Age spreads in star forming regions?

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.

The timescale of cluster formation is an essential parameter in order to understand the formation process of star clusters. Pre-main sequence (PMS) stars in nearby young open clusters reveal a large spread in brightness. If the spread were considered to be a result of a real spread in age, the corresponding cluster formation timescale would be about 5–20 Myr. Hence it could be interpreted that star formation in an open cluster is prolonged for up to a few tens of Myr. However, difficulties in reddening correction, observational errors, and systematic uncertainties introduced by imperfect evolutionary models for PMS stars can result in an artificial age spread. Alternatively, we can utilize Li abundance as a relative age indicator of PMS star to determine the cluster formation timescale. The optical spectra of 134 PMS stars in NGC 2264 have been obtained with MMT/Hectochelle. The equivalent widths have been measured for 86 PMS stars with a detectable Li line ( ). Li abundance under the condition of local thermodynamic equilibrium (LTE) was derived using the conventional curve of growth method. After correction for non-LTE effects, we find that the initial Li abundance of NGC 2264 is . From the distribution of the Li abundances, the underlying age spread of the visible PMS stars is estimated to be about 3–4 Myr and this, together with the presence of embedded populations in NGC 2264, suggests that the cluster formed on a timescale shorter than 5 Myr.

view Abstract Citations (213) References (72) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS NGC 6611: A Cluster Caught in the Act Hillenbrand, Lynne A. ; Massey, Philip ; Strom, Stephen E. ; Merrill, K. Michael Abstract We have combined optical CCD photometry and spectroscopy with infrared imaging photometry to study the young cluster NGC 6611. We use these data to derive improved values for the reddening law (R = 3.75) and the distance modulus (m - M = 11.5), and to construct a physical Hertzsprung-Russell (HR) diagram from which we can probe the ages, masses, and evolutionary states of this stellar ensemble. The HR diagram shows a strong population of high-mass stars, the most massive of which has a mass of roughly 80 solar mass, similar to what we find in other Galactic and Magellanic Cloud clusters and associations. The age of the massive stellar population in NGC 6611 is approximately 2 million yr, with an age spread of, perhaps, 1 million yr, although the data are also consistent with there being no discernible age spread among the most massive stars. However, the HR diagram does reveal that one star of somewhat lower mass (30 solar mass) must have formed approximately 6 million yr ago. The upper end of the mass function of NGC 6611 is found to have a slope of Gamma = -1.1 +/- 0.3, indistinguishable from a Salpeter slope, and similar to what we have found in other Galactic associations, but shallower than what we have found in the Magellanic Clouds. Our most significant result, however, is that we catch this cluster in the act of forming intermediate-mass (3-8 solar mass) stars. This is the first well-established case where large numbers of intermediate-mass stars have been seen on their way to the Zero Age Main Sequence (ZAMS). That intermediate-mass pre-main-sequence stars are indeed present is evidenced both by their location above the ZAMS in the HR diagram, and in some cases by their spectroscopic and infrared signatures of (possibly remnant protostellar) circumstellar material. The pre-main-sequence population ranges from as young as 0.25 million yr to at least 1 million yr of age. We find an highly unusual number (27) of emission-line stars, which appear quite similar in their optical and infrared continuum and optical spectroscopic properties to 'classical Be/Ae' stars (as opposed to Herbig Be/Ae stars). Our data are inconsistent with the traditional interpretation that these classical Be/Ae stars are slightly evolved stars undergoing mass loss. Instead, we offer the conjecture that these may be young stars whose circumstellar disks have become optically thin, and produce Balmer emission lines. The infrared data do indicate a number of stars, particularly among the embedded sample, whose colors are consistent with those of stars thought to be surrounded by optically thick circumstellar accretion disks. The identification of such disks around young massive stars continues to be rare, and implies that the disk survival times around intermediate- and high-mass stars are much shorter (less than 0.5 Myr) than those of disks surrounding lower-mass stars. Publication: The Astronomical Journal Pub Date: November 1993 DOI: 10.1086/116774 Bibcode: 1993AJ....106.1906H Keywords: Balmer Series; Distance; Galactic Clusters; Hertzsprung-Russell Diagram; Pre-Main Sequence Stars; Stellar Evolution; Stellar Mass; Charge Coupled Devices; Galactic Evolution; Infrared Photometry; Magellanic Clouds; Stellar Spectra; Stellar Spectrophotometry; Astronomy; GALAXIES: STAR CLUSTERS full text sources ADS | data products SIMBAD (1029) CDS (1)

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.

The possibility to estimate ages and masses of Young Stellar Objects (YSOs) from their location in the Hertzsprung-Russell diagram (HRD) or a color-magnitude diagram provides a very important tool for the investigation of fundamental questions related to the processes of star formation and early stellar evolution. Age estimates are essential for studies of the temporal evolution of circumstellar material around YSOs and the conditions for planet formation. The characterization of the age distribution of the YSOs in a star forming region allows researchers to reconstruct the star formation history and provides important information on the fundamental question of whether star formation is a slow or a fast process. However, the reliability of these age measurements and the ability to detect possible age spreads in the stellar population of star forming regions are fundamentally limited by several factors. The variability of YSOs, unresolved binary components, and uncertainties in the calibrations of the stellar parameters cause uncertainties in the derived luminosities that are usually much larger than the typical photometry errors. Furthermore, the pre-main sequence evolution track of a YSO depends to some degree on the initial conditions and the details of its individual accretion history. I discuss how these observational and model uncertainties affect the derived isochronal ages, and demonstrate how neglecting or underestimating these uncertainties can easily lead to severe misinterpretations, gross overestimates of the age spread, and ill-based conclusions about the star formation history. These effects are illustrated by means of Monte-Carlo simulations of observed star clusters with realistic observational uncertainties. The most important points are as follows. First, the observed scatter in the HRD must not be confused with a genuine age spread, but is always just an upper limit to the true age spread. Second, histograms of isochronal ages naturally show a decreasing number of stars for ages above the median, a pattern that can be misinterpreted as an accelerating star formation rate. Third, it is emphasized that many star forming regions consist of several sub-groups, which often have different ages. If these distinct stellar populations cannot be disentangled (e.g., due to projection effects) and the HRD of all stars in the region is used for an age analysis, it is very difficult (often impossible) to discern between the scenario of an extended period of star formation (i.e. a large age spread) and the alternative concept of a temporal sequence of several discrete star formation episodes. Considering these factors, most observations of star forming regions suggest that age spreads are usually smaller than the corresponding crossing times, supporting the scenario of fast and dynamic star formation.

We present the results of an optical multi-band survey for low-mass Pre-Main Sequence (PMS) stars and young Brown Dwarfs (BDs) in the Chamaeleon II (Cha II) dark cloud. This survey constitutes the complementary optical data to the c2d Spitzer Legacy survey in Cha II. Using the Wide-Field Imager (WFI) at the ESO 2.2m telescope, we surveyed a sky area of about 1.75 square degrees in Cha II. The region was observed in the Rc, Ic and z broad-bands, in H-alpha and in two medium-band filters centered at 856 and 914 nm. We select PMS star and young BD candidates using colour-magnitude diagrams (CMDs) and theoretical isochrones reproduced ad-hoc for the WFI at the ESO2.2m telescope system. The selection criteria are also reinforced by using the previously known PMS stars in Cha II to define the PMS locus on the CMDs and by investigating the infrared (IR) colours of the candidates. By exploiting the WFI intermediate-band photometry we also estimate the effective temperature and the level of H-alpha emission of the candidates. Our survey, which is one of the largest and deepest optical surveys conducted so far in Cha II, recovered the majority of the PMS stars and 10 member candidates of the cloud from previous IR surveys. In addition, the survey revealed 10 new potential members. From our photometric characterisation, we estimate that some 50% of the 20 candidates will result in true Cha II members. Based on our temperature estimates, we conclude that several of these objects are expected to be sub-stellar and give a first estimate of the fraction of sub-stellar objects.

The distance to the Orion Nebula cluster (ONC) is estimated using the rotational properties of its low-mass pre-main-sequence (PMS) stars. Rotation periods, projected equatorial velocities and distance-dependent radius estimates are used to form an observational sin i distribution (where i is the axial inclination), which is modelled to obtain the distance estimate. A distance of 440 ± 34 pc is found from a sample of 74 PMS stars with spectral types between G6 and M2, but this falls to 392 ± 32 pc when PMS stars with accretion discs are excluded on the basis of their near-infrared excess. Since the radii of accreting stars are more uncertain and probably systematically underestimated, then this closer distance is preferred. The quoted uncertainties include statistical errors and uncertainties due to a number of systematic effects including binarity and inclination bias. This method is geometric and independent of stellar evolution models, though does rely on the assumption of random axial orientations and the Cohen & Kuhi effective temperature scale for PMS stars. The new distance is consistent with, although lower and more precise, than most previous ONC distance estimates. A closer ONC distance implies smaller luminosities and an increased age based on the positions of PMS stars in the Hertzsprung–Russell diagram.

A growing disquiet has emerged in recent years that standard stellar models are at odds with observations of the colour-magnitude diagrams (CMDs) and lithium depletion patterns of pre main sequence (PMS) stars in clusters. In this work we select 1,246 high probability K/M-type constituent members of 5 young open clusters (5–125 Myr) in the Gaia-ESO Survey to test a series of models that use standard input physics and others that incorporate surface magnetic fields or cool starspots. We find that: standard models provide systematically under-luminous isochrones for low-mass stars in the CMD and fail to predict Li-depletion of the right strength at the right colour; magnetic models provide better CMD fits with isochrones that are ∼1.5 − 2 times older, and provide better matches to Li depletion patterns. We investigate how rotation periods, most of which are determined here for the first time from Transiting Exoplanet Survey Satellite data, correlate with CMD position and Li. Among the K-stars in the older clusters we find the brightest and least Li-depleted are the fastest rotators, demonstrating the classic ‘Li-rotation connection’ for the first time at ∼35 Myr in NGC 2547, and finding some evidence that it exists in the early M-stars of NGC 2264 at <10 Myr. However, the wide dispersion in Li depletion observed in fully-convective M-dwarfs in the γ Vel cluster at ∼20 Myr appears not to be correlated with rotation and is challenging to explain without a very large (>10 Myr) age spread.

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.

view Abstract Citations (150) References (71) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Multiwavelength Study of Star Formation in the L1495E Cloud in Taurus Strom, Karen M. ; Strom, Stephen E. Abstract We have carried out a deep (t=30000s) x-ray search of the eastern portion of the L1495 cloud centered on the well known weak line T Tauri star (WTTS) V410 Tau using the ROSAT PSPC. This deep exposure enabled a search for candidate pre-main sequence (PMS) objects in this cloud to a limit 20 times more sensitive than that typical of the fields examined with the Einstein searches. Despite assertions that the PMS population in Taurus-Auriga is nearly completely known, this x-ray survey revealed 8 new PMS objects in a region 50' in diameter, as compared to a previously known stellar population of 12 objects, including deeply embedded IRAS sources. Spectroscopic and photometric observations enable us to place these objects in the HR Diagram. The newly discovered objects are predominantly stars of spectral type M0 and later, and a large fraction (6/8) appear to be surrounded by circumstellar accretion disks as judged by their infrared excess and H alpha emission. We combined the data for these x-ray discovered objects with extant and new data for the previously identified PMS stars in this region to examine the history of star formation and the frequency distribution of stellar masses in this cloud. If the ``post ROSAT'' population is either complete or representative, we conclude (1) that star formation in L1495 East took place 1 x 10^6 yrs ago and that the spread in ages is small; (2) the frequency distribution of masses, Publication: The Astrophysical Journal Pub Date: March 1994 DOI: 10.1086/173886 arXiv: arXiv:astro-ph/9309037 Bibcode: 1994ApJ...424..237S Keywords: Interstellar Matter; Molecular Clouds; Pre-Main Sequence Stars; Star Formation; Stellar Envelopes; X Ray Astronomy; Accretion Disks; Hertzsprung-Russell Diagram; Stellar Spectrophotometry; Astrophysics; STARS: CIRCUMSTELLAR MATTER; ISM: INDIVIDUAL ALPHANUMERIC: L1495; STARS: FORMATION; STARS: PRE--MAIN-SEQUENCE; X-RAYS: STARS; Astrophysics E-Print: Latex, AASTeX macros, version 3 full text sources arXiv | ADS | data products SIMBAD (63)

This thesis presents a detailed investigation of the rotational behaviour of young pre-main sequence (PMS) stars in order to achieve a better understanding of their angular momentum evolution during this evolutionary stage. For that purpose I have carried out an extensive photometric monitoring program which enabled me to identify a total of 405 periodic and 184 irregular variable PMS stars in the young open cluster NGC 2264 (age: 2-4 Myr). Hence, I could increase the number of known PMS stars in NGC 2264 by more than a factor of three and the number of published periods by more than a factor of ten. Apart from the Orion Nebular Cluster (ONC, age: 1 Myr), with about 370 known rotation periods, NGC 2264 is therefore the only cluster for which a statistically significant number of rotation periods of PMS stars is known. For the first time it was possible to compare the rotational behaviour of stars of two young clusters with each other. I have shown that the period distribution of stars in NGC 2264 strongly depends on mass and quantitatively agrees with that of the ONC. However, the stars in NGC 2264 rotate with shorter rotation periods on average. A quantitative analysis which took into account the age ratio and the different stellar radii showed that a large fraction of stars spin up with conserved angular momentum while increasing in age. However, I also found that some stars clearly rotate with longer rotation periods even though they are aging. Apparently, these stars lose angular momentum, which is interpreted as a result of magnetic coupling to their circumstellar disk (``disk-locking''). The resulting locking period is about 8 days. My analysis further showed that ``disk-locking'' in NGC\,2264 or the ONC is important only for higher mass stars (M >= 0.3$\msun$). There is evidence that also the lower mass stars (M <= 0.3$\msun$) interact magnetically with their disks, but this interaction is in most cases not strong enough to remove angular momentum with sufficiently high rates to lock the star with a constant rotation period. These findings are closely connected with a surprising result. The typical brightness variations of the stars in NGC 2264 (caused by star spots) differ for the two mass regimes considered here. While the peak-to-peak variation of the higher mass stars is typically up to 0.2 mag, the lower mass stars show brightness modulations only up to 0.06 mag. I argue that this is evidence for a different magnetic field structure, caused by different dynamo processes, and could be the decisive factor for the different rotational behaviour of the lower mass stars.

We present the project we are carrying out at present, the search for and characterisation of pre-main sequence (PMS) stars among the members of Galactic young clusters. The observations of 10 southern clusters, nine of them located in the Carina-Sagittarius spiral arm of the Milky Way are presented. We aim at listing candidate PMS member stars in young clusters. The catalogued stars will serve as a basis for future spectroscopic studies of individual objects to determine the properties of stellar formation in the last phases before the main sequence stage. Properties such as the presence of residual envelopes or disks, age spread among PMS members, and the possible presence of several episodes of star formation in the clusters, are to be addressed. Multicolour photometry in the UBVRcIc system has been obtained for 10 southern young clusters in the fourth Galactic quadrant, located between Galactic longitudes l=238 and l=310. For six clusters in the sample, the observations presented here provide the first published study based on CCD photometry. A quantitative comparison is performed with post-MS isochrones, and PMS isochrones from three different evolutionary models are used in the photometric membership analysis for possible PMS stars. The observations produce photometric indices in the Johnson-Cousins photometric systems for a total of 26962 stars. Matching with the 2MASS data base provides astrometric calibration for all stars, and JHK 2MASS indices for 60 per cent of them. Post-MS cluster ages range from 4 to 60 Myr, whereas the photometric membership analysis assigns PMS membership to a total of 842 stars, covering an age range between 1 and 10 Myr. A catalogue, named DAY-I, with the information on the PMS candidate members has been ellaborated.

We use the Geneva &lt;scp&gt;syclist&lt;/scp&gt; isochrone models that include the effects of stellar rotation to investigate the role that rotation has on the resulting colour–magnitude diagram of young and intermediate age clusters. We find that if a distribution of rotation velocities exists within the clusters, rotating stars will remain on the main sequence for longer, appearing to be younger than non-rotating stars within the same cluster. This results in an extended main sequence turn-off (eMSTO) that appears at young ages (∼30 Myr) and lasts beyond 1 Gyr. If this eMSTO is interpreted as an age spread, the resulting age spread is proportional to the age of the cluster, i.e. young clusters (&lt;100 Myr) appear to have small age spreads (tens of Myr) whereas older clusters (∼1 Gyr) appear to have much large spreads, up to a few hundred Myr. We compare the predicted spreads for a sample of rotation rates to observations of young and intermediate age clusters, and find a strong correlation between the measured ‘age spread’ and the age of the cluster, in good agreement with models of stellar rotation. This suggests that the ‘age spreads’ reported in the literature may simply be the result of a distribution of stellar rotation velocities within clusters.

We revisit the problem of low-mass pre-main-sequence (PMS) stellar evolution and its observational consequences for where stars fall on the Hertzsprung-Russell diagram (HRD). In contrast to most previous work, our models follow stars as they grow from small masses via accretion, and we perform a systematic study of how the stars' HRD evolution is influenced by their initial radius, by the radiative properties of the accretion flow, and by the accretion history, using both simple idealized accretion histories and histories taken from numerical simulations of star cluster formation. We compare our numerical results to both non-accreting isochrones and to the positions of observed stars in the HRD, with a goal of determining whether both the absolute ages and the age dispersions inferred from non-accreting isochrones are reliable. We show that non-accreting isochrones can sometimes overestimate stellar ages for more massive stars (those with effective temperatures above \sim 3500 K), thereby explaining why non-accreting isochrones often suggest a systematic age difference between more and less massive stars in the same cluster. However, we also find the only way to produce a similar overestimate for the ages of cooler stars is by selecting parameters that are strongly inconsistent with both theoretical models and observational constraints. We therefore conclude that inferred ages and age spreads for cool stars are reliable, at least to the extent that the observed bolometric luminosities and temperatures are accurate. Finally, we note that the time-dependence of the mass accretion rate has remarkably little effect on low-mass stars' evolution on the HRD, and that such time-dependence may be neglected for all stars except those with effective temperatures above \sim 4000 K.

In light of recent substantial updates to spectral type estimations and newly established intrinsic colours, effective temperatures, and bolometric corrections for pre-main sequence (PMS) stars, we re-address the theory of accretion-disc regulated stellar angular momentum (AM) evolution. We report on the compilation of a consistent sample of fully convective stars within two of the most well-studied and youngest, nearby regions of star formation: the Orion Nebula Cluster (ONC) and Taurus-Auriga. We calculate the average specific stellar AM ($j_{\star}$) assuming solid body rotation, using surface rotation periods gathered from the literature and new estimates of stellar radii and ages. We use published Spitzer IRAC fluxes to classify our stars as Class II or Class III and compare their $j_{\star}$ evolution. Our results suggest that disc dispersal is a rapid process that occurs at a variety of ages. We find a consistent $j_{\star}$ reduction rate between the Class II and Class III PMS stars which we interpret as indicating a period of accretion disc-regulated AM evolution followed by near-constant AM evolution once the disc has dissipated. Furthermore, assuming our observed spread in stellar ages is real, we find the removal rate of $j_{\star}$ during the Class II phase is more rapid than expected by contraction at constant stellar rotation rate. A much more efficient process of AM removal must exist, most likely in the form of an accretion-driven stellar wind or other outflow from the star-disc interaction region or extended disc surface.