A WFI survey in the Chamaeleon II dark cloud

\n We report high-resolution spectroscopic observations, as well as \nhigh-resolution near infrared (IR) imaging of six stars previously \nidentified in a ROSAT pointed observation in the direction of \nthe B-type star β Cru, and classified as low-mass pre-main\nsequence (PMS) stars. Four of the stars \nare confirmed to be low-mass PMS stars, associated with the \nLower Centaurus-Crux group, while the other two are unrelated to the\nSco-Cen association. The confirmed PMS stars are most likely in their\npost-T Tauri evolutionary phase. \nAlthough future deep X-ray observations with high-resolution \nimagers might detect more new PMS stars, the possibility that the\nCrux PMS stars are part of a small aggregate, with β Crux \nitself approximately at the center, is rather unlikely, given the \nhigh velocity dispersion and the low spatial density of the confirmed\nPMS stars. \nInstead, these stars may be part of a moving group in a more \ndisperse and numerous population of low-mass PMS stars, distributed \nin the Lower Centaurus-Crux subgroup.\nNew PMS binaries and multiple systems were also discovered among \nthe stars in the sample, namely a close visual pair and a hierarchical \ntriple system in which one of the components is a double-lined \nspectroscopic binary (SB2). The detailed orbital solution is reported \nfor the inner short-period ($P_{\\rm orb} =$ 58.3 days) SB2.\nA preliminary orbital solution for the hierarchical triple system\nyields a systemic orbital period of about 4.6 years, which makes \nthis object a very suitable target for follow-up observations \nwith the Very-Large Telescope Interferometer (VLTI) in the coming \nyears.\n\n \n

We present the results of a multi-wavelength study of the star forming region in L1616. Our observations include ROSAT All-Sky Survey (RASS) and High Resolution Imager (HRI) X-ray observations, optical wide-field imaging and near-IR imaging data and optical long-slit and multi-object spectroscopic follow-up. 22 new low-mass pre-main sequence (PMS) stars are found to be distributed mainly to the East of the L1616 cometary cloud, in about a one-square-degree field. We find that the class-III infrared sources outnumber the class-II infrared sources by a factor of about three. The X-ray properties of the PMS stars in L1616 are quite similar to those of PMS stars detected in the Orion Nebula Cluster. The comparison of the position of the L1616 PMS stars in the HR diagram with theoretical PMS evolutionary tracks yields an average age of 1–2 Myr, with a very small age spread of about 1 Myr. Unlike the fossil star forming regions in Orion, L1616 appears to be a region of on-going star formation relatively far from the Orion A and B clouds. Given the small age spread, the spatial distribution of the PMS stars relative to the head of the cloud, as well as its cometary shape and high star formation efficiency, we conclude that the star formation in L1616 was most likely induced by a single event, the impact of the winds of the massive stars of the Orion OB association or a supernova explosion being the possible triggers. The Initial Mass Function (IMF) in L1616 is roughly consistent with that of the field in the mass range 0.3 < M/M_⊙ < 2.5. Several faint objects, detected in our optical images, are good candidates for young Brown Dwarfs (BDs). We might expect the number of BDs in L1616 to be intermediate between Taurus and the Trapezium.

The large-scale magnetic fields of several pre-main sequence (PMS) stars have been observed to be simple and axisymmetric, dominated by tilted dipole and octupole components. The magnetic fields of other PMS stars are highly multipolar and dominantly non-axisymmetric. Observations suggest that the magnetic field complexity increases as PMS stars evolve from Hayashi to Henyey tracks in the Hertzsprung–Russell diagram. Independent observations have revealed that X-ray luminosity decreases with age during PMS evolution, with Henyey track PMS stars having lower fractional X-ray luminosities (LX/L*) compared to Hayashi track stars. We investigate how changes in the large-scale magnetic field topology of PMS stars influences coronal X-ray emission. We construct coronal models assuming pure axisymmetric multipole magnetic fields, and magnetic fields consisting of a dipole plus an octupole component only. We determine the closed coronal emitting volume, over which X-ray-emitting plasma is confined, using a pressure balance argument. From the coronal volumes, we determine X-ray luminosities. We find that LX decreases as the degree ℓ of the multipole field increases. For dipole plus octupole magnetic fields, we find that LX tends to decrease as the octupole component becomes more dominant. By fixing the stellar parameters at values appropriate for a solar mass PMS star, varying the magnetic field topology results in two orders of magnitude variation in LX. Our results support the idea that the decrease in LX as PMS stars age can be driven by an increase in the complexity of the large-scale magnetic field.

In the first part of this thesis a new analysis of the Orion Nebula Cluster, one of the most studied star-forming regions in the Galaxy, is presented. Based on multi-band optical photometry and spectroscopy obtained with the Wide-Field Imager (WFI) at the ESO/MPI 2.2-m telescope at La Silla Observatory, I study the systematic effects that bias the derivation of stellar parameters of pre-main sequence (PMS) stars. I derive the new H-R diagram of the entire region, and assign masses and ages to all the members. The age is found to be ∼ 2 − 3 Myr, older than previously estimated. I also confirm the presence of an age spread, and show how the previously found mass-age correlation can be affected by the sample incompleteness and uncertainties in the evolutionary models. In the second part of this thesis, I carry out a research on the low-mass stellar population of the young stellar cluster LH 95 in the Large Magellanic Cloud, based on deep optical photometry with the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope; the deepest ever obtained toward this galaxy, down to V ≃ 28 mag. I isolate the PMS cluster population, and derive the first extragalactic Initial Mass Function (IMF) down to the subsolar regime. It shows a flattening below 1 M⊙, in agreement with the Galactic IMF once results are corrected for unresolved binarity. I study the age distribution of LH 95, introducing a statistical method to derive average age and age-spreads accounting simultaneously for unresolved binarity, differential extinction, variability, accretion and crowding of PMS stars. The best-fit solution for LH 95 suggests an age of ∼ 4 Myr with a gaussian age spread of σ ∼ 1.2 Myr. Finally, I study the early-type highmass stellar population of the cluster, through ground based spectroscopy obtained with the Fiber-fed Extended Range Optical Spectrograph (FEROS) at the ESO/MPI 2.2-m telescope at La Silla Observatory, and photometry from the 1-m telescope at Siding Spring Observatory. The derived stellar masses are combined with my results on the low-mass IMF of the cluster for the study of the most complete extragalactic IMF ever performed.

We present the first report on results of a near-infrared imaging survey of the Lupus 3 dark cloud. This cloud is known to be associated with a modest cluster of T Tauri stars from a previous optical Hα emission-line star survey. The survey covers 7' × 11', which corresponds to a projected area of ~0.35 × 0.55 pc at a distance of 150 pc. Mapping was carried out at J, H, and Ks, to 10 σ limiting magnitudes of J = 17.0, H = 16.5, and Ks = 15.5. A total of 229 sources brighter than Ks < 15.8 were detected at all bands with a 90% completeness limit. Source classification is performed based on the near-infrared colors. Ten sources are candidates of Lada's Class II pre–main-sequence (PMS) stars, as they have a color excess that cannot be explained by reddening resulting from interstellar dust. We also identified 11 Class I–like candidates that were not detected at J and have a large color excess (H-Ks ≥ 2), which is unlikely to arise from extinction in the Lupus dark cloud. There are four subclusters in this survey area of which three are embedded and mainly consist of the Class I–like candidates. The average density of PMS stars is around 500 pc-3, suggesting the presence of a modest cluster of embedded PMS stars. We estimate masses of the Class II candidates with aid of an evolutionary model of PMS stars. Ten of them have masses less than 0.08 M⊙ if we assume their age to be 106 yr. Hence, we consider them to be young brown dwarf (YBD) candidates. The relative population of YBDs in the Lupus 3 dark cloud is larger than in the Taurus.

A polytropic model is used to investigate the effects of dark photospheric spots on the evolution and radii of magnetically active, low-mass (M<0.5Msun), pre-main sequence (PMS) stars. Spots slow the contraction along Hayashi tracks and inflate the radii of PMS stars by a factor of (1-beta)^{-N} compared to unspotted stars of the same luminosity, where beta is the equivalent covering fraction of dark starspots and N \simeq 0.45+/-0.05. This is a much stronger inflation than predicted by the models of Spruit & Weiss (1986) for main sequence stars with the same beta, where N \sim 0.2 to 0.3. These models have been compared to radii determined for very magnetically active K- and M-dwarfs in the young Pleiades and NGC 2516 clusters, and the radii of tidally-locked, low-mass eclipsing binary components. The binary components and ZAMS K-dwarfs have radii inflated by \sim 10 per cent compared to an empirical radius-luminosity relation that is defined by magnetically inactive field dwarfs with interferometrically measured radii; low-mass M-type PMS stars, that are still on their Hayashi tracks, are inflated by up to \sim 40 per cent. If this were attributable to starspots alone, we estimate that an effective spot coverage of 0.35 < beta < 0.51 is required. Alternatively, global inhibition of convective flux transport by dynamo-generated fields may play a role. However, we find greater consistency with the starspot models when comparing the loci of active young stars and inactive field stars in colour-magnitude diagrams, particularly for the highly inflated PMS stars, where the large, uniform temperature reduction required in globally inhibited convection models would cause the stars to be much redder than observed.

\n We present the results of an extensive search for periodic and irregular variable \npre-main sequence (PMS) stars in the young (2–4 Myr) open cluster <ext-link ext-link-type="aoi">NGC 2264</ext-link>, based \non photometric monitoring using the Wide Field Imager (WFI) on the 2.2 m telescope on La Silla (Chile). In total, about 10 600 stars with $I_{{\\rm C}}$ magnitudes between 9.8 mag \nand 21 mag have been monitored in our $34\\arcmin$ $\\times$ $33\\arcmin$ field. Time series data \nwere obtained in the $I_{{\\rm C}}$ band in 44 nights between Dec. 2000 and March 2001; \naltogether we obtained 88 data points per star. Using two different time series analysis \ntechniques (Scargle periodogram and CLEAN) we found 543 periodic variable stars with \nperiods between 0.2 days and 15 days. Also, 484 irregular variable stars were identified \nusing a $\\chi^2$-test. In addition we have carried out nearly simultaneous observations in V, $R_{{\\rm C}}$ and a narrow-band Hα filter. The photometric data enable us to \nreject background and foreground stars from our sample of variable stars according to their \nlocation in the $I_{{\\rm C}}$ vs. $(R_{{\\rm C}}-I_{{\\rm C}})$ colour–magnitude and \n$(R_{{\\rm C}}-{\\rm H}\\alpha)$ vs. $(R_{{\\rm C}}-I_{{\\rm C}})$ colour–colour diagrams. We \nidentified 405 periodic variable and 184 irregular variable PMS stars as cluster members \nusing these two different tests. In addition 35 PMS stars for which no significant variability \nwere detected could be identified as members using an $\\rm H\\alpha$ emission index criterion. \nThis yields a total of 624 PMS stars in <ext-link ext-link-type="aoi">NGC 2264</ext-link>, of which only 182 were previously known. \nMost of the newly found PMS stars are fainter than $I_{{\\rm C}} \\simeq 15~{\\rm mag}$\nand of late spectral type ($\\ga$M 2). We find that the periodic variables, as a group, have \na smaller degree of variability and smaller $\\rm H\\alpha$ index than the irregular variables. \nThis suggests that the sample of periodic variables is biased towards weak-line T Tauri\nstars (WTTSs) while most of the irregular variables are probably classical T Tauri \nstars (CTTSs). We have quantified this bias and estimated that the expected fraction of WTTSs \namong PMS stars in the cluster is $77\\%$. This is relatively close to the fraction of WTTSs \namong the periodic variables which is $85\\%$. We also estimated the total fraction of \nvariables in the cluster using only two well selected concentrations of PMS stars called \n<ext-link ext-link-type="aoi">NGC 2264</ext-link> N &amp; S in which we can easily estimate the total number of PMS stars. We find that \nat least $74\\%$ of the PMS stars in the cluster with $I_{{\\rm C}}\\leq 18.0~{\\rm mag}$ \nwere found to be variable (either periodic or irregular) by our study. This number shows \nthat our search for PMS stars in <ext-link ext-link-type="aoi">NGC 2264</ext-link> through extensive and accurate photometric \nmonitoring is very efficient in detecting most PMS stars down to at least \n$I_{{\\rm C}}=18.0~{\\rm mag}$.\n\n \n

We present the results of a photometric BVI survey of 0.9 deg 2 around the Wolf-Rayet binary γ 2 Vel and its early-type common proper motion companion γ 1 Vel (together referred to as the y Vel system). Several hundred pre-main-sequence (PMS) stars are identified and the youth of a subset of these is spectroscopically confirmed by the presence of lithium in their atmospheres, Ha emission and high levels of X-ray activity. We show that the PMS stars are kinematically coherent and spatially concentrated around y Vel. The PMS stars have similar proper motions to y Vel, to main-sequence (MS) stars around y Vel and to early-type stars of the wider Vela OB2 association of which γ 2 Vel is the brightest member. The ratio of MS stars to low-mass (0.1-0.6M ⊙ ) PMS stars is consistent with a Kroupa mass function. MS fitting to stars around y Vel gives an association distance modulus of 7.76 ± 0.07 mag, which is consistent with a similarly determined distance for Vela OB2 and also with interferometric distances to γ 2 Vel. High-mass stellar models indicate an age of 3-4Myr for γ 2 Vel, but the low-mass PMS stars have ages of ≃10 Myr according to low-mass evolutionary models and 5-10 Myr by empirically placing them in an age sequence with other clusters based on colour-magnitude diagrams and lithium depletion. We conclude that the low-mass PMS stars form a genuine association with y Vel, and this is a subcluster within the larger Vela OB2 association. We speculate that γ 2 Vel formed after the bulk of the low-mass stars, expelling gas, terminating star formation and unbinding the association. The velocity dispersion of the PMS stars is too low for this star-forming event to have produced all the stars in the extended Vela OB2 association. Instead, star formation must have been initiated at several sites within a molecular cloud either sequentially or simultaneously after some triggering event.

?????We present the initial results of a deep CCD survey for very low mass pre?main-sequence (PMS) stars in selected fields of the Taurus molecular cloud complex. The results reported herein span a little over half a square degree in the dark clouds L1495, L1529, L1551, and B209. Our survey is complete down to IC ~ 19 at (R - I)C ~ 2.5, which enables us to probe well below the hydrogen burning limit at 1?2 Myr. From follow-up spectroscopic observations we have identified nine new low-mass T Tauri stars (TTSs). A large fraction of the new PMS objects (5/9) have very low masses as inferred from their late spectral types (?M5), and comparison with recent evolutionary tracks and Pleiades brown dwarfs suggests that our M6?M6.5 new TTSs are very young brown dwarfs. Two of the new TTSs may constitute a new, moderately embedded, binary classical T Tauri system. The new young stars represent a ~38% increase in the known PMS population of our survey area and a factor of ~2 increase in the number of known late-type TTSs. In spite of our sensitivity, we detect no young stars with spectral types later than ~M7. Our results illustrate the importance of spectroscopy in eliminating foreground M stars.

About a dozen substellar companions orbiting young stellar objects or pre-main sequence stars at several hundred au have been identified in the last decade. These objects are interesting both due to the uncertainties surrounding their formation, and because their large separation from the host star offers the potential to study the atmospheres of young giant planets and brown dwarfs. Here, we present X-shooter spectroscopy of SR 12 C, a 2 Myr young brown dwarf orbiting SR 12 at an orbital separation of 1083 au. We determine the spectral type, gravity, and effective temperature via comparison with models and observational templates of young brown dwarfs. In addition, we detect and characterize accretion using several accretion tracers. We find SR 12 C to be a brown dwarf of spectral type L0 1, log g = 4 0.5, an effective temperature of 2600 100 K. Our spectra provide clear evidence for accretion at a rate of 10 -10 M yr -1 . This makes SR 12 one of the few sub-stellar companions with a reliable estimate for its accretion rate. A comparison of the ages and accretion rates of sub-stellar companions with young isolated brown dwarfs does not reveal any significant differences. If further accretion rate measurements of a large number of substellar companions can confirm this trend, this would hint towards a similar formation mechanism for substellar companions at large separations and isolated brown dwarfs.

As a part of an ongoing effort to characterise the young stellar populations in the Large Magellanic Cloud, we present HST-WFPC2 broad and narrow band imaging of two fields with recent star formation activity in the Tarantula region. A population of objects with Hα and/or Balmer continuum excess was identified. On account of the intense Hα emission (equivalent widths up to several tens of Å), its correlation with the Balmer continuum excess and the stars' location on the HR diagram, we interpret them as low mass (~) Pre-Main Sequence stars. In this framework, the data show that coeval high and low mass stars have significantly different spatial distributions, implying that star formation processes for different ranges of stellar masses are rather different and/or require different initial conditions. We find that the overall slope of the mass function of the young population is somewhat steeper than the classical Salpeter value and that the star formation density of this young component is , i.e. intermediate between the value for an active spiral disk and that of a starburst region. The uncertainties associated with the determination of the slope of the mass function and the star formation density are thoroughly discussed.

We present results of optical spectroscopic and BVR_CI_C photometric observations of 77 pre-main sequence (PMS) stars in the Cepheus flare region. A total of 64 of these are newly confirmed PMS stars, originally selected from various published candidate lists. We estimate effective temperatures and luminosities for the PMS stars, and comparing the results with pre-main sequence evolutionary models we estimate stellar masses of 0.2-2.4M_sun and stellar ages of 0.1-15 Myr. Among the PMS stars, we identify 15 visual binaries with separations of 2-10 arcsec. From archival IRAS, 2MASS, and Spitzer data, we construct their spectral energy distributions and classify 5% of the stars as Class I, 10% as Flat SED, 60% as Class II, and 3% as Class III young stellar objects (YSOs). We identify 12 CTTS and 2 WTTS as members of NGC 7023, with mean age of 1.6 Myr. The 13 PMS stars associated with L1228 belong to three small aggregates: RNO 129, L1228A, and L1228S. The age distribution of the 17 PMS stars associated with L1251 suggests that star formation has propagated with the expansion of the Cepheus flare shell. We detect sparse aggregates of 6-7 Myr old PMS stars around the dark clouds L1177 and L1219, at a distance of 400 pc. Three T Tauri stars appear to be associated with the Herbig Ae star SV Cep at a distance of 600 pc. Our results confirm that the molecular complex in the Cepheus flare region contains clouds of various distances and star forming histories.

This overview summarizes the age dating methods available for young sub-solar mass stars. Pre-main sequence age diagnostics include the Hertzsprung-Russell (HR) diagram, spectroscopic surface gravity indicators, and lithium depletion; asteroseismology is also showing recent promise. Near and beyond the zero-age main sequence, rotation period or vsiniand activity (coronal and chromospheric) diagnostics along with lithium depletion serve as age proxies. Other authors in this volume present more detail in each of the aforementioned areas. Herein, I focus on pre-main sequence HR diagrams and address the questions: Do empirical young cluster isochrones match theoretical isochrones? Do isochrones predict stellar ages consistent with those derived via other independent techniques? Do the observed apparent luminosity spreads at constant effective temperature correspond to true age spreads? While definitive answers to these questions are not provided, some methods of progression are outlined.

We discuss new constraints on the population of compact ionized sources in the Trapezium Cluster thought to arise from the ionization by the central OB stars of circumstellar disks around low-mass pre-main sequence stars. We present new HST Planetary Camera observations of two of these candidate disk sources, resolving extended nebulosity around them. One source shows a small-scale (greater than 100 AU) bow-shock structure, previously seen on larger scales by O'Dell et al. We show that the circumstellar disk model is the most likely one for the majority of sources, although it remains plausible that some of the larger objects could be equilibrium globules. We combine the most complete censuses of compact radio sources and stars in the core region to derive the fraction of the stellar population that may be associated with a circumstellar disk. Our estimate of 25-75 percent is comparable to that found for PMS stars in the Taurus-Auriga dark clouds, indicating that the dense cluster environment of the Trapezium has not drastically reduced the frequency of disks seen around pre-main sequence stars.

We present a study of the mass accretion rates of pre-main sequence (PMS) stars in the cluster Trumpler 14 (Tr 14) in the Carina Nebula. Using optical multiband photometry we were able to identify 356 PMS stars showing H excess emission with equivalent width EW(H) > 20 . We interpret this observational feature as an indication that these objects are still actively accreting gas from their circumstellar medium. From a comparison of the HR diagram with PMS evolutionary models we derive ages and masses of the PMS stars. We find that most of the PMS objects are younger than 10 Myr with a median age of 3 Myr. Surprisingly, we also find that 20% of the mass accreting objects are older than 10 Myr. For each PMS star in Trumpler 14 we determine the mass accretion rate ( acc ) and discuss its dependence on mass and age. We finally combine the optical photometry with near-IR observations to build the spectral energy distribution (SED) for each PMS star in Tr 14. The analysis of the SEDs suggests the presence of transitional discs in which a large amount of gas is still present and sustains accretion onto the PMS object at ages older than 10 Myr. Our results, discussed in light of recent recent discoveries with Herschel of transitional discs containing a massive gas component around the relatively old PMS stars TW Hydrae, 49 Ceti, and HD 95086, support a new scenario n which old and evolved debris discs still host a significant amount of gas.