Nearby Field Stars Research Articles

Searching for Binary Asteroids in Pan-STARRS1 Archival Images

We developed two different point-spread function (PSF) analysis techniques for discovering wide separation binary asteroids in wide-field surveys. We then applied these techniques to images of main belt asteroids in the 4–60 km size range captured by Pan-STARRS1. Johnston lists fewer than 10 known binaries in this size range with separations greater than 10% of the primary’s Hill radius, so discovering more wide binary asteroids is crucial for understanding the limits of binary stability and improving our knowledge of asteroid masses. We analyzed each image by (i) comparing the major axis orientation of the asteroid’s elliptical PSF to its nonsidereal rate on the sky and (ii) comparing the one-dimensional median profile created by collapsing the image along the asteroid’s direction of motion to that of nearby field stars. For both methods, we flagged any results that deviated significantly from the expected measurements of single asteroids, and those targets with the most flags were identified as binary candidates for confirmation with high-acuity imaging.

Orbital variability of the optical linear polarization of the γ-ray binary LS I +61° 303 and new constraints on the orbital parameters

We studied the variability of the linear polarization and brightness of the γ-ray binary LS I +61° 303. High-precision BVR photopolarimetric observations were carried out with the Dipol-2 polarimeter on the 2.2 m remotely controlled UH88 telescope at Mauna Kea Observatory and the 60 cm Tohoku telescope at Haleakala bservatory (Hawaii) over 140 nights in 2016−2019. We also determined the degree and angle of the interstellar polarization toward LS I +61° 303 using two out of four nearby field stars that have Gaia’s parallaxes. After subtracting the interstellar polarization, we determined the position angle of the intrinsic polarization θ ≃ 11°, which can either be associated with the projection of the Be star’s decretion disk axis on the plane of sky, or can differ from it by 90°. Using the Lomb-Scargle method, we performed timing analyses and period searches of our polarimetric and photometric data. We found statistically significant periodic variability of the normalized Stokes parameters q and u in all passbands. The most significant period of variability, PPol = 13.244 ± 0.012 d, is equal to one half of the orbital period Porb = 26.496 d. The fits of the polarization variability curves with Fourier series show a dominant contribution from the second harmonic which is typical for binary systems with circular orbits and nearly symmetric distribution of light scattering material with respect to the orbital plane. The continuous change of polarization with the orbital phase implies co-planarity of the orbit of the compact object and the Be star’s decretion disk. Using a model of Thomson scattering by a cloud that orbits the Be star, we obtained constraints on the orbital parameters, including a small eccentricity e < 0.2 and periastron phase of ϕp ≈ 0.6, which coincides with the peaks in the radio, X-ray, and TeV emission. These constraints are independent of the assumption about the orientation of the decretion disk plane on the sky. We also extensively discuss the apparent inconsistency with the previous measurements of the orbital parameters from radial velocities. By folding the photometry data acquired during a three-year time span with the orbital period, we found a linear phase shift of the moments of the brightness maximum, confirming the possible existence of superorbital variability.

BAFFLES: Bayesian Ages for Field Lower-mass Stars

Abstract Age is a fundamental parameter of stars, yet in many cases, ages of individual stars are presented without robust estimates of the uncertainty. We have developed a Bayesian framework, BAFFLES, to produce the age posterior for a star from its calcium emission strength (log( )) or lithium abundance (Li EW) and B − V color. We empirically determine the likelihood functions for calcium and lithium as functions of age from literature measurements of stars in benchmark clusters with well-determined ages. We use a uniform prior on age, which reflects a uniform star formation rate. The age posteriors we derive for several test cases are consistent with literature ages found from other methods. BAFFLES represents a robust method to determine the age posterior probability distribution for any field star with 0.45 ≤ B − V ≤ 0.9 and a measurement of and/or 0.35 ≤ B − V ≤ 1.9 and measured Li EW. We compile colors, , and Li EW from over 2630 nearby field stars from the literature, and present the derived BAFFLES age posterior for each star.

Combining high-contrast imaging and radial velocities to constrain the planetary architectures of nearby stars

Context. Nearby stars are prime targets for exoplanet searches and characterization using a variety of detection techniques. Combining constraints from the complementary detection methods of high-contrast imaging (HCI) and radial velocity (RV) can further constrain the planetary architectures of these systems because these methods place limits at different regions of the companion mass and semi-major axis parameter space. Compiling a census of the planet population in the solar neighborhood is important to inform target lists for future space missions that will specifically target nearby stars to search for Earth analogs. Aims. We aim to constrain the planetary architectures from the combination of HCI and RV data for six nearby stars within 6 pc: τ Ceti, Kapteyn’s star, AX Mic, 40 Eri, HD 36395, and HD 42581. We explored where HCI adds information to constraints from the long-term RV monitoring data for these stars. Methods. We compiled the sample from stars with available archival VLT/NACO HCI data at L′ band (3.8 μm), where we expect substellar companions to be brighter for the typically older ages of nearby field stars (>1 Gyr). The NACO data were fully reanalyzed using the state-of-the-art direct imaging pipeline PynPoint and combined with RV data from HARPS, Keck/HIRES, and CORALIE. A Monte Carlo approach was used to assess the completeness in the companion mass and semi-major axis parameter space from the combination of the HCI and RV data sets. Results. We find that the HCI data add significant information to the RV constraints, increasing the completeness for certain companions masses and semi-major axes by up to 68–99% for four of the six stars in our sample, and by up to 1–13% for the remaining stars. The improvements are strongest for intermediate semi-major axes (15–40 AU), corresponding to the semi-major axes of the ice giants in our own solar system. The HCI mass limits reach 5–20 MJup in the background-limited regime, depending on the age of the star. Conclusions. Through the combination of HCI and RV data, we find that stringent constraints can be placed on the possible substellar companions in these systems. Applying these methods systematically to nearby stars will quantify our current knowledge of the planet population in the solar neighborhood and inform future observations.

The Evolutionary Status of GK Subgiants

Abstract Snowden & Young suggested that the reason why there are GK subgiants is because they are members of binaries, which would bring them above the main sequence in an Hertzsprung–Russell (HR) diagram. They studied a sample of 30 G0-K1 IV stars and were disappointed to find only two to be spectroscopic binaries. With more accurate radial velocities I found seven binaries in their samples of subgiants and control stars; orbital elements are given for those seven. Using Hipparcos parallaxes and SIMBAD data, I found that nearly all of the G0-K1 IV stars fall on the evolutionary tracks by Garardi et al. for Population I stars with masses of 0.9–1.9 M ⊙ and ages of up to 1010 yr, which are normal parameters for nearby field stars. Therefore there is no problem regarding the existence of GK subgiants.

Faint warm debris disks around nearby bright stars explored by AKARI and IRSF

Context: Debris disks are important observational clues for understanding planetary-system formation process. In particular, faint warm debris disks may be related to late planet formation near 1 AU. A systematic search of faint warm debris disks is necessary to reveal terrestrial planet formation. Aims: Faint warm debris disks show excess emission that peaks at mid-IR wavelengths. Thus we explore debris disks using the AKARI mid-IR all-sky point source catalog (PSC), a product of the second generation unbiased IR all-sky survey. Methods : We investigate IR excess emission for 678 isolated main-sequence stars for which there are 18 micron detections in the AKARI mid-IR all-sky catalog by comparing their fluxes with the predicted fluxes of the photospheres based on optical to near-IR fluxes and model spectra. The near-IR fluxes are first taken from the 2MASS PSC. However, 286 stars with Ks<4.5 in our sample have large flux errors in the 2MASS photometry due to saturation. Thus we have measured accurate J, H, and Ks band fluxes, applying neutral density (ND) filters for Simultaneous InfraRed Imager for Unbiased Survey (SIRIUS) on IRSF, the \phi 1.4 m near-IR telescope in South Africa, and improved the flux accuracy from 14% to 1.8% on average. Results: We identified 53 debris-disk candidates including eight new detections from our sample of 678 main-sequence stars. The detection rate of debris disks for this work is ~8%, which is comparable with those in previous works by Spitzer and Herschel. Conclusion: The importance of this study is the detection of faint warm debris disks around nearby field stars. At least nine objects have a large amount of dust for their ages, which cannot be explained by the conventional steady-state collisional cascade model.

A measurement of interstellar polarization and an estimation of Galactic extinction for the direction of the X-ray black hole binary V404 Cygni

Abstract V404 Cygni is a well-known candidate for the black hole binary thought to have relativistic jets. It showed extreme outbursts in 2015 June, characterized by a large amplitude and short time variation of flux in the radio, optical, and X-ray bands. Not only disk emission but also synchrotron radiation from the relativistic jets was suggested by radio observations. However, it is difficult to measure the accurate spectral shape in the optical/near-infrared band because there are uncertainties of interstellar extinction. To estimate the extinction value for V404 Cygni, we performed photopolarimetric and spectroscopic observations of V404 Cygni and nearby field stars. Here, we estimate the Galactic extinction using interstellar polarization based on the observation that the origin of the optical polarization is the interstellar medium, and investigate the properties of interstellar polarization around V404 Cygni. We found a good correlation between the color excess and polarization degree in the field stars. We also confirmed that the wavelength dependence of the polarization degree in the highly polarized field stars was similar to that of V404 Cygni. Using the highly polarized field stars, we estimated the color excess for the (B − V) color and the extinction in the V band to be E(B − V) = 1.2 ± 0.2 and 3.0 &amp;lt; A(V) &amp;lt; 3.6, respectively. A tendency for a bluer peak of polarization (λmax &amp;lt; 5500 Å) was commonly seen in the highly polarized field stars, suggesting that the dust grains toward this region are generally smaller than the Galactic average. The corrected spectral energy distribution of V404 Cygni in the near-infrared and optical bands in our results indicated a spectral break between 2.5 × 1014 Hz and 3.7 × 1014 Hz, which might originate in the synchrotron self-absorption.

Asteroseismic Properties of Solar-type Stars Observed with the NASA K2 Mission: Results from Campaigns 1–3 and Prospects for Future Observations

We present an asteroseismic analysis of 33 solar-type stars observed in short cadence (SC) during Campaigns (C) 1–3 of the NASA K2 mission. We were able to extract both average seismic parameters and individual mode frequencies for stars with dominant frequencies up to ∼3300 μHz, and we find that data for some targets are good enough to allow for a measurement of the rotational splitting. Modeling of the extracted parameters is performed by using grid-based methods using average parameters and individual frequencies together with spectroscopic parameters. For the target selection in C3, stars were chosen as in C1 and C2 to cover a wide range in parameter space to better understand the performance and noise characteristics. For C3 we still detected oscillations in 73% of the observed stars that we proposed. Future K2 campaigns hold great promise for the study of nearby clusters and the chemical evolution and age–metallicity relation of nearby field stars in the solar neighborhood. We expect oscillations to be detected in ∼388 SC targets if the K2 mission continues until C18, which will greatly complement the ∼500 detections of solar-like oscillations made for SC targets during the nominal Kepler mission. For ∼30–40 of these, including several members of the Hyades open cluster, we furthermore expect that inference from interferometry should be possible.

First multicolour polarimetry of TeV γ-ray binary HESS J0632+057 close to periastron passage

We present the results of UBVRI polarimetry of the TeV gamma-ray binary HESS J0632+057 obtained on 2015 March 24 (JD 2457106) and 2015 December 12 (JD 2457369). The detected polarisation values of HESS J0632+057, just after periastron passage (March 24), are higher than all previously published values (p(V) ~ 4.2%) and the position angle (theta ~ 171-172 degrees) is also different by ~6-10 degrees from previously published values. The data obtained just before the subsequent periastron passage (December 12) show statistically lower polarisation in all photometric bands (p(V) ~ 3.9%) and a different position angle theta ~ 167-168 degrees. From observations of a nearby field star, the interstellar component of the measured polarisation was estimated as p_is(V) ~ 0.65% and theta_is ~ 153 degrees. This estimate was used with the previous "V"-band estimation by "field-stars method" (p(V) ~ 2% and theta_is ~ 165 degrees) of Yudin (2014) to identify the wavelength dependence of the intrinsic polarisation in HESS J0632+057. It was found that after subtraction of the interstellar component (for both p_is estimates), the wavelength dependence of the intrinsic polarisation in HESS J0632+057 is essentially flat. We propose that the formation of an additional source of polarisation or some perturbation of circumstellar material at this orbital phase can explain the changes in the level of polarisation in HESS J0632+057 close to periastron passage.

Evidence for a chemical enrichment coupling of globular clusters and field stars in the Fornax dSph

The globular cluster H4, located in the center of the Fornax dwarf spheroidal galaxy, is crucial for understanding the formation and chemical evolution of star clusters in low-mass galactic environments. H4 is peculiar because the cluster is significantly more metal-rich than the galaxy's other clusters, is located near the galaxy center, and may also be the youngest cluster in the galaxy. In this study, we present detailed chemical abundances derived from high-resolution (R~28000) spectroscopy of an isolated H4 member star for comparison with a sample of 22 nearby Fornax field stars. We find the H4 member to be depleted in the alpha-elements Si, Ca, and Ti with [Si/Fe]=-0.35+-0.34, [Ca/Fe]=+0.05+-0.08, and [Ti/Fe]=-0.27+-0.23, resulting in an average [alpha/Fe]=-0.19+-0.14. If this result is representative of the average cluster properties, H4 is the only known system with a low [alpha/Fe] ratio and a moderately low metallicity embedded in an intact birth environment. For the field stars we find a clear sequence, seen as an early depletion in [alpha/Fe] at low metallicities, in good agreement with previous measurements. H4 falls on top of the observed field star [alpha/Fe] sequence and clearly disagrees with the properties of Milky Way halo stars. We therefore conclude that within a galaxy, the chemical enrichment of globular clusters may be closely linked to the enrichment pattern of the field star population. The low [alpha/Fe] ratios of H4 and similar metallicity field stars in Fornax give evidence that slow chemical enrichment environments, such as dwarf galaxies, may be the original hosts of alpha-depleted clusters in the halos of the Milky Way and M31.

FRIENDS OF HOT JUPITERS. III. AN INFRARED SPECTROSCOPIC SEARCH FOR LOW-MASS STELLAR COMPANIONS

Surveys of nearby field stars indicate that stellar binaries are common, yet little is known about the effects that these companions may have on planet formation and evolution. The Friends of Hot Jupiters project uses three complementary techniques to search for stellar companions to known planet-hosting stars: radial velocity monitoring, adaptive optics imaging, and near-infrared spectroscopy. In this paper, we examine high-resolution K band infrared spectra of fifty stars hosting gas giant planets on short-period orbits. We use spectral fitting to search for blended lines due to the presence of cool stellar companions in the spectra of our target stars, where we are sensitive to companions with temperatures between 3500-5000 K and projected separations less than 100 AU in most systems. We identify eight systems with candidate low-mass companions, including one companion that was independently detected in our AO imaging survey. For systems with radial velocity accelerations, a spectroscopic non-detection rules out scenarios involving a stellar companion in a high inclination orbit. We use these data to place an upper limit on the stellar binary fraction at small projected separations, and show that the observed population of candidate companions is consistent with that of field stars and also with the population of wide-separation companions detected in our previous AO survey. We find no evidence that spectroscopic stellar companions are preferentially located in systems with short-period gas giant planets on eccentric and/or misaligned orbits.

PHOTOMETRIC STUDY ON STELLAR MAGNETIC ACTIVITY. I. FLARE VARIABILITY OF RED DWARF STARS IN THE OPEN CLUSTER M37

Based on one-month long MMT time-series observations of the open cluster M37, we monitored light variations of nearly 2500 red dwarfs and successfully identified 420 flare events from 312 cluster M dwarf stars. For each flare light curve, we derived observational and physical parameters, such as flare shape, peak amplitude, duration, energy, and peak luminosity. We show that cool stars produce serendipitous flares energetic enough to be observed in the $r$-band, and their temporal and peak characteristics are almost the same as those in traditional $U$-band observations. We also found many large-amplitude flares with inferred $\Delta u > 6$ mag in the cluster sample which had been rarely reported in previous ground-based observations. Following the ergodic hypothesis, we investigate in detail statistical properties of flare parameters over a range of energy ($E_{r}$ $\simeq$ $10^{31}-10^{34}$ erg). As expected, there are no statistical differences in the distributions of flare timescales, energies, and frequencies among stars of the same age and mass group. We note that our sample tend to have longer rise and decay timescales compared to those seen in field flare stars of the same spectral type and be more energetic. Flare frequency distributions follow power-law distributions with slopes $\beta \sim0.62-1.21$ for all flare stars and $\beta \sim0.52-0.97$ for stars with membership information ($P_{mem} \geq 0.2$). These are in general agreement with previous works on flare statistics of young open clusters and nearby field stars. Our results give further support to the classical age-activity relations.

STATISTICAL PROPERTIES OF GALACTIC δ SCUTI STARS: REVISITED

We present statistical characteristics of 1,578 {\delta} Scuti stars including nearby field stars and cluster member stars within the Milky Way. We obtained 46% of these stars (718 stars) from the works done by Rodr\'{i}guez and collected the remaining 54% stars (860 stars) from other literatures. We updated the entries with the latest information of sky coordinate, color, rotational velocity, spectral type, period, amplitude and binarity. The majority of our sample are well characterized in terms of typical period range (0.02-0.25 days), pulsation amplitudes (<0.5 mag) and spectral types (A-F type). Given this list of {\delta} Scuti stars, we examined relations between their physical properties (i.e., periods, amplitudes, spectral types and rotational velocities) for field stars and cluster members, and confirmed that the correlations of properties are not significantly different from those reported in the Rodr\'{i}guez's works. All the {\delta} Scuti stars are cross-matched with several X-ray and UV catalogs, resulting in 27 X-ray and 41 UV-only counterparts. These counterparts are interesting targets for further study because of their rarity and uniqueness in showing {\delta} Scuti-type variability and X-ray/UV emission at the same time. The compiled catalog can be accessed through the web interface http://stardb.yonsei.ac.kr/DeltaScuti

Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems

This paper describes Herschel observations of the nearby (8.5pc) G5V multi-exoplanet host star 61 Vir at 70-500micron carried out by the DEBRIS survey. These reveal emission that is extended out to >15arcsec with a morphology that can be fitted by a nearly edge-on (77deg inclination) radially broad (from 30AU to >100AU) debris disk of fractional luminosity 2.7x10^-5, with two unrelated sources nearby that are more prominent at longer wavelengths. Chance alignment with a background object seen at 1.4GHz provides potential for confusion, but the star's 1.4"/yr proper motion allows Spitzer 70micron images to confirm that what we are interpreting as disk emission really is circumstellar. Although the exact shape of the disk's inner edge is not well constrained, the region inside 30AU must be significantly depleted in planetesimals. This is readily explained if there are additional planets in the 0.5-30AU region, but is also consistent with collisional erosion. We also find tentative evidence that the presence of detectable debris around nearby stars correlates with the presence of the lowest mass planets that are detectable in current radial velocity surveys. Out of an unbiased sample of the nearest 60 G stars, 11 are known to have planets, of which 6 (including 61 Vir) have planets that are all less massive than Saturn, and 4 of these have evidence for debris. The debris toward one of these planet-hosts (HD20794) is reported here for the first time. This fraction (4/6) is higher than that expected for nearby field stars (15%), and implies that systems that form low-mass planets are also able to retain bright debris disks. We suggest that this correlation could arise because such planetary systems are dynamically stable and include regions that are populated with planetesimals in the formation process where the planetesimals can remain unperturbed over Gyr timescales.

DISCOVERY OF A WOLF-RAYET STAR THROUGH DETECTION OF ITS PHOTOMETRIC VARIABILITY

We report the serendipitous discovery of a heavily reddened Wolf-Rayet star that we name WR142b. While photometrically monitoring a cataclysmic variable, we detected weak variability in a nearby field star. Low-resolution spectroscopy revealed a strong emission line at 7100 Ang., suggesting an unusual object and prompting further study. A spectrum taken with the Hobby-Eberly Telescope confirms strong HeII emission and a NIV 7112 Ang. line consistent with a nitrogen-rich Wolf-Rayet star of spectral class WN6. Analysis of the HeII line strengths reveals no detectable hydrogen in WR142b. A blue-sensitive spectrum obtained with the Large Binocular Telescope shows no evidence for a hot companion star. The continuum shape and emission line ratios imply a reddening of E(B-V)=2.2 to 2.5 mag. If not for the dust extinction, this new Wolf-Rayet star could be visible to the naked eye.

Microlensing of Kepler Stars as a Method of Detecting Primordial Black Hole Dark Matter

If the dark matter consists of primordial black holes (PBHs), we show that gravitational lensing of stars being monitored by NASA's Kepler search for extrasolar planets can cause significant numbers of detectable microlensing events. A search through the roughly 150,000 light curves would result in large numbers of detectable events for PBHs in the mass range 5×10(-10) M(⊙) to 10(-4) M(⊙). Nondetection of these events would close almost 2 orders of magnitude of the mass window for PBH dark matter. The microlensing rate is higher than previously noticed due to a combination of the exceptional photometric precision of the Kepler mission and the increase in cross section due to the large angular sizes of the relatively nearby Kepler field stars. We also present a new formalism for calculating optical depth and microlensing rates in the presence of large finite-source effects.

Observational detection of eclipses of J5 Amalthea by the Galilean satellites

We carried out observations of the small jovian satellite Amalthea (J5) as it was being eclipsed by the Galilean satellites near the 2009 equinox of Jupiter in order to apply the technique of mutual event photometry to the astrometric determination of this satellite's position. The observations were carried out during the period 06/2009-09/2009 from the island of Maui, Hawaii and Siding Spring, Australia with the 2m Faulkes Telescopes North and South respectively. We observed in the near-infrared part of the spectrum using a PanStarrs-Z filter with Jupiter near the edge of the field in order to mitigate against the glare from the planet. Frames were acquired at rates >1/min during eclipse times predicted using recent JPL ephemerides for the satellites. Following subtraction of the sky background from these frames, differential aperture photometry was carried out on Amalthea and a nearby field star. We have obtained three lightcurves which show a clear drop in the flux from Amalthea, indicating that an eclipse took place as predicted. These were model-fitted to yield best estimates of the time of maximum flux drop and the impact parameter. These are consistent with Amalthea's ephemeris but indicate that Amalthea is slightly ahead of, and closer to Jupiter than, its predicted position by approximately half the ephemeris uncertainty in these directions. We argue that a ground-based campaign of higher-cadence photometry accurate at the 5% level or better during the next season of eclipses in 2014-15 should yield positions to within 0".5 and affect a corresponding improvement in Amalthea's ephemeris.

The Unevolved Main Sequence of Nearby Field Stars and the Open Cluster Distance Scale

ABSTRACTThe slope and zero point of the unevolved main sequence as a function of metallicity are investigated using a homogeneous catalog of nearby field stars with absolute magnitudes defined with revised Hipparcos parallaxes, Tycho-2 photometry, and precise metallicities from high-dispersion spectroscopy. (B - V) –temperature relations are derived from 1746 stars between [Fe/H] = -0.5 and +0.6 and 372 stars within 0.05 dex of solar abundance; for Teff = 5770 K, the solar color is B - V = 0.652 ± 0.002 (SEM). From over 500 cool dwarfs between [Fe/H] = -0.5 and +0.5, at fixed MV = 0.213 ± 0.005, with a very weak dependenceupon the adopted main-sequence slope with B - V at a given [Fe/H]. At Hyades metallicity, this translates into at fixed B - V = 0.98 ± 0.02, midway between the range of values empirically derived from smaller and/or less homogenous samples and model isochrones. From field stars of similar metallicity, the Hyades ([Fe/H] = +0.13) with no reddening has (m - M)0 = 3.33 ± 0.02 and M67, with E(B - V) = 0.041, AV = 3.1E(B - V), and [Fe/H] = 0.00, has (m - M)0 = 9.71 ± 0.02 (SEM), where the errors quoted refer to internal errors alone. At the extreme end of the age and metallicity scale, with E(B - V) = 0.125 ± 0.025 and [Fe/H] = +0.39 ± 0.06, comparison of the fiducial relation for NGC 6791 to 19 field stars with (B - V) above 0.90 and [Fe/H] = +0.25 or higher, adjusted to the metallicity of NGC 6791, leads to (m - M)0 = 13.07 ± 0.09, internal and systematic errors included.

THE YOUNG, MASSIVE, STAR CLUSTER SANDAGE-96 AFTER THE EXPLOSION OF SUPERNOVA 2004dj IN NGC 2403

The bright Type II-plateau supernova (SN) 2004dj occurred within the young, massive stellar cluster Sandage-96 in a spiral arm of NGC 2403. New multi-wavelength observations obtained with several ground-based and space-based telescopes are combined to study the radiation from Sandage-96 after SN 2004dj faded away. Sandage-96 started to dominate the flux in the optical bands starting September 2006 (~800 d after explosion). The optical fluxes are equal to the pre-explosion ones within the observational uncertainties. An optical Keck spectrum obtained ~900 d after explosion shows the dominant blue continuum from the cluster stars shortward of 6000 \AA as well as strong SN nebular emission lines redward. The integrated spectral energy distribution (SED) of the cluster has been extended into the ultraviolet region by archival XMM-Newton and new Swift observations, and compared with theoretical models. The outer parts of the cluster have been resolved by the Hubble Space Telescope, allowing the construction of a color-magnitude diagram. The fitting of the cluster SED with theoretical isochrones results in cluster ages between 10--40 Myr, depending on metallicity and the model family. The isochrone fitting indicates that the resolved part of the cluster has a bimodal age distribution: a younger population at ~10--16 Myr, and an older one at ~32--100 Myr which is similar to the age distribution of the nearby field stars. These stars may have been captured from the field during the cluster formation. The young age of Sandage-96 suggest 12 < M_prog < 20 M_\odot as the most probable mass range for the progenitor of SN 2004dj. This is consistent with, but perhaps slightly higher than, most of the other Type II-plateau SN progenitor masses determined so far.

Identification of the TW Hydrae association member 2M1235-39: a tertiary component of the HR 4796 system

Aims. We seek to determine whether the late-type star 2MASS J12354893-3950245 (2M1235-39) is a member of the TW Hya Association (TWA), a hypothesis suggested by its association with a bright X-ray source detected serendipitously by ROSAT and XMM-Newton and its (~3 ' ) proximity to the well-studied (A+M binary) system HR 4796. Methods. We used optical spectroscopy to establish the Li and H α line strengths of 2M1235-39, and determined its proper motion via optical imaging. We also considered its X-ray and near-IR fluxes relative to the M star HR 4796B. Results. The optical spectrum of 2M1235-39 displays strong Li absorption and H α emission (equivalent widths of 630 mA and –6.7 A, respectively). Comparison of the spectrum with that of a nearby field star, along with the DENIS catalog IJK magnitudes, indicates the spectral type of 2M1235-39 is M4.5. We measure a proper motion for 2M1235-39 that agrees, within the errors, with that of HR 4796. Conclusions. The Li absorption and H α emission line strengths of 2M1235-39, its near-IR and X-ray fluxes, and its proper motion all indicate that 2M1235-39 is a TWA member. Most likely this star is a wide (13 500 AU) separation, low-mass, tertiary component of the HR 4796 system.