underlying initial mass function…
In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cygnus OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun, and hosts hundreds of massive stars and thousands of low mass members. In this paper, we analyze the spatial variation of the disk fraction in Cygn…
Constraining young massive cluster properties with radio-continuum observations: The Arches cluster
Instituto de Astrofísica de Andalucía (CSIC)（安达卢西亚天体物理研究所）；Centro de Astrobiología, CSIC-INTA（天体生物学中心）；School of Sciences, European University Cyprus（塞浦路斯欧洲大学理学院）
Young Moving Groups (YMGs) are close (<100pc), coherent collections of young (<100Myr) stars that appear to have formed in the same star-forming molecular cloud. As such we would expect their individual initial mass functions (IMFs) to be similar to other star-forming regions, and by extension the Galactic field. Their close proximity to the Sun and their young ages means that YMGs are promising locations to search for young forming exoplanets. However, due to their low numbers of stars, stochastic sampling of the IMF means their stellar populations could vary significantly. We determine the r…
We present Plateau de Bure interferometer observations obtained in continuum at 1.3 and 3.5 mm towards the six most massive and young (IR-quiet) dense cores in Cygnus X. Located at only 1.7 kpc, the Cygnus X region offers the opportunity of reaching small enough scales (of the order of 1700 AU at 1.3 mm) to separate individual collapsing objects. The cores are sub-fragmented with a total of 23 fragments inside 5 cores. Only the most compact core, CygX-N63, could actually be a single massive protostar with an envelope mass as large as 60 Msun. The fragments in the other cores have sizes and sep…
In deep near-infrared imaging of the low-metallicity (${\rm [O/H]}=-0.7$ dex) H II region Sh 2-127 (S127) with Subaru/MOIRCS, we detected two young clusters with 413 members (S127A) in a slightly extended H II region and another with 338 members (S127B) in a compact H II region. The limiting magnitude was $K=21.3$ mag (10$σ$), corresponding to a mass detection limit of $\sim$0.2 $M_\odot$. These clusters are an order of magnitude larger than previously studied young low-metallicity clusters and larger than the majority of solar neighborhood young clusters. Fits to the K-band luminosity functio…
Low mass star formation inside massive clusters is crucial to understand the effect of cluster environment on processes like circumstellar disk evolution, planet and brown dwarf formation. The young massive association of Cygnus OB2, with a strong feedback from massive stars, is an ideal target to study the effect of extreme environmental conditions on its extensive low-mass population. We aim to perform deep multi-wavelength studies to understand the role of stellar feedback on the IMF, brown dwarf fraction and circumstellar disk properties in the region. We introduce here, the deepest and wi…
The evolution of HII regions/supershells can trigger a new generation of stars/clusters at their peripheries, with environmental conditions that may affect the initial mass function, disk evolution and star formation efficiency. In this paper we study the stellar content and star formation processes in the young cluster Stock 8, which itself is thought to be formed during the expansion of a supershell. We present deep optical photometry along with JHK and 3.6, 4.5 μm photometry from UKIDSS and Spitzer-IRAC. We use multi-color criteria to identify the candidate young stellar objects in the regi…
The origin of the lowest mass free-floating objects -- brown dwarfs and planetary-mass objects -- is one of the major unsolved problems in star formation. Establishing a census of young substellar objects is a fundamental prerequisite for distinguishing between competing theoretical scenarios. Such a census allows us to probe the initial mass function (IMF), binary statistics, and properties of accretion disks. Our SONYC (Substellar Objects in Nearby Young Clusters) survey relies on extremely deep wide-field optical and near-infrared imaging, with follow-up spectroscopy, in combination with Sp…
The supermassive black hole at the center of the Milky Way plays host to a massive, young cluster that may have formed in one of the most inhospitable environments in the Galaxy. We present new measurements of the global properties of this cluster, including the initial mass function (IMF), age, and cluster mass. These results are based on Keck laser-guide-star adaptive optics observations used to identify the young stars and measure their Kp-band luminosity function as presented in Do et al. 2013. A Bayesian inference methodology is developed to simultaneously fit the global properties of the…
Three presumably young eclipsing binary systems in the direction of the Cygnus OB1, OB3 and OB9 associations are studied. Component spectra are reconstructed and their orbits are determined using light curves and spectra disentangling techniques. V443 Cyg and V456 Cyg have circular orbits, while the light curve of V2107\,Cyg imposes a slightly eccentric orbit ($e=0.045\pm0.03)$. V443 Cyg harbours F-type stars, and not young early-A stars as previously suggested in the literature based on photometry solely. It appears to be situated in the foreground (distance $0.6\pm0.2$ kpc) of the young stel…
We present the first results from a 124 night J, H, K near-infrared monitoring campaign of the dark cloud L 1003 in Cygnus OB7, an active star-forming region. Using 3 seasons of UKIRT observations spanning 1.5 years, we obtained high-quality photometry on 9,200 stars down to J=17 mag, with photometric uncertainty better than 0.04 mag. On the basis of near-infrared excesses from disks, we identify 30 pre-main sequence stars, including 24 which are newly discovered. We analyze those stars and find the NIR excesses are significantly variable. All 9,200 stars were monitored for photometric variabi…
We have studied the star formation history and the initial mass function (IMF) using the age and mass derived from spectral energy distribution (SED) fitting and from color-magnitude diagrams. We also examined the physical and structural parameters of more than 1,000 pre-main sequence stars in NGC 2264 using the on-line SED fitting tool (SED fitter) of Robitaille et al. The cumulative distribution of stellar ages showed a distinct difference among SFRs. The results indicate that star formation in NGC 2264 started at the surface region (Halo and Field regions) about 6 - 7 Myr ago, propagated in…
University of Split, Faculty of Science（斯普利特大学理学院）；Univ. Grenoble Alpes, CNRS, IPAG（格勒诺布尔阿尔卑斯大学，法国国家科学研究中心，天体物理与行星学研究所）；Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências, Universidade de Lisboa（里斯本大学理学院，空间与天体物理研究所）；European Southern Observatory（欧洲南方天文台）；Max-Planck-Institut für Astronomie（马克斯·普朗克天文学研究所）；Istituto Nazionale di Astrofisica (INAF) - Osservatorio Astronomico di Palermo（意大利国家天体物理研究所（INAF）- 巴勒莫天文台）
Context. Brown dwarfs represent a sizable fraction of the stellar content of our Galaxy and populate the transition between the stellar and planetary mass regime. There is however no agreement on the processes responsible for their formation. Aims. We have conducted a large survey of the young, nearby cluster IC 348, to uncover its low-mass brown dwarf population and study the cluster properties in the substellar regime. Methods. Deep optical and near-IR images taken with MegaCam and WIRCam at the Canada-France-Hawaii Telescope (CFHT) were used to select photometric candidate members. A spectr…
In the last decades, the early pre main sequence stellar rotational evolution picture has been constrained by studies targeting different young regions at a variety of ages. Observational studies suggest a mass-rotation dependence, and for some mass ranges a connection between rotation and the presence of a circumstellar disk. Not still fully explored, though, is the role of environmental conditions on the rotational regulation. We investigate the rotational properties of candidate members of the young massive association Cygnus OB2. The Stetson variability index, Lomb-Scargle periodogram, Sau…
We obtained deep near-infrared images of Sh 2-208, one of the lowest-metallicity HII regions in the Galaxy, [O/H] = -0.8 dex. We detected a young cluster in the center of the HII region with a limiting magnitude of K = 18.0 mag (10sigma), which corresponds to a mass detection limit of ~0.2 M_sun. This enables the comparison of star-forming properties under low metallicity with those of the solar neighborhood. We identified 89 cluster members. From the fitting of the K-band luminosity function (KLF), the age and distance of the cluster are estimated to be ~0.5 Myr and ~4 kpc, respectively. The …
[Abridged] Twenty-six high-luminosity IRAS sources believed to be collection of stars in the early phases of high-mass star formation have been observed in the NIR (J, H, K) to characterize the clustering properties of their young stellar population and gain insight into the initial conditions of star formation in these clusters (Initial Mass Function [IMF], Star Formation History [SFH]), and to deduce mean values for cluster ages. K luminosity functions (KLFs) are compared with simulated ones from a model that generates populations of synthetic clusters starting from assumptions on the IMF, t…
Understanding the properties of young open clusters, such as the Initial Mass Function (IMF), star formation history and dynamic evolution, is crucial to obtain reliable theoretical predictions of the mechanisms involved in the star formation process. We want to obtain a list, as complete as possible, of confirmed members of the young open cluster Gamma Velorum, with the aim of deriving general cluster properties such as the IMF. We used all available spectroscopic membership indicators within the Gaia-ESO public archive together with literature photometry and X-ray data and, for each method, …
The Cygnus region, which dominates the local spiral arm of the Galaxy, is one of the nearest complexes of massive star formation. Its massive stellar content, regions of ongoing star formation, and molecular gas have been studied in detail. However, little is known of the history of the region beyond the past 10 Myr. The brightness and spectroscopic characteristics of red supergiants make it easy to identify them and build up a virtually complete sample of such stars at the distance of the Cygnus region, thus providing a record of massive star formation extending several tens of Myr into the p…
In the star formation process, the vital impact of environmental factors such as feedback from massive stars and stellar density on the form of the initial mass function (IMF) at low-mass end is yet to be understood. Hence a systematic, highly sensitive observational analysis of a sample of regions under diverse environmental conditions is essential. We analyse the IMF of eight young clusters ($<$5 Myr), namely IC1848-West, IC1848-East, NGC 1893, NGC 2244, NGC 2362, NGC 6611, Stock 8 and Cygnus OB2, which are located at the Galactocentric distance ($R_g$) range $\sim$6-12 kpc along with nearby…
Cygnus OB2 provides a unique insight into the high-mass stellar content in one of the largest groups of young massive stars in our Galaxy. Although several studies of its massive population have been carried out over the last decades, an extensive spectroscopic study of the whole known O-star population in the association is still lacking. In this work, we created the most complete spectroscopic census of O stars carried out so far in Cygnus OB2 using already existing and new spectroscopy. We present the spectra for 78 O-type stars, from which we identify new binary systems, obtain the distrib…
As a young massive cluster in the Central Molecular Zone, the Arches cluster is a valuable probe of the stellar Initial Mass Function (IMF) in the extreme Galactic Center environment. We use multi-epoch Hubble Space Telescope observations to obtain high-precision proper motion and photometric measurements of the cluster, calculating cluster membership probabilities for stars down to 1.8 M$_{\odot}$ between cluster radii of 0.25 pc -- 3.0 pc. We achieve a cluster sample with just ~8% field contamination, a significant improvement over photometrically-selected samples which are severely compromi…
Cygnus OB2 is the nearest example of a massive star forming region, containing over 50 O-type stars and hundreds of B-type stars. We have analysed the properties of young stars in two fields in Cyg OB2 using the recently published deep catalogue of Chandra X-ray point sources with complementary optical and near-IR photometry. Our sample is complete to 1 Msun (excluding A and B-type stars that do not emit X-rays), making this the deepest study of the stellar properties and star formation history in Cyg OB2 to date. From Siess et al. (2000) isochrone fits to the near-IR color-magnitude diagram, …
Young open clusters located in the outer Galaxy provide us with an opportunity to study star formation activity in a different environment from the solar neighborhood. We present a UBVI and H alpha photometric study of the young open clusters NGC 1624 and NGC 1931 that are situated toward the Galactic anticenter. Various photometric diagrams are used to select the members of the clusters and to determine the fundamental parameters. NGC 1624 and NGC 1931 are, on average, reddened by <E(B-V)> = 0.92 +/- 0.05 and 0.74 +/- 0.17 mag, respectively. The properties of the reddening toward NGC 1931 ind…
To study star formation in low metallicity environments ([M/H] ~ -1 ,dex), we obtained deep near-infrared (NIR) images of Sh 2-207 (S207), which is an HII region in the outer Galaxy with spectroscopically determined metallicity of [O/H] ~= -0.8 dex. We identified a young cluster in the western region of S207 with a limiting magnitude of Ks =19.0 mag (10 sigma) that corresponds to a mass detection limit of <~0.1 M_sun and enables the comparison of star-forming properties under low metallicity with those of the solar neighborhood. From the fitting of the K-band luminosity function (KLF), the age…
We present a survey of molecular outflows across the dark cloud complex in the Cygnus region, based on 46.75 deg^2 field of CO isotopologues data from Milky Way Imaging Scroll Painting (MWISP) survey. A supervised machine learning algorithm, Support Vector Machine (SVM), is introduced to accelerate our visual assessment of outflow features in the data cube of 12CO and 13CO J = 1-0 emission. A total of 130 outflow candidates are identified, of which 77 show bipolar structures and 118 are new detections. Spatially, these outflows are located inside dense molecular clouds and some of them are fou…
We have performed an unbiased search for outflows from young stars in Cygnus-X using 42 deg$^2$ of data from the UKIRT Widefield Infrared Survey for H$_2$ (UWISH2 survey), to identify shock-excited near-IR H$_2$ emission in the 1$-$0 S(1) 2.122 $μ$m line. We uncovered 572 outflows, of which 465 are new discoveries, increasing the number of known objects by more than 430%. This large and unbiased sample allows us to statistically determine the typical properties of outflows from young stars. We found 261 bipolar outflows and 16% of these are parsec-scale. The typical bipolar outflow is 0.45pc i…
I review efforts to determine the form and any lower limit to the initial mass function in the Galactic disk, using observations of low-mass stars and brown dwarfs in the field, young clusters and star forming regions. I focus on the methodologies that have been used and the uncertainties that exist due to observational limitations and to systematic uncertainties in calibrations and theoretical models. I conclude that whilst it is possible that the low-mass IMFs deduced from the field and most young clusters are similar, there are too many problems to be sure; there are examples of low-mass cl…
We use deep Hubble Space Telescope photometry of the rich, young (~20-45 Myr-old) star cluster NGC 1818 in the Large Magellanic Cloud to derive its stellar mass function (MF) down to ~0.15 Msun. This represents the deepest robust MF thus far obtained for a stellar system in an extragalactic, low-metallicity ([Fe/H]~-0.4 dex) environment. Combining our results with the published MF for masses above 1.0 Msun, we obtain a complete present-day MF. This is a good representation of the cluster's initial MF (IMF), particularly at low masses, because our observations are centred on the cluster's uncro…
We investigate the young stellar populations associated with DR 18 and ECX 6-21, which are two isolated globules irradiated by the O-type stars of the Cygnus OB2 association. Both are HII regions containing obvious tracers of recent and ongoing star formation. We also study smaller isolated molecular structures in their surroundings. Both globules contain their own embedded populations, with a higher fraction of the less-evolved classes. Masses and temperatures are estimated under the assumption of a common age of 1 Myr, which has been found to appropriately represent the general Cygnus OB2 YS…
We present new images and photometry of the massive star forming complex Cygnus X obtained with IRAC and MIPS on the Spitzer Space Telescope. A combination of IRAC, MIPS, UKIDSS, and 2MASS data are used to identify and classify young stellar objects. Of the 8,231 sources detected exhibiting infrared excess in Cygnus X North, 670 are classified as Class I and 7,249 are classified as Class II. Using spectra from the FAST spectrograph at the Fred L. Whipple Observatory and Hectospec on the MMT, we spectrally typed 536 sources in the Cygnus X complex to identify the massive stars. We find that YSO…
RCW 38 is a deeply embedded young (~1 Myr), massive star cluster located at a distance of 1.7 kpc. Twice as dense as the Orion Nebula Cluster, orders of magnitude denser than other nearby star forming regions, and rich in massive stars, RCW 38 is an ideal place to look for potential differences in brown dwarf formation efficiency as a function of environment. We present deep, high resolution adaptive optics data of the central ~0.5x0.5 pc^2 obtained with NACO at the Very Large Telescope. Through comparison with evolutionary models we determine masses and extinction for ~480 candidate members, …
We have compiled a significantly updated and comprehensive census of massive stars in the nearby Cygnus OB2 association by gathering and homogenising data from across the literature. The census contains 169 primary OB stars, including 52 O-type stars and 3 Wolf-Rayet stars. Spectral types and photometry are used to place the stars in a Hertzprung-Russell diagram, which is compared to both non-rotating and rotating stellar evolution models, from which stellar masses and ages are calculated. The star formation history and mass function of the association are assessed, and both are found to be he…
We exploit the ability of the Hubble Space Telescope to probe near infrared water absorption present in the atmosphere of low-mass stars, brown dwarfs and planetary mass objects to create a very pure sample of Orion Nebula Cluster (ONC) members, not affected by contamination from background stars and galaxies which lack water absorption. Thanks to these data we infer the Initial Mass Function (IMF) of the ONC in the $0.005 - 1.4$M$_{\odot}$ regime, i.e. down to few Jupiter masses. The young age of the ONC, $\sim1$ Myr, provides a snapshot of the outcome of star formation for the present-day co…
We study the dynamical evolution of the young star cluster Arches and its dependence on the assumed initial stellar mass function (IMF). We perform many direct $N$-body simulations with various initial conditions and two different choices of IMFs. One is a standard Kroupa IMF without any mass segregation. The other is a radially dependent IMF, as presently observed in the Arches. We find that it is unlikely for the Arches to have attained the observed degree of mass segregation at its current age starting from a standard non-segregated Kroupa IMF. We also study the possibility of a collisional…
Open clusters and associations are groups of young stars, respectively bound and unbound, that share the same origin and disperse over time into the galactic field. As such, their formation and evolution are the key to understand the origin and properties of galactic stellar populations. Moreover, since their members have about the same age, they are ideal laboratories to study the properties of young stars and constrain stellar evolution theories. In this contribution, I present our current knowledge on open clusters and associations. I focus on the methods used to derive the statistical prop…
We present a UBV I and H alpha photometric study of the young open cluster NGC 1893 in the H II region W8 (IC 410 or Sh 2-236). A total of 65 early-type members are selected from photometric diagrams. A mean reddening of the stars is <E(B-V)> = 0.563 +/- 0.083 mag. The published photometric data in the near- and mid-infrared passbands are used to test the reddening law toward the cluster, and we confirm that the reddening law is normal (R_V = 3.1). Zero-age main sequence fitting gives a distance modulus of V_0 - M_V = 12.7 +/- 0.2 mag, equivalent to 3.5 +/- 0.3 kpc. From H alpha photometry 125…
The initial mass function (IMF) succinctly characterizes a stellar population, provides a statistical measure of the end result of the star-formation process, and informs our under- standing of the structure and dynamical evolution of stellar clusters, the Milky Way, and other galaxies. Detecting variations in the form of the IMF could provide powerful insights into the processes that govern the formation and evolution of stars, clusters, and galaxies. In this contribution, we review measurements of the IMF in resolved stellar populations, and critically assess the evidence for systematic IMF …
We investigate the star formation activity in a young star forming cluster embedded at the edge of the RCW 41 HII region. As a complementary goal, we aim at demonstrating the gain provided by Wide-Field Adaptive Optics instruments to study young clusters. We used deep, JHKs images from the newly commissioned Gemini-GeMS/GSAOI instrument, complemented with Spitzer IRAC observations, in order to study the photometric properties of the young stellar cluster. GeMS is an AO instrument, delivering almost diffraction limited images over a field of 2' across. The exquisite angular resolution allows us…
Cygnus OB2 is the most massive association within 2kpc from the Sun, hosting hundreds of massive stars, thousands of young low mass members, and some sights of active star formation in the surrounding cloud. Recently, 10 photoevaporating proplyd-like objects with tadpole-shaped morphology were discovered in the outskirts of the OB association, approximately 6-14pc away from its center. The classification of these objects is ambiguous, being either evaporating residuals of the parental cloud which are hosting a protostar inside, or disk-bearing stars with an evaporating disk, such as the evapor…
Few topics in astronomy initiate such vigorous discussion as whether or not the initial mass function (IMF) of stars is universal, or instead sensitive to the initial conditions of star formation. The distinction is of critical importance: the IMF influences most of the observable properties of stellar populations and galaxies, and detecting variations in the IMF could provide deep insights into the process by which stars form. In this contribution, we take a critical look at the case for IMF variations, with a view towards whether other explanations are sufficient given the evidence. Studies …
The determination of the lower-end of the initial mass function (IMF) provides strong constraints on star formation theories. We report here on a search for isolated planetary-mass objects in the 3 Myr-old star-forming region IC 348. Deep, narrowband CH4off and CH4on images were obtained with CFHT/WIRCam over 0.11 sq.deg. in the central part of IC 348 to identify young T-dwarfs from their 1.6 um methane absorption bands. We report three faint T-dwarf candidates with CH4on-CH4off colours > 0.4 mag. Extinction was estimated for each candidate and lies in the range Av ~ 5-12 mag. Comparisons with…
We report on Smoothed Particle Hydrodynamics (SPH) simulations of the impact on a turbulent $\sim2\times10^{3}$ M$_{\odot}$ star--forming molecular cloud of irradiation by an external source of ionizing photons. We find that the ionizing radiation has a significant effect on the gas morphology, but a less important role in triggering stars. The rate and morphology of star formation are largely governed by the structure in the gas generated by the turbulent velocity field, and feedback has no discernible effect on the stellar initial mass function. Although many young stars are to be found in d…
