arXiv subjects

P. A. B. Galli

Publications and source records attributed to P. A. B. Galli.

30 records · Page 2Linked to original sources

arXiv2020

Lupus DANCe. Census of stars and 6D structure with Gaia-DR2 data

Context: Lupus is recognised as one of the closest star-forming regions, but the lack of trigonometric parallaxes in the pre-Gaia era hampered many studies on the kinematic properties of this region and led to incomplete censuses of its stellar population. Aims: We use the second data release of the Gaia space mission combined with published ancillary radial velocity data to revise the census of stars and investigate the 6D structure of the Lupus complex. Methods: We performed a new membership analysis of the Lupus association based on astrometric and photometric data over a field of 160 deg2 around the main molecular clouds of the complex and compared the properties of the various subgroups in this region. Results: We identified 137 high-probability members of the Lupus association of young stars, including 47 stars that had never been reported as members before. Many of the historically known stars associated with the Lupus region identified in previous studies are more likely to be field stars or members of the adjacent Scorpius-Centaurus association. Our new sample of members covers the magnitude and mass range from G=8 to G=18 mag and from 0.03 to 2.4Msun, respectively. We compared the kinematic properties of the stars projected towards the molecular clouds Lupus 1 to 6 and showed that these subgroups are located at roughly the same distance (about 160~pc) and move with the same spatial velocity. Our age estimates inferred from stellar models show that the Lupus subgroups are coeval (with median ages ranging from about 1 to 3 Myr). The Lupus association appears to be younger than the population of young stars in the Corona-Australis star-forming region recently investigated by our team using a similar methodology. The initial mass function of the Lupus association inferred from the distribution of spectral types shows little variation compared to other star-forming regions.

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arXiv2020

The lithium-rotation connection in the newly discovered young stellar stream Psc-Eri (Meingast 1)

Context. As a fragile element, lithium is a sensitive probe of physical processes occurring in stellar interiors. Aims. We aim at investigating the relationship between lithium abundance and rotation rate in low-mass members of the newly discovered 125~Myr-old Psc-Eri stellar stream. Methods. We obtained high resolution optical spectra and measure the equivalent width of the 607.8 nm LiI line for 40 members of the Psc-Eri stream, whose rotational periods have been derived by arXiv:1905.10588. Results. We show that a tight correlation exists between lithium content and rotation rate among the late-G to early K-type stars of the Psc-Eri stream. Fast rotators are systematically Li-rich, while slow rotators are Li-depleted. This trend mimics the one previously reported for the similar age Pleiades cluster. Conclusions. The lithium-rotation connection thus seems to be universal over a restricted effective temperature range for low-mass stars at or close to the zero-age main sequence, and does not depend on environmental conditions.

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arXiv2020

Corona-Australis DANCe. I. Revisiting the census of stars with Gaia-DR2 data

Corona-Australis is one of the nearest regions to the Sun with recent and ongoing star formation, but the current picture of its stellar (and substellar) content is not complete yet. We take advantage of the second data release of the Gaia space mission to revisit the stellar census and search for additional members of the young stellar association in Corona-Australis. We applied a probabilistic method to infer membership probabilities based on a multidimensional astrometric and photometric data set over a field of 128 deg$^{2}$ around the dark clouds of the region. We identify 313 high-probability candidate members to the Corona-Australis association, 262 of which had never been reported as members before. Our sample of members covers the magnitude range between $G\gtrsim5$ mag and $G\lesssim20$ mag, and it reveals the existence of two kinematically and spatially distinct subgroups. There is a distributed `off-cloud' population of stars located in the north of the dark clouds that is twice as numerous as the historically known `on-cloud' population that is concentrated around the densest cores. By comparing the location of the stars in the HR-diagram with evolutionary models, we show that these two populations are younger than 10 Myr. Based on their infrared excess emission, we identify 28 Class II and 215 Class III stars among the sources with available infrared photometry, and we conclude that the frequency of Class II stars (i.e. `disc-bearing' stars) in the on-cloud region is twice as large as compared to the off-cloud population. The distance derived for the Corona-Australis region based on this updated census is $d=149.4^{+0.4}_{-0.4}$ pc, which exceeds previous estimates by about 20 pc.In this paper we provide the most complete census of stars in Corona-Australis available to date that can be confirmed with Gaia data.

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arXiv2019

Structure and kinematics of the Taurus star-forming region from Gaia-DR2 and VLBI astrometry

Aims:We take advantage of the second data release of the Gaia space mission and the state-of-the-art astrometry delivered from very long baseline interferometry observations to revisit the structure and kinematics of the nearby Taurus star-forming region. Methods: We apply a hierarchical clustering algorithm for partitioning the stars in our sample into groups (i.e., clusters) that are associated with the various molecular clouds of the complex, and derive the distance and spatial velocity of individual stars and their corresponding molecular clouds. Results: We show that the molecular clouds are located at different distances and confirm the existence of important depth effects in this region reported in previous studies. For example, we find that the L 1495 molecular cloud is located at $d=129.9^{+0.4}_{-0.3}$ pc, while the filamentary structure connected to it (in the plane of the sky) is at $d=160.0^{+1.2}_{-1.2}$ pc. We report B 215 and L 1558 as the closest ($d=128.5^{+1.6}_{-1.6}$ pc) and most remote ($d=198.1^{+2.5}_{-2.5}$ pc) substructures of the complex, respectively. The median inter-cloud distance is 25 pc and the relative motion of the subgroups is on the order of a few km/s. We find no clear evidence for expansion (or contraction) of the Taurus complex, but signs of the potential effects of a global rotation. Finally, we compare the radial velocity of the stars with the velocity of the underlying $^{13}$CO molecular gas and report a mean difference of $0.04\pm0.12$ km/s (with r.m.s. of 0.63 km/s) confirming that the stars and the gas are tightly coupled.

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arXiv2018

The seven sisters DANCe IV. Bayesian hierarchical model

Aims. We develop, test and characterise of a new statistical tool (intelligent system) for the sifting and analysis of nearby young open cluster (NYOC) populations. Methods. Using a Bayesian formalism, this statistical tool is able to obtain the posterior distributions of parameters governing the cluster model. It also uses hierarchical bayesian models to establish weakly informative priors, and incorporates the treatment of missing values and non-homogeneous (heteroscedastic) observational uncertainties. Results. From simulations, we estimate that this statistical tool renders kinematic (proper motion) and photometric (luminosity) distributions of the cluster population with a contamination rate of $5.8 \pm 0.2$ %. The luminosity distributions and present day mass function agree with the ones found by Bouy et al. (2015b) on the completeness interval of the survey. At the probability threshold of maximum accuracy, the classifier recovers $\sim$ 90% of Bouy et al. (2015b) candidate members and finds 10% of new ones. Conclusions. A new statistical tool for the analysis of NYOC is introduced, tested and characterised. Its comprehensive modelling of the data properties allows it to get rid of the biases present in previous works. In particular, those resulting from the use of only completely observed (non-missing) data and the assumption of homoskedastic uncertainties. Also, its Bayesian framework allows it to properly propagate observational uncertainties into membership probabilities and cluster velocity and luminosity distributions. Our results are in a general agreement with those from the literature, although we provide the most up-to-date and extended list of candidate members of the Pleiades cluster.

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arXiv2017

On the origin of the wide-orbit circumbinary giant planet HD 106906: A dynamical scenario and its impact on the disk

A giant planet has been recently resolved at a projected distance of 730 au from the tight pair of young ($\sim$ 13 Myr) intermediate-mass stars HD 106906AB in the Lower Centaurus Crux (LCC) group. The stars are surrounded by a debris disk which displays a ring-like morphology and strong asymmetries at multiple scales. We aim at studying the likelihood of a scenario where the planet formed closer to the stars in the disk, underwent inward disk-induced migration, and got scattered away by the binary star before being stabilized by a close encounter (fly-by). We performed semi-analytical calculations and numerical simulations (Swift_HJS package) to model the interactions between the planet and the two stars. We accounted for the migration as a simple force. We studied the LCC kinematics to set constraints on the local density of stars, and therefore on the fly-by likelihood. We performed N-body simulations to determine the effects of the planet trajectories (ejection and secular effects) onto the disk morphology. The combination of the migration and mean-motion resonances with the binary star (often 1:6) can eject the planet. Nonetheless, we estimate that the fly-by hypothesis decreases the scenario probability to less than $10^{-7}$ for a derived local density of stars of 0.11 stars/pc$^{3}$. We show that the concomitant effect of the planet and stars trajectories induce spiral-features in the disk which may correspond to the observed asymmetries. Moreover, the present disk shape suggests that the planet is on an eccentric orbit. The scenario we explored is a natural hypothesis if the planet formed within a disk. Conversely, its low probability of occurrence and the fact that HD 106906 b shares some characteristics with other systems in Sco-Cen (e.g. HIP 78530, in terms of mass ratio and separation) may indicate an alternative formation pathway for those objects.

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arXiv2016

A revised moving cluster distance to the Pleiades open cluster

Aims: We provide a new distance estimate for the Pleiades based on the moving cluster method, which will be useful to further discuss the so-called Pleiades distance controversy and compare it with the very precise parallaxes from the Gaia space mission. Methods: We apply a refurbished implementation of the convergent point search method to an updated census of Pleiades stars to calculate the convergent point position of the cluster from stellar proper motions. Then, we derive individual parallaxes for 64 cluster members using radial velocities compiled from the literature, and approximate parallaxes for another 1146 stars based on the spatial velocity of the cluster. This represents the largest sample of Pleiades stars with individual distances to date. Results: The parallaxes derived in this work are in good agreement with previous results obtained in different studies (excluding Hipparcos) for individual stars in the cluster. We report a mean parallax of $7.44\pm 0.08$~mas and distance of $134.4^{+2.9}_{-2.8}$pc that is consistent with the weighted mean of $135.0\pm 0.6$pc obtained from the non-Hipparcos results in the literature. Conclusions: Our result for the distance to the Pleiades open cluster is not consistent with the Hipparcos catalog, but favors the recent and more precise distance determination of $136.2\pm 1.2$pc obtained from Very Long Baseline Interferometry observations. It is also in good agreement with the mean distance of $133\pm 5$pc obtained from the first trigonometric parallaxes delivered by the Gaia satellite for the brightest cluster members in common with our sample.

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arXiv2016

Proper motion survey and kinematic analysis of the Rho Ophiuchi embedded cluster

We aim at performing a kinematic census of young stellar objects (YSOs) in the Rho Ophiuchi F core and partially in the E core of the L1688 dark cloud. We run a proper motion program at the ESO New Technology Telescope (NTT) with the Son of ISAAC (SOFI) instrument over nine years in the near-infrared. We complemented these observations with various public image databases to enlarge the time base of observations and the field of investigation to 0.5 deg X 0.5 deg. We derived positions and proper motions for 2213 objects. From these, 607 proper motions were derived from SOFI observations with a ~1.8 mas/yr accuracy while the remaining objects were measured only from auxiliary data with a mean precision of about ~3 mas/yr. We performed a kinematic analysis of the most accurate proper motions derived in this work, which allowed us to separate cluster members from field stars and to derive the mean properties of the cluster. From the kinematic analysis we derived a list of 68 members and 14 candidate members, comprising 26 new objects with a high membership probability. These new members are generally fainter than the known ones. We measured a mean proper motion of (mu_RA*, mu_DEC)=(-8.2, -24.3)+/-0.8 mas/yr for the L1688 dark cloud. A supervised classification was applied to photometric data of members to allocate a spectral energy distribution (SED) classification to the unclassified members.} We kinematically confirmed that the 56 members that were known from previous studies of the Rho Ophiuchi F cluster and that were also part of our survey are members of the cluster, and we added 26 new members. We defined the evolutionary status of the unclassified members of the cluster. We showed that a large part (23) of these new members are probably brown dwarfs, which multiplies the number of known substellar objects in the cluster by a factor of 3.3.

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arXiv2015

Evolution of the T Tauri star population in the Lupus association

Aims: In a recent study, we derived individual distances for 109 pre-main sequence stars that define the Lupus kinematic association of young stars. Here, we use these new distances to derive the masses and ages of Lupus T Tauri stars with the aim of better constraining the lifetime of their circumstellar disks. Methods: Using the photometric and spectroscopic information available in the literature, we computed the photospheric luminosity of 92 T Tauri stars in the Lupus association. Then, we estimated their masses and ages from theoretical evolutionary models. Based on Monte Carlo simulations and statistical tests, we compare the mass and age distribution of the classical T Tauri stars (CTTS) and weak-line T Tauri (WTTS) in our sample. Results: We show that the CTTSs are on average younger than the WTTSs and that the probability that both T~Tauri subclasses are drawn from the same mass and age parental distribution is very low. Our results favor the scenario proposed earlier for the Taurus-Auriga association, where the CTTSs evolve into WTTSs when their disks are fully accreted by the star. Based on an empirical disk model, we find that the average disk lifetime for the T Tauri stars in the Lupus association is $τ_{d}=3\times10^{6}\,(M_*/M_{\odot})^{0.55}$ yr. Conclusions: We find evidence that the average lifetime of the circumstellar disks in the Lupus association is shorter than in the Taurus-Auriga association and discuss the implications of this result.

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arXiv2014

The TW Hydrae Association : trigonometric parallaxes and kinematic analysis

The nearby TW Hydrae Association (TWA) is currently a benchmark for the study of formation and evolution of young low-mass stars, circumstellar disks and the imaging detection of planetary companions. For such studies, it is crucial to evaluate the distance to group members in order to access their physical properties. Membership of several stars is strongly debated and age estimates vary from one author to another with doubts about coevality. We revisit the kinematic properties of the TWA in light of new trigonometric parallaxes and proper motions to derive the dynamical age of the association and physical parameters of kinematic members. Using observations performed with the NTT/ESO telescope we measured trigonometric parallaxes and proper motions for 13 stars in TWA. With the convergent point method we identify a co-moving group with 31 TWA stars. We deduce kinematic distances for 7 members of the moving group that lack trigonometric parallaxes. A traceback strategy is applied to the stellar space motions of a selection of 16 of the co-moving objects with accurate and reliable data yielding a dynamical age for the association of t = 7.5 +/- 0.7 Myr. Using our new parallaxes and photometry available in the literature we derive stellar ages and masses from theoretical evolutionary models. With new parallax and proper motion measurements from this work and current astrometric catalogs we provide an improved and accurate database for TWA stars to be used in kinematical analysis. We conclude that the dynamical age obtained via traceback strategy is consistent with previous age estimates for the TWA, and is also compatible with the average ages derived in the present paper from evolutionary models for pre-main sequence stars.

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arXiv2013

A kinematic study and membership analysis of the Lupus star-forming region

This paper is dedicated to investigating the kinematic properties of the Lupus moving group of young stars with the primary objective of deriving individual parallaxes for each group member. We identify those stars in the Lupus star-forming region that define the comoving association of young stars by utilizing our new and improved convergent point search method that allows us to derive the precise position of the convergent point of the comoving association from the stars' proper motions. We used published proper motion catalogs and searched the literature for radial velocities, which are needed to compute individual parallaxes. We supplemented the radial velocity data with new measurements from spectroscopic observations performed with the FEROS spectrograph mounted on the MPG/ESO 2.2m telescope at La Silla. We identify a comoving group with 109 pre-main sequence stars and candidates that define the kinematic properties of the Lupus low-mass star-forming region. We derive individual parallaxes for stars with known radial velocity and tentative parallaxes for the remaining group members by assuming that all stars share the same space motion. The convergent point method, combined with the k-NN algorithm, makes it possible to distinguish the Lupus and Upper Centaurus Lupus stars from the adjacent Scorpius-Centaurus association. We find significant depth effects in this region and show that the classical T Tauri stars, located in the close vicinity of the Lupus molecular clouds, form a background population, while the weak-emission line T Tauri stars are dispersed not only in angular extent but also in depth. The newly derived individual parallaxes will be used in a forthcoming paper to refine the masses and ages of Lupus T Tauri stars, with the aim of better constraining the lifetimes of their circumstellar, protoplanetary disks.

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arXiv2011

A new method for calculating the convergent point of a moving group

Context: Convergent point (CP) search methods are important tools for studying the kinematic properties of open clusters and young associations whose members share the same spatial motion. Aims: We present a new CP search strategy based on proper motion data. We test the new algorithm on synthetic data and compare it with previous versions of the CP search method. As an illustration and validation of the new method we also present an application to the Hyades open cluster and a comparison with independent results. Methods: The new algorithm rests on the idea of representing the stellar proper motions by great circles over the celestial sphere and visualizing their intersections as the CP of the moving group. The new strategy combines a maximum-likelihood analysis for simultaneously determining the CP and selecting the most likely group members and a minimization procedure that returns a refined CP position and its uncertainties. The method allows one to correct for internal motions within the group and takes into account that the stars in the group lie at different distances. Results: Based on Monte Carlo simulations, we find that the new CP search method in many cases returns a more precise solution than its previous versions. The new method is able to find and eliminate more field stars in the sample and is not biased towards distant stars. The CP solution for the Hyades open cluster is in excellent agreement with previous determinations.

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