Low-mass Pre-main Sequence Stars Research Articles

Evidence for transit-timing variations of the 11 Myr exoplanet TOI-1227 b

TOI-1227 b is an 11 Myr old validated transiting planet in the middle of its contraction phase, with a current radius of 0.85 Rj. It orbits a low-mass pre-main sequence star (0.170 M⊙, 0.56 R⊙) every 27.4 days. The magnetic activity of its young host star induces radial velocity jitter and prevents good measurements of the planetary mass. We gathered additional transit observations of TOI-1227 b with space- and ground-based telescopes, and we detected highly significant transit-timing variations (TTVs). Their amplitude is about 40 min and their dominant timescale is longer than 3.7 yr. Their most probable origin is dynamical interactions with additional planets in the system. We modeled the TTVs with inner and outer perturbers near first and second order resonances; several orbital configurations provide an acceptable fit. More data are needed to determine the actual orbital configuration and eventually measure the planetary masses. These TTVs and an updated transit chromaticity analysis reinforce the evidence that TOI-1227 b is a planet.

Accretion and magnetism on young eccentric binaries: DQ Tau and AK Sco

ABSTRACT The accretion and ejection of mass in pre-main-sequence (PMS) stars are key processes in stellar evolution as they shape the stellar angular momentum transport necessary for the stars’ stability. Magnetospheric accretion on to classical T Tauri stars and low-mass PMS stars has been widely studied in the single-star case. This process cannot be directly transferred to PMS binary systems, as tidal and gravitation effects, and/or accretion from a circumbinary disc (with variable separation of the components in the case of eccentric orbits) are in place. This work examines the accretion process of two PMS eccentric binaries, DQ Tau and AK Sco, using high-resolution spectropolarimetric time series. We investigate how magnetospheric accretion can be applied to these systems by studying the accretion-related emission lines and the magnetic field of each system. We discover that both systems are showing signs of magnetospheric accretion, despite their slightly different configurations, and the weak magnetic field of AK Sco. Furthermore, the magnetic topology of DQ Tau A shows a change relative to the previous orbital cycle studied: previously dominated by the poloidal component, it is now dominated by the toroidal component. We also report an increase of the component’s accretion and the absence of an accretion burst at the apastron, suggesting that the component’s magnetic variation might be the cause of the inter-cycle variations of the system’s accretion. We conclude on the presence of magnetospheric accretion for both systems, together with gravitational effects, especially for AK Sco, composed of more massive components.

Time-series photometry and multiwavelength characterization of the young stellar cluster Mon R2

ABSTRACT Using the Las Cumbres Observatory Global Telescope Network (LCOGT), we have obtained multi-epoch photometry of the young cluster Mon R2. We have monitored over 6000 sources with i-band between 13 and 23 mag within a 26 × 26 arcmin2 field of view. For each star, we collected ∼1500 photometric points covering a temporal window of 23 d. Based on these data, we have measured rotation-modulated of 136 stars and identified around 90 additional variables, including 14 eclipsing binary candidates. Moreover, we found 298 other variables with photometric high-scatter. In addition, we have obtained r-band and Hα narrow-band photometry of the cluster with LCOGT and low-resolution optical spectroscopy of 229 stars with GMOS-Gemini. We used the Gaia data from the periodic stars and objects with Hα or IR-excesses, which are mostly low-mass pre-main sequence stars (<1 M⊙) in the cluster to estimate the distance (825 ± 51 pc) and the mean proper motions (μαcos(δ) = −2.75 mas yr−1 and μδ = 1.15 mas yr−1) of its members. This allows us to use the Gaia data to identify additional Mon R2 member candidates. We also used Pan-STARRS photometry from our LCOGT sources to construct a more precise H-R diagram, from which we estimate the mean age of the cluster and identify other possible members including eleven spectroscopy brown dwarf with M7 to M9 GMOS spectral types. Finally, we combined our membership lists with Spitzer infrared photometry to investigate the incidence of stars with discs and the effect these have on stellar rotation.

The Effelsberg survey of FU Orionis and EX Lupi objects

Context. FU Orionis (FUor) and EX Lupi (EXor) type objects are two groups of peculiar and rare pre-main sequence low-mass stars that are undergoing powerful accretion outbursts during their early stellar evolution. Although water masers are widespread in star-forming regions and serve as powerful probes of mass accretion and ejection on small scales, little is known about the prevalence of water masers toward FUors and EXors. Aims. We aim to perform the first systematic search for the 22.2 GHz water maser line in FUors and EXors in order to determine its overall incidence in these eruptive variables and to facilitate high angular-resolution follow-up observations. Methods. We used the Effelsberg 100-m radio telescope to observe the H2O (616 – 523) transition at 22.2GHz toward a sample of 51 eruptive young stellar objects. Results. We detected a total of five water masers in our survey; three are associated with eruptive stars, equivalent to a detection rate of ~6% for our sample of eruptive sources. These detections include one EXor, V512 Per (also known as SVS 13 or SVS 13A), and two FUors, Z CMa, and HH 354 IRS. This is the first reported detection of water maser emission towards HH 354 IRS. We also detected water maser emission in our pointing towards the FUor binary RNO 1B/1C, which most likely originates from the nearby deeply embedded source IRAS 00338+6312 (~4″ from RNO 1B/1C). Emission was also detected from H2O(B), (also known as SVS 13C), a Class 0 source ~30″ from the EXor V512 Per. The peak flux density of H2O(B) in our observations, 498.7 Jy, is the highest observed to date, indicating that we have serendipitously detected a water maser flare in this source. In addition to the two non-eruptive Class 0 sources (IRAS 00338+6312 and H2O(B)/SVS 13C), we detected maser emission towards one Class 0/I (HH 354 IRS) and two Class I (V512 Per/SVS 13A and Z CMa) eruptive stars. Conclusions. Despite the low detection rate, we demonstrate the presence of 22.2 GHz water maser emission in both FUor and EXor systems, paving the way to radio interferometric observations to study the environments of these eruptive stars on small scales. Comparisons of our data with historical observations spanning several decades suggest that multiple water maser flares have occurred in both V512 Per and H2O(B).

Magneto-convective Modeling of Low-mass Pre-main Sequence Stars: Postponing the Depletion of Lithium

Standard evolutionary models of pre-main sequence (PMS) low-mass stars often predict radii R and Li abundances A(7Li) which are too small to agree with empirical data, while effective temperatures T eff are predicted to be too large. There are also discrepancies in replicating the stellar age. Discrepancies in R, T eff, and A(7Li) are reported by Stassun et al. for both components (A/B) of the eclipsing binary 2M1222-57 belonging to the young Lower Centaurus Crux association. Using the output of a magneto-convective stellar evolution model which we applied in an early study (in 2010) of PMS stars in a different young association, we show that the discrepancies reported by Stassun et al. in the 2M1222-57 system are alleviated. The models are instructive in showing why larger lithium abundances occur in magnetic PMS stars: it takes longer for the central gas in such a star to become hot enough to destroy lithium.

H2O masers and host environments of FU Orionis and EX Lupi type low-mass eruptive YSOs

AbstractThe FU Orionis (FUor) and EX Lupi (EXor) type objects are rare pre-main sequence low-mass stars undergoing accretion outbursts. Maser emission is widespread and is a powerful probe of mass accretion and ejection on small scales in star forming region. However, very little is known about the overall prevalence of water masers towards FUors/Exors. We present results from our survey using the Effelsberg 100-m telescope to observe the largest sample of FUors and EXors, plus additional Gaia alerted sources (with the potential nature of being eruptive stars), a total of 51 targets, observing the 22.2 GHz H2O maser, while simultaneously covering the NH3 23 GHz.

Infrared variability of young solar analogues in the Lagoon Nebula

ABSTRACT T Tauri stars are low-mass pre-main sequence stars that are intrinsically variable. Due to the intense magnetic fields they possess, they develop dark spots on their surface that, because of rotation, introduce a periodic variation of brightness. In addition, the presence of surrounding discs could generate flux variations by variable extinction or accretion. Both can lead to a brightness decrease or increase, respectively. Here, we have compiled a catalogue of light curves for 379 T Tauri stars in the Lagoon Nebula (M8) region, using VVVX survey data in the Ks-band. All these stars were already classified as pre-MS stars based on other indicators. The data presented here are spread over a period of about eight years, which gives us a unique follow-up time for these sources at this wavelength. The light curves were classified according to their degree of periodicity and asymmetry, to constrain the physical processes responsible for their variation. Periods were compared with the ones found in literature, on a much shorter baseline. This allowed us to prove that for 126 stars, the magnetically active regions remain stable for several years. Besides, our near-IR data were compared with the optical Kepler/K2 light curves, when available, giving us a better understanding of the mechanisms responsible for the brightness variations observed and how they manifest at different bands. We found that the periodicity in both bands is in fairly good agreement, but the asymmetry will depend on the amplitude of the bursts or dips events and the observation cadence.

A Study of the Photometric and Spectroscopic Variations of the Prototypical FU Orionis–type Star V1057 Cyg

Abstract Among the low-mass pre-main sequence stars, a small group called FU Orionis–type objects (FUors) are notable for undergoing powerful accretion outbursts. V1057 Cyg, a classical example of an FUor, went into outburst around 1969–1970, after which it faded rapidly, making it the fastest-fading FUor known. Around 1995, a more rapid increase in fading occurred. Since that time, strong photometric modulations have been present. We present nearly 10 yr of source monitoring at Piszkéstető Observatory, complemented with optical/NIR photometry and spectroscopy from the Nordic Optical Telescope, Bohyunsan Optical Astronomy Observatory, Transiting Exoplanet Survey Satellite, and Stratospheric Observatory for Infrared Astronomy. Our light curves show continuation of significant quasi-periodic variability in brightness over the past decade. Our spectroscopic observations show strong wind features, shell features, and forbidden emission lines. All of these spectral lines vary with time. We also report the first detection of [S ii], [N ii], and [O iii] lines in the star.

Time-domain Study of the Young Massive Cluster Westerlund 2 with the Hubble Space Telescope. I

Abstract Time-domain studies of pre-main-sequence (PMS) stars have long been used to investigate star properties during their early evolutionary phases and to trace the evolution of circumstellar environments. Historically these studies have been confined to the nearest, low-density, star-forming regions. We used the Wide Field Camera 3 on board the Hubble Space Telescope to extend, for the first time, the study of PMS variability to one of the few young massive clusters in the Milky Way, Westerlund 2. Our analysis reveals that at least one-third of the intermediate- and low-mass PMS stars in Westerlund 2 are variable. Based on the characteristics of their light curves, we classified ∼11% of the variable stars as weak-line T Tauri candidates, ∼52% as classical T Tauri candidates, ∼5% as dippers, and ∼26% as bursters. In addition, we found that 2% of the stars below 6 M ⊙ (∼6% of the variables) are eclipsing binaries, with orbital periods shorter than 80 days. The spatial distribution of the different populations of variable PMS stars suggests that stellar feedback and UV radiation from massive stars play an important role in the evolution of circumstellar and planetary disks.

Resolving the FU Orionis System with ALMA: Interacting Twin Disks?

Abstract FU Orionis objects are low-mass pre-main sequence stars characterized by dramatic outbursts several magnitudes in brightness. These outbursts are linked to episodic accretion events in which stars gain a significant portion of their mass. The physical processes behind these accretion events are not yet well understood. The archetypal FU Ori system, FU Orionis, is composed of two young stars with detected gas and dust emission. The continuum emitting regions have not been resolved until now. Here, we present 1.3 mm observations of the FU Ori binary system using the Atacama Large Millimeter/submillimeter Array. The disks are resolved at 40 mas resolution. Radiative transfer modeling shows that the emission from FU Ori north (primary) is consistent with a dust disk with a characteristic radius of ∼11 au. The ratio between the major and minor axes shows that the inclination of the disk is ∼37°. FU Ori south is consistent with a dust disk of similar inclination and size. Assuming the binary orbit shares the same inclination angle as the disks, the deprojected distance between the north and south components is 0.″6, i.e., ∼250 au. Maps of 12CO emission show a complex kinematic environment with signature disk rotation at the location of the northern component, and also (to a lesser extent) for FU Ori south. The revised disk geometry allows us to update FU Ori accretion models, yielding a stellar mass and mass accretion rate of FU Ori north of 0.6 M ⊙ and 3.8 × 10−5 M ⊙ yr−1, respectively.

An extensive VLT/X-shooter library of photospheric templates of pre-main sequence stars

Studies of the formation and evolution of young stars and their disks rely on the knowledge of the stellar parameters of the young stars. The derivation of these parameters is commonly based on comparison with photospheric template spectra. Furthermore, chromospheric emission in young active stars impacts the measurement of mass accretion rates, a key quantity to study disk evolution. Here we derive stellar properties of low-mass pre-main sequence stars without disks, which represent ideal photospheric templates for studies of young stars. We also use these spectra to constrain the impact of chromospheric emission on the measurements of mass accretion rates. The spectra in reduced, flux-calibrated, and corrected for telluric absorption form are made available to the community. We derive the spectral type for our targets by analyzing the photospheric molecular features present in their VLT/X-Shooter spectra by means of spectral indices and comparison of the relative strength of photospheric absorption features. We also measure effective temperature, gravity, projected rotational velocity, and radial velocity from our spectra by fitting them with synthetic spectra with the ROTFIT tool. The targets have negligible extinction and spectral type from G5 to M8. We perform synthetic photometry on the spectra to derive the typical colors of young stars in different filters. We measure the luminosity of the emission lines present in the spectra and estimate the noise due to chromospheric emission in the measurements of accretion luminosity in accreting stars. We provide a calibration of the photospheric colors of young PMS stars as a function of their spectral type in a set of standard broad-band optical and near-infrared filters. For stars with masses of ~ 1.5Msun and ages of ~1-5 Myr, the chromospheric noise converts to a limit of measurable mass accretion rates of ~ 3x10^-10 Msun/yr.

Mid-infrared interferometric variability of DG Tauri: Implications for the inner-disk structure

Context. DG Tau is a low-mass pre-main sequence star, whose strongly accreting protoplanetary disk exhibits a so-far enigmatic behavior: its mid-infrared thermal emission is strongly time-variable, even turning the 10 $\mu$m silicate feature from emission to absorption temporarily. Aims. We look for the reason for the spectral variability at high spatial resolution and at multiple epochs. Methods. We study the temporal variability of the mid-infrared interferometric signal, observed with the VLTI/MIDI instrument at six epochs between 2011 and 2014. We fit a geometric disk model to the observed interferometric signal to obtain spatial information about the disk. We also model the mid-infrared spectra by template fitting to characterize the profile and time dependence of the silicate emission. We use physically motivated radiative transfer modeling to interpret the mid-infrared interferometric spectra. Results. The inner disk (r<1-3 au) spectra exhibit a 10 $\mu$m absorption feature related to amorphous silicate grains. The outer disk (r>1-3 au) spectra show a crystalline silicate feature in emission, similar to the spectra of comet Hale-Bopp. The striking difference between the inner and outer disk spectral feature is highly unusual among T Tauri stars. The mid-infrared variability is dominated by the outer disk. The strength of the silicate feature changed by more than a factor of two. Between 2011 and 2014 the half-light radius of the mid-infrared-emitting region decreased from 1.15 to 0.7 au. Conclusions. For the origin of the absorption we discuss four possible explanations: a cold obscuring envelope, an accretion heated inner disk, a temperature inversion on the disk surface and a misaligned inner geometry. The silicate emission in the outer disk can be explained by dusty material high above the disk plane, whose mass can change with time, possibly due to turbulence in the disk.

X-shooter spectroscopy of young stellar objects

Hydrogen recombination emission lines commonly observed in accreting young stellar objects represent a powerful tracer for the gas conditions in the circumstellar structures. Here we perform a study of the HI decrements and line profiles, from the Balmer and Paschen lines detected in the X-Shooter spectra of a homogeneous sample of 36 T Tauri stars in Lupus, the accretion and stellar properties of which were already derived in a previous work. We aim to obtain information on the gas physical conditions to derive a consistent picture of the HI emission mechanisms in pre-main sequence low-mass stars. We have empirically classified the sources based on their HI line profiles and decrements. We identified four Balmer decrement types (classified as 1, 2, 3, and 4) and three Paschen decrement types (A, B, and C), characterised by different shapes. We first discussed the connection between the decrement types and the source properties and then compared the observed decrements with predictions from recently published local line excitation models. One third of the objects show lines with narrow symmetric profiles, and present similar Balmer and Paschen decrements (straight decrements, types 2 and A). Lines in these sources are consistent with optically thin emission from gas with hydrogen densities of order 10^9 cm^-3 and 5000<T<15000 K. These objects are associated with low mass accretion rates. Type 4 (L-shaped) Balmer and type B Paschen decrements are found in conjunction with very wide line profiles and are characteristic of strong accretors, with optically thick emission from high-density gas (log n_H > 11 cm^-3). Type 1 (curved) Balmer decrements are observed only in three sub-luminous sources viewed edge-on, so we speculate that these are actually reddened type 2 decrements. About 20% of the objects present type 3 Balmer decrements (bumpy), which cannot be reproduced with current models.

X-ray and optical spectroscopy of the massive young open cluster IC 1805

Very young open clusters are ideal places to study the X-ray properties of a homogeneous population of early-type stars. In this respect, the IC1805 open cluster is very interesting as it hosts the O4If$^+$ star HD15570 thought to be in an evolutionary stage intermediate between a normal O-star and a Wolf-Rayet star. Such a star could provide a test for theoretical models aiming at explaining the empirical scaling relation between the X-ray and bolometric luminosities of O-type stars. We have observed IC1805 with XMM-Newton and further collected optical spectroscopy of some of the O-star members of the cluster. The optical spectra allow us to revisit the orbital solutions of BD+60$^{\circ}$ 497 and HD15558, and provide the first evidence of binarity for BD+60$^{\circ}$ 498. X-ray emission from colliding winds does not appear to play an important role among the O-stars of IC1805. Notably, the X-ray fluxes do not vary significantly between archival X-ray observations and our XMM-Newton pointing. The very fast rotator BD+60$^{\circ}$ 513, and to a lesser extent the O4If$^+$ star HD15570 appear somewhat underluminous. Whilst the underluminosity of HD15570 is only marginally significant, its amplitude is found to be compatible with theoretical expectations based on its stellar and wind properties. A number of other X-ray sources are detected in the field, and the brightest objects, many of which are likely low-mass pre-main sequence stars, are analyzed in detail.

TheGaia-ESO Survey: A lithium-rotation connection at 5 Myr?

We investigate here the lithium content of young low-mass stars in the 5~Myr-old star forming region NGC~2264 and its relationship with rotation. We combine lithium equivalent width measurements, EW(Li), from the Gaia-ESO Survey with the determination of rotational periods from the CSI~2264 survey. We consider only bona fide non accreting cluster members in order to minimize uncertainties on EW(Li). We report the existence of a relationship between lithium content and rotation in NGC~2264 at an age of 5~Myr. The Li-rotation connection is seen over a restricted temperature range, Teff=3800--4400K, where fast rotators are Li-rich compared to slow ones. This correlation is similar to, albeit of lower amplitude than, the Li-rotation connection previously reported for K dwarfs in the 125 Myr-old Pleiades cluster. We investigate whether the non-standard pre-main sequence models developed so far to explain the Pleiades results, which are based on episodic accretion, pre-main sequence core-envelope decoupling, and/or radius inflation due to enhanced magnetic activity, can account for an early development of the Li-rotation connection. While radius inflation appears to be the most promising possibility, each of these models has issues. We therefore also discuss external causes that might operate during the first few Myr of pre-main sequence evolution, such as planet engulfment and/or steady disk accretion, as possible candidates for the common origin for Li-excess and fast rotation in young low-mass pre-main sequence stars. The emergence of a connection between lithium content and rotation rate at such an early age as 5~Myr suggests a complex link between accretion processes, early angular momentum evolution, and possibly planet formation, which likely impacts early stellar evolution and still is to be fully deciphered.

THE INTERMEDIATE-MASS STAR-FORMING REGION LYNDS 1340. AN OPTICAL VIEW

ABSTRACT We have performed an optical spectroscopic and photometric search for young stellar objects associated with the molecular cloud Lynds 1340, and examined the structure of the cloud by constructing an extinction map, based on SDSS data. The new extinction map suggests a shallow, strongly fragmented cloud, having a mass of some 3700 M ☉. Longslit spectroscopic observations of the brightest stars over the area of L1340 revealed that the most massive star associated with L1340 is a B4-type, ∼5 M ☉ star. The new spectroscopic and photometric data of the intermediate-mass members led to a revised distance of pc, and revealed seven members of the young stellar population with M ≳ 2 M ☉. Our search for Hα emission line stars, conducted with the Wide Field Grism Spectrograph 2 on the 2.2 m telescope of the University of Hawaii and covering a 30′ × 40′ area, resulted in the detection of 75 candidate low-mass pre-main sequence stars, 58 of which are new. We constructed spectral energy distributions (SEDs) of our target stars, based on SDSS, 2MASS, Spitzer, and WISE photometric data, derived their spectral types, extinctions, and luminosities from BVRIJ fluxes, estimated masses by means of pre-main sequence evolutionary models, and examined the disk properties utilizing the 2–24 μm interval of the SED. We measured the equivalent width of the Hα lines and derived accretion rates. The optically selected sample of pre-main sequence stars has a median effective temperature of 3970 K, a stellar mass of 0.7 M ☉, and an accretion rate of 7.6 × 10−9 M ☉ yr−1.

OPTICAL SPECTROSCOPY OF X-RAY-SELECTED YOUNG STARS IN THE CARINA NEBULA

We present low-resolution optical spectra for 29 X-ray sources identified as either massive star candidates or low-mass pre-main-sequence (PMS) star candidates in the clusters Trumpler 16 and Trumpler 14 of the Carina Nebula. Spectra of two more objects (one with an X-ray counterpart, and one with no X-ray counterpart), not originally our targets, but found close (~3'') to two of our targets, are presented as well. Twenty early-type stars, including an O8 star, seven B1–B2 stars, two B3 stars, a B5 star, and nine emission-line stars, are identified. Eleven T Tauri stars, including eight classical T Tauri stars (CTTSs) and three weak-lined T Tauri stars, are identified. The early-type stars in our sample are more reddened compared to the previously known OB stars of the region. The Chandra hardness ratios of our T Tauri stars are found to be consistent with the Chandra hardness ratios of T Tauri stars of the Orion Nebula Cluster. Most early-type stars are found to be nonvariable in X-ray emission, except the B2 star J104518.81–594217.9, the B3 star J104507.84–594134.0, and the Ae star J104424.76–594555.0, which are possible X-ray variables. J104452.20–594155.1, a CTTS, is among the brightest and the hardest X-ray sources in our sample, appears to be a variable, and shows a strong X-ray flare. The mean optical and near-infrared photometric variability in the V and Ks bands, of all sources, is found to be ~0.04 and 0.05 mag, respectively. The T Tauri stars show significantly larger mean variation, ~0.1 mag, in the Ks band. The addition of one O star and seven B1–B2 stars reported here contributes to an 11% increase of the known OB population in the observed field. The 11 T Tauri stars are the first ever confirmed low-mass PMS stars in the Carina Nebula region.

NEAR-IR PHOTOMETRIC AND OPTICAL SPECTROSCOPIC STUDY OF THE FU ORIONIS OBJECT V582 AURIGAE

We carried out near-IR photometric and optical spectroscopic observations of V582 Aur, which isa FU Orionis type object, to investigate any periodic and/or aperiodic variations. We obtained lightcurves on the scale of a night and a year, in J, H and Ks bands with KASINICS (KASI Near InfraredCamera System) attached to the BOAO (Bohyun-san Optical Astronomy Observatory) 1.8-m reector inYoungcheon, South Korea and examined photometric variations on the two time scales. So far we have notfound any periodic brightness variations on the scale of a night. On the other hand, we have found thatthere seems to be a periodic brightness variation with a period of approximately 45 days. In addition,high-resolution optical spectroscopic observations of V582 Aur were performed from February 2013 toMay 2014 with the high-resolution echelle spectrograph BOES attached to the BOAO 1.8-m reector.We analyzed several spectral lines to understand the physical state of V582 Aur. The P Cyg pro les areclearly shown in the H line and Na I D line.Key words: Pre-main sequence variables: FU Orionis type object: Periodic brightness variation: PCyg pro le1. INTRODUCTIONV582 Aur, which is located in an active star formationregion near the Auriga OB2 association, is a new FUorobject candidate. The increase in brightness of this ob-ject began in late 1984 or early 1985 and it reached amaximum brightness in January 1986. After the max-imum light, the brightness of this star has remained ina state of maximum brightness. The light variation ofV582 Aur was rst reported by Anton Khruslov (Samus,2009) and it was con rmed as an FUor object by Semkovet al. (2011; 2013). An FUor object is a low-mass pre-main sequence star de ned by Herbig (1977). FUors gothrough an outburst with an amplitude of V ˇ 4{6mag. They are located in star forming regions and areassociated with reection nebula. In addition, they showthe spectra of F-G supergiants during the outburst, astrong Li I 6707A absorption line, and near-infrared COband (Herbig, 1977; Reipurth & Aspin, 2010). One ofthe most important spectroscopic features of FUors isthe P Cyg pro le, which is commonly displayed in theNa I D and H lines, occurring due to strong winds fromthe center of star. Some FUors were found to exhibitperiodic spectroscopic (Herbig et al., 2003; Powell et

Multiple populations in globular clusters: the distinct kinematic imprints of different formation scenarios

Several scenarios have been proposed to explain the presence of multiple stellar populations in globular clusters. Many of them invoke multiple generations of stars to explain the observed chemical abundance anomalies, but it has also been suggested that self-enrichment could occur via accretion of ejecta from massive stars onto the circumstellar disc of low-mass pre-main sequence stars. These scenarios imply different initial conditions for the kinematics of the various stellar populations. Given some net angular momentum initially, models for which a second generation forms from gas that collects in a cooling flow into the core of the cluster predict an initially larger rotational amplitude for the polluted stars compared to the pristine stars. This is opposite to what is expected from the accretion model, where the polluted stars are the ones crossing the core and are on preferentially radial (low-angular momentum) orbits, such that their rotational amplitude is lower. Here we present the results of a suite of $N$-body simulations with initial conditions chosen to capture the distinct kinematic properties of these pollution scenarios. We show that initial differences in the kinematics of polluted and pristine stars can survive to the present epoch in the outer parts of a large fraction of Galactic globular clusters. The differential rotation of pristine and polluted stars is identified as a unique kinematic signature that could allow us to distinguish between various scenarios, while other kinematic imprints are generally very similar from one scenario to the other.

The effect of star-spots on the ages of low-mass stars determined from the lithium depletion boundary

In a coeval group of low-mass stars, the luminosity of the sharp transition between stars that retain their initial lithium and those at slightly higher masses in which Li has been depleted by nuclear reactions, the lithium depletion boundary (LDB), has been advanced as an almost model-independent means of establishing an age scale for young stars. Here we construct polytropic models of contracting pre-main sequence stars (PMS) that have cool, magnetic starspots blocking a fraction $\beta$ of their photospheric flux. Starspots slow the descent along Hayashi tracks, leading to lower core temperatures and less Li destruction at a given mass and age. The age, $\tau_{\rm LDB}$, determined from the luminosity of the LDB, $L_{\rm LDB}$, is increased by a factor $(1-\beta)^{-E}$ compared to that inferred from unspotted models, where $E \simeq 1 + d\log \tau_{\rm LDB}/d \log L_{\rm LDB}$ and has a value $\sim 0.5$ at ages $< 80$ Myr, decreasing to $\sim 0.3$ for older stars. Spotted stars have virtually the same relationship between K-band bolometric correction and colour as unspotted stars, so this relationship applies equally to ages inferred from the absolute K magnitude of the LDB. Low-mass PMS stars do have starspots, but the appropriate value of $\beta$ is highly uncertain with a probable range of $0.1 < \beta < 0.4$. For the smaller $\beta$ values our result suggests a modest systematic increase in LDB ages that is comparable with the maximum levels of theoretical uncertainty previously claimed for the technique. The largest $\beta$ values would however increase LDB ages by 20-30 per cent and demand a re-evaluation of other age estimation techniques calibrated using LDB ages.