Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Hot Stars of Old Open Clusters: M67, NGC 188 and NGC6791
50
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
We analyze ultraviolet (~1500 A) images of the old open clusters M67, NGC 188, and NGC 6791 obtained with Ultraviolet Imaging Telescope (UIT) during the second flight of the Astro observatory in March 1995. Twenty stars are detected on the UIT image of M67, including 11 blue stragglers, seven white dwarf candidates, and the yellow giant -- white dwarf binary S1040. The ultraviolet photometry of the blue stragglers F90 (S975) and F131 (S1082) suggests that these stars have hot subluminous companions. We present a semi-empirical integrated ultraviolet spectrum of M67, and show that the blue stragglers dominate the integrated spectrum at wavelengths shorter than 2600 A. The number of white dwarfs in M67 is roughly consistent with the number expected from white dwarf cooling models. Eight candidate sdB/sdO stars are detected in NGC 6791, and two are detected in NGC 188. The luminosity range 1.10 < log L/Lsun < 1.27, derived from the ultraviolet photometry of the six sdB candidates, is consistent with theoretical models of metal-rich hot horizontal branch (HB) stars. The fraction of hot HB stars in both NGC 6791 and NGC 188 is about 30%, implying that the integrated spectra of both clusters should show a UV turnup at least as strong as that observed in any elliptical galaxy.
rate research
Read More
This paper provides a detailed comparison of the differences in parameters derived for a star cluster from its color-magnitude diagrams depending on the filters and models used. We examine the consistency and reliability of fitting three widely-used stellar evolution models to fifteen combinations of optical and near-IR photometry for the old open cluster NGC 188. The optical filter response curves match those of the theoretical systems and are thus not the source of fit inconsistencies. NGC 188 is ideally suited to the present study thanks to a wide variety of high-quality photometry and available proper motions and radial velocities which enable us to remove non-cluster members and many binaries. Our Bayesian fitting technique yields inferred values of age, metallicity, distance modulus, and absorption as a function of the photometric band combinations and stellar models. We show that the historically-favored three band combinations of UBV and VRI can be meaningfully inconsistent with each other and with longer baseline datasets such as UBVRIJHKs. Differences among model sets can also be substantial. For instance, fitting Yi et al. (2001) and Dotter et al. (2008) models to UBVRIJHKs photometry for NGC 188 yields the following cluster parameters: age={5.78+ 0.03, 6.45+-0.04} Gyr, [Fe/H]={+0.125+-0.003, -0.077+-0.003} dex, m-M={11.441+-0.007, 11.525+-0.005} mag, and Av={0.162+-0.003, 0.236+-0.003} mag, respectively. Within the formal fitting errors, these two fits are substantially and statistically different. Such differences amongst fits using different filters and models are a cautionary tale regarding our current ability to fit star cluster color-magnitude diagrams. Additional modeling of this kind, with more models and star clusters, and future GAIA parallaxes are critical for isolating and quantifying the most relevant uncertainties in stellar evolutionary models.
The binary fractions of open and globular clusters yield powerful constraints on their dynamical state and evolutionary history. We apply publicly available Bayesian analysis code to a $UBVRIJHK_{S}$ photometric catalog of the old open cluster NGC 188 to detect and characterize photometric binaries along the cluster main sequence. This technique has the advantage that it self-consistently handles photometric errors, missing data in various bandpasses, and star-by-star prior constraints on cluster membership. Simulations are used to verify uncertainties and quantify selection biases in our analysis, illustrating that among binaries with mass ratios >0.5, we recover the binary fraction to better than 7% in the mean, with no significant dependence on binary fraction and a mild dependence on assumed mass ratio distribution. Using our photometric catalog, we recover the majority (65%$pm$11%) of spectroscopically identified main sequence binaries, including 8 of the 9 with spectroscopically measured mass ratios. Accounting for incompleteness and systematics, we derive a mass ratio distribution that rises toward lower mass ratios (within our $q >$0.5 analysis domain). We observe a raw binary fraction for solar-type main sequence stars with mass ratios $q >$0.5 of 42%$pm$4%, independent of the assumed mass ratio distribution to within its uncertainties, consistent with literature values for old open clusters but significantly higher than the field solar-type binary fraction. We confirm that the binaries identified by our method are more concentrated than single stars, in agreement with previous studies, and we demonstrate that the binary nature of those candidates which remain unidentified spectroscopically is strongly supported by photometry from Gaia DR2.
We present the UV photometry of the old open cluster NGC188 obtained using images acquired with Ultraviolet Imaging Telescope (UVIT) on board the ASTROSAT satellite, in two far-UV (FUV) and one near-UV (NUV) filters. UVIT data is utilised in combination with optical photometric data to construct the optical and UV colour-magnitude diagrams (CMDs). In the FUV images, we detect only hot and bright blue straggler stars (BSSs), one hot subdwarf, and one white dwarf (WD) candidate. In the NUV images, we detect members up to a faintness limit of ~22 mag including 21 BSSs, 2 yellow straggler stars (YSSs), and one WD candidate. This study presents the first NUV-optical CMDs, and are overlaid with updated BaSTI-IAC isochrones and WD cooling sequence, which are found to fit well to the observed CMDs. We use spectral energy distribution (SED) fitting to estimate the effective temperatures, radii, and luminosities of the UV-bright stars. We find the cluster to have an HB population with three stars (Teff = 4750 - 21000 K). We also detect two yellow straggler stars, with one of them with UV excess connected to its binarity and X-ray emission.
Determinations of beryllium abundance in stars, together with lithium, provide a key tool to investigate the so far poorly understood extra-mixing processes at work in stellar interiors. We measured Be in three open clusters,complementing existing Be surveys, and aiming at gathering a more complete empirical scenario of the evolution of Be as a function of stellar age and temperature. Specifically, we analyzed VLT/UVES spectra of members of NGC 2516, the Hyades, and M 67 to determine their Be and Li abundances. In the first two clusters we focused on stars cooler than 5400 K, while the M 67 sample includes stars warmer than 6150 K, as well as two subgiants and two blue stragglers. We also computed the evolution of Be for a 0.9 Mo star based on standard evolutionary models. We find different emprical behaviours for stars in different temperature bins and ages. Stars warmer than 6150 K show Be depletion and follow a Be vs. Li correlation while Be is undepleted in stars in the ~6150-5600 K range. NGC 2516 members cooler than 5400 K have not depleted any Be, but older Hyades of similar temperature do show some depletion. Be is severely depleted in the subgiants and blue stragglers. The results for warm stars are in agreement with previous studies, supporting the hypothesis that mixing in this temperature regime is driven by rotation. The same holds for the two subgiants that have evolved from the Li gap. This mechanism is instead not the dominant one for solar-type stars. We show that Be depletion of cool Hyades cannot simply be explained by the effect of increasing depth of the convective zone. Finally, the different Be content of the two blue stragglers suggests that they have formed by two different processes (i.e., collisions vs. binary merging).
We present the results of a 47-ks Chandra-ACIS observation of the old open cluster M67. We detected 25 proper-motion cluster members (including ten new sources) and 12 sources (all new) that we suspect to be members from their locations close to the main sequence (1 < B-V < 1.7). Of the detected members, 23 are binaries. Among the new sources that are members and probable members are four spectroscopic binaries with P_orb < 12 d, two contact binaries and two periodic photometric variables with P_ph < 8.4 d. Their X-rays are likely the result of coronal activity enhanced by tidally locked rapid rotation. The X-rays of the new source S997, a blue straggler in a wide eccentric orbit, are puzzling. Spectral fits show that the X-rays of the brightest sources S1063 (a binary with a sub-subgiant), S1082 (a triple blue straggler with a close binary) and S1040 (a circular binary of a giant and a cool white dwarf), are consistent with coronal emission. We detected a new bright source that must have brightened at least about ten times since the time of the ROSAT observations. It is not clear whether its faint blue optical counterpart belongs to M67. We discuss the possibility that this source is a low-mass X-ray binary in quiescence, which would be the first of its kind in an open cluster. In addition to cluster members, we detected about 100 background sources, many of which we identify with faint objects in the ESO Imaging Survey.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
