Age Distribution Of Stars Research Articles

Age distribution of stars in boxy/peanut/X-shaped bulges formed without bar buckling

ABSTRACT Some barred galaxies, including the Milky Way, host a boxy/peanut/X-shaped bulge (BPX-shaped bulge). Previous studies suggested that the BPX-shaped bulge can either be developed by bar buckling or by vertical inner Lindblad resonance (vILR) heating without buckling. In this paper, we study the observable consequence of a BPX-shaped bulge built up quickly after bar formation via vILR heating without buckling, using an N-body/hydrodynamics simulation of an isolated Milky Way-like galaxy. We found that the BPX-shaped bulge is dominated by stars born prior to bar formation. This is because the bar suppresses star formation, except for the nuclear stellar disc (NSD) region and its tips. The stars formed near the bar ends have higher Jacobi energy, and when these stars lose their angular momentum, their non-circular energy increases to conserve Jacobi energy. This prevents them from reaching the vILR to be heated to the BPX-shaped bulge region. By contrast, the NSD forms after the bar formation. From this simulation and general considerations, we expect that the age distributions of the NSD and BPX-shaped bulge formed without bar buckling do not overlap each other. Then, the transition age between these components betrays the formation time of the bar, and is testable in future observations of the Milky Way and extragalactic barred galaxies.

Similarities behind the high- and low-α disc: small intrinsic abundance scatter and migrating stars

ABSTRACT The detailed age-chemical abundance relations of stars measure time-dependent chemical evolution. These trends offer strong empirical constraints on nucleosynthetic processes, as well as the homogeneity of star-forming gas. Characterizing chemical abundances of stars across the Milky Way over time has been made possible very recently, thanks to surveys like Gaia, APOGEE, and Kepler. Studies of the low-α disc have shown that individual elements have unique age–abundance trends and the intrinsic dispersion around these relations is small. In this study, we examine and compare the age distribution of stars across both the high and low-α disc and quantify the intrinsic dispersion of 16 elements around their age–abundance relations at [Fe/H] = 0 using APOGEE DR16. We examine the age–metallicity relation and visualize the temporal and spatial distribution of disc stars in small chemical cells. We find: (1) the high-α disc has shallower age–abundance relations compared to the low-α disc, but similar median intrinsic dispersions of ∼0.03 dex; (2) turnover points in the age-[Fe/H] relations across radius for both the high- and low-α disc. The former constrains the mechanisms that set similar intrinsic dispersions, regardless of differences in the enrichment history, for stars in both disc, and the latter indicates the presence of radial migration in both disc. Our study is accompanied by an age catalogue for 64 317 stars in APOGEE derived using the cannon with a median uncertainty of 1.5 Gyr (26 per cent; APO-CAN stars), and a red clump catalogue of 22 031 stars with a contamination rate of 2.7 per cent.

Lessons on early structure formation from a mature galaxy cluster observed at cosmic noon

ABSTRACT We demonstrate a new approach of indirectly constraining both early star and structure formation via mature galaxy clusters at cosmic noon (z ∼ 2), using the cluster XLSSC 122 as an example. With the standard Press Schechter formalism, we infer a rapid evolution of the star formation efficiency (the ratio of stellar to halo mass) from 10−4 to 0.01 during z ∼ 20−13, based on the age distribution of stars in post-starburst galaxies of XLSSC 122, measured by Hubble Space Telescope photometry assuming no dust extinction. Here, we consider all low-mass haloes, including minihaloes, that host the first stars and galaxies ($5\times 10^5\ \rm M_{\odot }\lesssim M_{\rm halo}\lesssim 10^{10}\ \rm M_{\odot }$). We also place new constraints on fuzzy dark matter models of ma ≲ 5 × 10−21 eV/c2 for the ultralight boson mass, from the abundance of galaxies with star formation at z ≳ 13 in XLSSC 122. Our exploratory results are consistent with existing constraints. More comprehensive results will be obtained if our approach is extended to a large sample of clusters or field post-starburst galaxies at cosmic noon, with improved modelling of halo and stellar populations.

Analysis of Membership Probability in Nearby Young Moving Groups with Gaia DR2

Abstract We analyze the membership probability of young stars belonging to nearby moving groups with Gaia DR2 data. The sample of 1429 stars was identified from “The Catalog of Suspected Nearby Young Moving Group Stars.” Good-quality parallax and proper motion values were retrieved for 890 stars from the Gaia DR2 database. The analysis for membership probability is performed in the framework of the LACEwING algorithm. From the analysis it is confirmed that 279 stars do not belong to any of the known moving groups. We estimated the U, V, W space velocity values for 250 moving group members, which were found to be more accurate than previous values listed in the literature. The velocity ellipses of all the moving groups are well constrained within the “good box,” a widely used criterion to identify moving group members. The age of moving group members are uniformly estimated from the analysis of the Gaia color–magnitude diagram with MIST isochrones. We found a spread in the age distribution of stars belonging to some moving groups, which needs to be understood from further studies.

Redistribution of stars and gas in the star formation deserts of barred galaxies

ABSTRACT Bars strongly influence the distribution of gas and stars within the central regions of their host galaxies. This is particularly pronounced in the star formation desert (SFD) which is defined as two symmetrical regions either side of the bar that show a deficit in young stars. Previous studies proposed that, if star formation is truncated because of the influence of the bar, then the age distribution of stars within the SFD could be used to determine the epoch of bar formation. To test this, we study the properties of SFDs in six galaxies from zoom-in cosmological re-simulations. Age maps reveal old regions on both sides of the bars, with a lack of stars younger than 10 Myr, confirming the SFD phenomenon. Local star formation is truncated in the SFDs because after the bar forms, gas in these regions is removed on 1 Gyr time-scales. However, the overall age distribution of stars in the SFD does not show a sharp truncation after bar formation but rather a gradual downturn in comparison to that of the bar. This more subtle signature may still give information on bar formation epochs in observed galaxies, but the interpretation will be more difficult than originally hoped. The gradual drop in the SFD age distribution, instead of a truncation, is due to radial migration of stars born in the disc. The SFD is thus one of the only regions where an uncontaminated sample of stars only affected by radial migration can be studied.

The Gaia-ESO Survey: Age spread in the star forming region NGC 6530 from the HR diagram and gravity indicators

Context. In very young clusters, stellar age distribution is empirical proof of the duration of star cluster formation and thus it gives indications of the physical mechanisms involved in the star formation process. Determining the amount of interstellar extinction and the correct reddening law are crucial steps to derive fundamental stellar parameters and in particular accurate ages from the Hertzsprung-Russell diagram. Aims. In this context, we seek to derive accurate stellar ages for NGC 6530, the young cluster associated with the Lagoon Nebula to infer the star formation history of this region. Methods. We used the Gaia-ESO survey observations of the Lagoon Nebula, together with photometric literature data and Gaia DR2 kinematics, to derive cluster membership and fundamental stellar parameters. Using spectroscopic effective temperatures, we analysed the reddening properties of all objects and derived accurate stellar ages for cluster members. Results. We identified 652 confirmed and 9 probable members. The reddening inferred for members and non-members allows us to distinguish foreground objects, mainly main-sequence stars, and background objects, mainly giants, and to trace the three-dimensional structure of the nebula. This classification is in agreement with the distances inferred from Gaia DR2 parallaxes for these objects. Finally, we derive stellar ages for 382 confirmed cluster members for which we obtained the individual reddening values. In addition, we find that the gravity-sensitive γ index distribution for the M-type stars is correlated with stellar age. Conclusions. For all members with Teff < 5500 K, the mean logarithmic age is 5.84 (units of years) with a dispersion of 0.36 dex. The age distribution of stars with accretion or discs, i.e. classical T Tauri stars with excess (CTTSe), is similar to that of stars without accretion and without discs, i.e. weak T Tauri stars with photospheric emission (WTTSp). We interpret this dispersion as evidence of a real age spread since the total uncertainties on age determinations, derived from Monte Carlo simulations, are significantly smaller than the observed spread. This conclusion is supported by evidence of the decrease of the gravity-sensitive γ index as a function of stellar ages. The presence of a small age spread is also supported by the spatial distribution and kinematics of old and young members. In particular, members with accretion or discs, formed in the last 1 Myr, show evidence of subclustering around the cluster centre, in the Hourglass Nebula and in the M8-E region, suggesting a possible triggering of star formation events by the O-type star ionization fronts.

ROTATING STARS FROM KEPLER OBSERVED WITH GAIA DR1

ABSTRACT Astrometric data from the recent Gaia Data Release 1 have been matched against the sample of stars from Kepler with known rotation periods. A total of 1299 bright rotating stars were recovered from the subset of Gaia sources with good astrometric solutions, most with temperatures above 5000 K. From these sources, 894 were selected as lying near the main sequence using their absolute G-band magnitudes. These main-sequence stars show a bimodality in their rotation period distribution, centered roughly around a 600 Myr rotation isochrone. This feature matches the bimodal period distribution found in cooler stars with Kepler, but was previously undetected for solar-type stars due to sample contamination by subgiants. A tenuous connection between the rotation period and total proper motion is found, suggesting that the period bimodality is due to the age distribution of stars within ∼300 pc of the Sun, rather than a phase of rapid angular momentum loss. This work emphasizes the unique power for understanding stellar populations that is created by combining temporal monitoring from Kepler with astrometric data from Gaia.

Galactic Archaeology with TESS: Prospects for Testing the Star Formation History in the Solar Neighbourhood

A period of quenching between the formation of the thick and thin disks of the Milky Way has been recently proposed to explain the observed age-[{\alpha}/Fe] distribution of stars in the solar neighbourhood. However, robust constraints on stellar ages are currently available for only a limited number of stars. The all-sky survey TESS (Transiting Exoplanet Survey Satellite) will observe the brightest stars in the sky and thus can be used to investigate the age distributions of stars in these components of the Galaxy via asteroseismology, where previously this has been difficult using other techniques. The aim of this preliminary study was to determine whether TESS will be able to provide evidence for quenching periods during the star formation history of the Milky Way. Using a population synthesis code, we produced populations based on various stellar formation history models and limited the analysis to red-giant-branch stars. We investigated the mass-Galactic-disk-height distributions, where stellar mass was used as an age proxy, to test for whether periods of quenching can be observed by TESS. We found that even with the addition of 15% noise to the inferred masses, it will be possible for TESS to find evidence for/against quenching periods suggested in the literature (e.g. between 7 and 9 Gyr ago), therefore providing stringent constraints on the formation and evolution of the Milky Way.

Age consistency between exoplanet hosts and field stars

Transiting planets around stars are discovered mostly through photometric surveys. Unlike radial velocity surveys, photometric surveys do not tend to target slow rotators, inactive or metal-rich stars. Nevertheless, we suspect that observational biases could also impact transiting-planet hosts. This paper aims to evaluate how selection effects reflect on the evolutionary stage of both a limited sample of transiting-planet host stars (TPH) and a wider sample of planet-hosting stars detected through radial velocity analysis. Then, thanks to uniform derivation of stellar ages, a homogeneous comparison between exoplanet hosts and field star age distributions is developed. Stellar parameters have been computed through our custom-developed isochrone placement algorithm, according to PARSEC models. The notable aspects of our algorithm include the treatment of element diffusion, activity checks in terms of $\log{R'_{HK}}$ and $v\sin{i}$, and the evaluation of the stellar evolutionary speed in the Hertzsprung-Russel diagram in order to better constrain age. Working with TPH, the observational stellar mean density $\rho_{\star}$ allows us to compute stellar luminosity even if the distance is not available, by combining $\rho_{\star}$ with the spectroscopic $\log{g}$. The median value of the TPH ages is $\sim5$ Gyr. Even if this sample is not very large, however, the result is very similar to what we found for the sample of spectroscopic hosts, whose modal and median values are [3, 3.5) Gyr and $\sim4.8$ Gyr, respectively. Thus, these stellar samples suffer almost the same selection effects. We also conclude that the age of our Sun is consistent with the age distribution of solar neighbourhood MS stars with spectral types from late F to early K, regardless of whether they harbour planets or not. We considered the possibility that our selected samples are older than the average disc population.

Exploring the vertical age structure of the Galactic disc

AbstractWhile in external or high-redshift galaxies we can only measure integrated stellar properties at best, the Milky Way offers us the unique opportunity to study its individual baryonic components, including stars. We use oscillations measured in red giant stars by the Kepler satellite to derive stellar ages and explore the vertical age structure across few kpc of the Milky Way disc. We find that old stars dominate at increasing Galactic heights, whereas closer to the plane a rich zoology of ages exists. The age distribution of stars shows a smooth distribution over the last 10 Gyr, which together with a flat age-metallicity relation is consistent with a quiescent evolution for the Milky Way disc since a redshift of about two.

STELLAR AGE SPREADS IN CLUSTERS AS IMPRINTS OF CLUSTER-PARENT CLUMP DENSITIES

It has recently been suggested that high-density clusters have stellar age distributions narrower than that of the Orion Nebula Cluster, indicating a possible trend of narrower age distributions for denser clusters. We show this effect to likely arise from star formation being faster in gas with a higher density. We model the star formation history of molecular clumps in equilibrium by associating a star formation efficiency (SFE) per free-fall time, \eff, to their volume density profile. Our model predicts a steady decline of the star formation rate (SFR), which we quantify with its half-life time, namely, the time needed for the SFR to drop to half its initial value. Given the uncertainties affecting the SFE per free-fall time, we consider two distinct values: 0.1 and 0.01. For isothermal spheres, \eff=0.1 leads to a half-life time of order the clump free-fall time, \tff. Therefore, the age distributions of stars formed in high-density clumps have smaller full-widths at half-maximum than those of stars formed in low-density clumps. When \eff=0.01, the half-life time is 10 times longer. We explore what happens if the star formation duration is shorter than 10\tff, that is, if the half-life time of the SFR cannot be defined. There, we build on the invariance of the shape of the young cluster mass function to show that an anti-correlation between clump density and star formation duration is expected. Therefore, regardless of whether the star formation duration is longer than the SFR half-life time, denser molecular clumps yield narrower star age distributions in clusters. Published densities and stellar age spreads of young clusters actually suggest that the time-scale for star formation is of order 1-4\tff. We conclude that there is no need to invoke the existence of multiple cluster formation mechanisms to explain the observed range of stellar age spreads in clusters.

The dependence of stellar age distributions on giant molecular cloud environment

Abstract In this Letter, we analyse the distributions of stellar ages in giant molecular clouds (GMCs) in spiral arms, interarm spurs and at large galactic radii, where the spiral arms are relatively weak. We use the results of numerical simulations of galaxies, which follow the evolution of GMCs and include star particles where star formation events occur. We find that GMCs in spiral arms tend to have predominantly young (<10 Myr) stars. By contrast, clouds which are the remainders of spiral arm giant molecular asssociations that have been sheared into interarm GMCs contain fewer young (<10 Myr) stars and more ∼20 Myr stars. We also show that clouds which form in the absence of spiral arms, due to local gravitational and thermal instabilities, contain preferentially young stars. We propose that the age distributions of stars in GMCs will be a useful diagnostic to test different cloud evolution scenarios, the origin of spiral arms and the success of numerical models of galactic star formation. We discuss the implications of our results in the context of Galactic and extragalactic molecular clouds.

No wide spread of stellar ages in the Orion Nebula Cluster

The wide luminosity dispersion seen for stars at a given effective temperature in the H-R diagrams of young clusters and star forming regions is often interpreted as due to significant (~10 Myr) spreads in stellar contraction age. In the scenario where most stars are born with circumstellar discs, and that disc signatures decay monotonically (on average) over timescales of only a few Myr, then any such age spread should lead to clear differences in the age distributions of stars with and without discs. We have investigated large samples of stars in the Orion Nebula Cluster (ONC) using three methods to diagnose disc presence from infrared measurements. We find no significant difference in the mean ages or age distributions of stars with and without discs, consistent with expectations for a coeval population. Using a simple quantitative model we show that any real age spread must be smaller than the median disc lifetime. For a log-normal age distribution, there is an upper limit of <0.14 dex (at 99% confidence) to any real age dispersion, compared to the ~=0.4 dex implied by the H-R diagram. If the mean age of the ONC is 2.5 Myr, this would mean at least 95% of its low-mass stellar population has ages between 1.3--4.8 Myr. We suggest that the observed luminosity dispersion is caused by a combination of observational uncertainties and physical mechanisms that disorder the conventional relationship between luminosity and age for pre main-sequence stars. This means that individual stellar ages from the H-R diagram are unreliable and cannot be used to directly infer a star formation history. Irrespective of what causes the wide luminosity dispersion, the finding that any real age dispersion is less than the median disc lifetime argues strongly against star formation scenarios for the ONC lasting longer than a few Myr.

THE HISTORY OF STAR FORMATION IN GALAXY DISKS IN THE LOCAL VOLUME AS MEASURED BY THE ADVANCED CAMERA FOR SURVEYS NEARBY GALAXY SURVEY TREASURY

We present a measurement of the age distribution of stars residing in spiral disks and dwarf galaxies. We derive a complete star formation history of the ~140 Mpc^3 covered by the volume-limited sample of galaxies in the Advanced Camera for Surveys (ACS) Nearby Galaxy Survey Treasury (ANGST). The total star formation rate density history is dominated by the large spirals in the volume, although the sample consists mainly of dwarf galaxies. Our measurement shows a factor of ~3 drop at z~2, in approximate agreement with results from other measurement techniques. While our results show that the overall star formation rate density has decreased since z~1, the measured rates during this epoch are higher than those obtained from other measurement techniques. This enhanced recent star formation rate appears to be largely due to an increase in the fraction of star formation contained in low-mass disks at recent times. Finally, our results indicate that despite the differences at recent times, the epoch of formation of ~50% of the stellar mass in dwarf galaxies was similar to that of ~50% of the stellar mass in large spiral galaxies (z>~2), despite the observed galaxy-to-galaxy diversity among the dwarfs.

Kinematical &amp; Chemical Characteristics of the Thin and Thick Disks

AbstractI discuss how the chemical abundance distributions, kinematics and age distributions of stars in the thin and thick disks of the Galaxy can be used to decipher the merger history of the Milky Way, a typical large galaxy. The observational evidence points to a rather quiescent past merging history, unusual in the context of the ‘consensus’ cold-dark-matter cosmology favoured from observations of structure on scales larger than individual galaxies.

Lithium-depleted stars in the young ${\mathsf \sigma}$ Orionis cluster

Context. Knowledge of the age distribution of stars in young clusters and associations is crucial to constrain models of star formation. HR diagrams of different young clusters and associations suggest the presence of age spreads, but the influence of errors on the derived ages is still largely debated. Determination of lithium abundances in low-mass stars represents an alternative and robust way to infer stellar ages.Aims. We measured lithium in a sample of low mass members of the young (4–5 Myr) σ Ori cluster with the main goal of investigating its star formation history.Methods. Using the FLAMES multi-object spectrograph on VLT/UT2, we obtained spectra of 98 candidate cluster members. The spectra were used to determine radial velocities, to infer the presence of Hα emission, and to measure the strength of the Li i 670.8 nm absorption line.Results. Using radial velocities, Hα and Li, together with information on X-ray emission, we identified 59 high probability cluster members. Three of them show severe Li depletion. The nuclear ages inferred for these highly depleted stars exceed 10–15 Myr; for two of them these values are in good agreement with the isochronal age, while for the third star the nuclear age exceeds the isochronal one.

Formation History of Stars and Star Clusters in Nearby Galaxies

We present first results from an HST/ACS imaging survey of stars and star clusters in five nearby spiral galaxies. This contribution concentrates on NGC 1313, a highly distorted late-type barred spiral. We compare the field star and cluster formation histories in our three ACS pointings for this galaxy. In one pointing, both the cluster and field star age distributions show clear evidence for a ramp-up in the star formation rate about 108 years ago.

The Outer Regions of the Nearby Sc Galaxies NGC 2403 and M33: Evidence for an Intermediate-Age Population at Large Radii

Deep g'r'i'z' images obtained with the Gemini Multiobject Spectrograph (GMOS) on Gemini North are used to investigate the stellar content in the outer regions of the nearby Sc galaxies NGC 2403 and M33. The field observed in NGC 2403 covers galactocentric distances between 5 and 11 kpc perpendicular to the line of sight (R) and 7 and 19 kpc along the plane of the disk (R). The red giant branch (RGB) tip occurs at i' = 23.6 ? 0.1, and the Cepheid and RGB-tip distance scales for NGC 2403 are in good agreement. The number density of bright main-sequence stars in this field experiences a steep cutoff at R ~ 10 kpc, which is consistent with the expected truncation radius of the disk predicted from studies of edge-on spiral galaxies. While very young stars are restricted to R < 10 kpc, a population of bright asymptotic giant branch (AGB) stars is present throughout the entire GMOS field, indicating that star formation occurred outside of the present-day star-forming disk of NGC 2403 during intermediate epochs. The AGB stars are not in a tidal stream; in fact, the ratio of AGB stars above the RGB tip to those below the RGB tip does not change with radius, indicating that the bright AGB stars are uniformly mixed with the fainter stellar content throughout the field. The AGB luminosity function (LF) scales with r-band surface brightness over a wide range of radii throughout the main body of NGC 2403, indicating that the age distribution of stars in the outer regions of the present-day star-forming disk is not skewed to younger values than in the inner disk. Based on the color of stars on the upper portions of the RGB, it is concluded that metallicity changes across the field, with [Fe/H] = -0.8 ? 0.1(random) ? 0.3(systematic) at R = 5 kpc, and [Fe/H] = -2.2 ? 0.2(random) ? 0.8(systematic) at R = 11 kpc. The M33 field samples R between 8 and 10 kpc and R between 14 and 17 kpc. Bright AGB stars are detected in this field, and the ratio of bright AGB stars to stars on the upper RGB is at least as large as that measured in the outer regions of NGC 2403; thus, an intermediate-age population occurs well outside of the young star-forming disk of M33. The color of stars on the upper RGB suggests that [Fe/H] = -1.0 ? 0.3(random) ? 0.3(systematic) in this field. The globular cluster systems of both NGC 2403 and M33 contain objects that formed during intermediate epochs, and it is suggested that the luminous AGB stars detected in the current study are the field counterparts of these clusters. The detection of intermediate-age stars in the outer regions of these galaxies is consistent with models in which late-type spiral galaxies formed more slowly than earlier-type systems.

Star Formation: The Current Frontier

ASTRONOMY Star formation occurs by the collapse of a cloud of dust and gas. Spiral galaxies, which formed from collapsed clouds, have a dense nucleus or bulge dominated by older stars formed by an early, rapid burst of star formation. The spiral arms consist of gas, dust, and younger stars swirling outward away from the center. Many processes may trigger star formation, including gravitational interactions or shock waves from supernovae, but distinguishing between these processes is difficult without gaining more information about the composition and age distributions of stars in the galaxies. Davidge has begun a survey in the near-infrared wavelength range of the Sc galaxies, the most “open” class of spiral galaxies, with the Canada-France-Hawaii Telescope (CFHT) adaptive optics bonnette (AOB) to determine the stellar structure of the centers of these galaxies and understand star formation processes. This initial study focuses on M33, the Triangulum galaxy, which is the third largest spiral galaxy in the Local Group (after the Milky Way and Andromeda). He has determined that there was a burst of star formation near the nucleus of M33 that occurred between 1 to 3 billion years ago. He also noted a recent episode of star formation in the nucleus, which is somewhat unexpected because M33 does not have a supermassive black hole at its center that might trigger star formation. Finally, he notes that the source of the gas for the recent star formation episode cannot be derived from the disk, which is chemically distinct from the young stars, so the collapsing gas is probably concentrated in the center. These results suggest that star formation is complex and variable, but more observations are needed to distinguish between the possible mechanisms that initiate the collapse.— LR Astron. J. 119 , 748 (2000).

Chemo-Dynamical Models and the Star Formation History of Galaxies

Sandage (1986) showed what the star formation rate (SFR) of galaxies of different Hubble type might look like. His curves are based on the study of Gallagher et al. (1984), who determined the SFR at three different epochs of galactic evolution. Sandage's figure establishes a connection of SFR and Hubble type but, as was already mentioned by Sandage, it “contains no physics”. In order to explore the background of this connection, however, it is necessary either to improve the observations or to model the evolution of galaxies self-consistently. However, the results of modelling the SFR are not reliable, if simplified models are used, which only describe some structural properties of galaxies. It is necessary to apply self-consistent models which take into consideration all relevant interaction processes between the gaseous and stellar components of a galaxy. Such models can be checked by comparison with observations like density and abundance distributions, star-gas content, velocities, velocity dispersions, mass-luminosity relations and age distributions of stars. A detailed model will show, whether the initial conditions, the feedback mechanisms during the evolution or the environment determine the evolution of a galaxy.