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Register a new userHunting for Brown Dwarfs in the Globular Cluster M4: 2nd epoch HST NIR observations
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We present an analysis of the second epoch HST WFC3 F110W near-Infrared (NIR) imaging data of the globular cluster M4. The new dataset suggests that one of the previously suggested four brown dwarf candidates in this cluster is indeed a high-probability cluster member. The position of this object in the NIR colour magnitude diagrams (CMDs) is in the white dwarf/brown dwarf area. The source is too faint to be a low-mass main sequence star, but, according to theoretical considerations, also most likely somewhat too bright to be a bona-fide brown dwarf. Since we know that the source is a cluster member, we determined a new optical magnitude estimate at the position the source should have in the optical image. This new estimate places the source closer to the white dwarf sequence in the optical-NIR CMD and suggests that it might be a very cool (T_eff < 4500 K) white dwarf at the bottom of the white dwarf cooling sequence in M4, or a white dwarf/brown dwarf binary. We cannot entirely exclude the possibility that the source is a very massive, bright brown dwarf, or a very low-mass main sequence star, however, we conclude that we still have not convincingly detected a brown dwarf in a globular cluster, but we expect to be very close to the start of the brown dwarf cooling sequence in this cluster. We also note that the main sequence ends at F110W approx.22.5 mag in the proper-motion cleaned CMDs, where completeness is still high.
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We present an analysis of deep HST/WFC3 near-IR (NIR) imaging data of the globular cluster M4. The best-photometry NIR colour-magnitude diagram (CMD) clearly shows the main sequence extending towards the expected end of the Hydrogen-burning limit and going beyond this point towards fainter sources. The white dwarf sequence can be identified. As such, this is the deepest NIR CMD of a globular cluster to date. Archival HST optical data were used for proper-motion cleaning of the CMD and for distinguishing the white dwarfs (WDs) from brown dwarf (BD) candidates. Detection limits in the NIR are around F110W approx 26.5 mag and F160W approx27 mag, and in the optical around F775W approx 28 mag. Comparing our observed CMDs with theoretical models, we conclude that we have reached beyond the H-burning limit in our NIR CMD and are probably just above or around this limit in our optical-NIR CMDs. Thus, any faint NIR sources that have no optical counterpart are potential BD candidates, since the optical data are not deep enough to detect them. We visually inspected the positions of NIR sources which are fainter than the H-burning limit in F110W and for which the optical photometry did not return a counterpart. We found in total five sources for which we did not get an optical measurement. For four of these five sources, a faint optical counterpart could be visually identified, and an upper optical magnitude was estimated. Based on these upper optical magnitude limits, we conclude that one source is likely a WD, one source could either be a WD or BD candidate, and the remaining two sources agree with being BD candidates. For only one source no optical counterpart could be detected, which makes this source a good BD candidate. We conclude that we found in total four good BD candidates.
Using WFPC2 on the Hubble Space Telescope, we have isolated a sample of 258 white dwarfs (WDs) in the Galactic globular cluster M4. Fields at three radial distances from the cluster center were observed and sizeable WD populations were found in all three. The location of these WDs in the color-magnitude diagram, their mean mass of 0.51($ pm 0.03$)M$_{odot}$, and their luminosity function confirm basic tenets of stellar evolution theory and support the results from current WD cooling theory. The WDs are used to extend the cluster main-sequence mass function upward to stars that have already completed their nuclear evolution. The WD/red dwarf binary frequency in M4 is investigated and found to be at most a few percent of all the main-sequence stars. The most ancient WDs found are about 9 Gyr old, a level which is set solely by the photometric limits of our data. Even though this is less than the age of M4, we discuss how these cooling WDs can eventually be used to check the turnoff ages of globular clusters and hence constrain the age of the Universe.
Globular clusters are the oldest conglomerates of stars in our Galaxy and can be useful laboratories to test theories from stellar evolution to cosmology. In this paper, we present a new method to estimate the absolute age of a globular cluster from observations of its brown dwarfs. The transition region between the end of the main sequence and the brown dwarf regime is characterized by a dearth of objects as function of magnitude. The brightest of the cooling brown dwarfs is easily identified by an increase in density in the color-magnitude diagram as you go fainter in magnitudes, and these brightest brown dwarfs get fainter with age. By identifying the brightest brown dwarfs, it is thus possible to determine the age of a globular cluster within a 1 Gyr precision with four-sigma confidence. This new method, which is independent of current methods of age estimation and which does not rely on the knowledge of the clusters distance from Earth, will become feasible thanks to the high spatial resolution and incredible infrared sensitivity of the James Webb Space Telescope.
We report new metallicities for stars of Galactic globular cluster M4 using the largest number of stars ever observed at high spectral resolution in any cluster. We analyzed 7250 spectra for 2771 cluster stars gathered with the VLT FLAMES+GIRAFFE spectrograph at VLT. These medium resolution spectra cover by a small wavelength range, and often have very low signal-to-noise ratios. We attacked this dataset by reconsidering the whole method of abundance analysis of large stellar samples from beginning to end. We developed a new algorithm that automatically determines the atmospheric parameters of a star. Nearly all data preparation steps for spectroscopic analyses are processed on the syntheses, not the observed spectra. For 322 Red Giant Branch stars with $V leq 14.7$ we obtain a nearly constant metallicity, $<[{rm Fe}/{rm H}]> = -1.07$ ($sigma$ = 0.02). No difference in the metallicity at the level of $0.01 ~textrm{dex}$ is observed between the two RGB sequences identified by cite{Monelli:2013us}. For 1869 Subgiant and Main Sequence Stars $V > 14.7$ we obtain $<[{rm Fe}/{rm H}]> = -1.16$ ($sigma$ = 0.09) after fixing the microturbulent velocity. These values are consistent with previous studies that have performed detailed analyses of brighter RGB stars at higher spectroscopic resolution and wavelength coverage. It is not clear if the small mean metallicity difference between brighter and fainter M4 members is real or is the result of the low signal-to-noise characteristics of the fainter stars. The strength of our approach is shown by recovering a metallicity close to a single value for more than two thousand stars, using a dataset that is non-optimal for atmospheric analyses. This technique is particularly suitable for noisy data taken in difficult observing conditions.
With the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope, we have discovered in M4 (NGC 6121, C 1620-264) the first extensive sequence of cooling white dwarfs seen in a globular cluster. Adopting a distance modulus of (m-M)_V = 12.65 and a reddening of E(B-V) = 0.37, we show that the sequence, which extends over 9 < M_U < 13, is comprised of white dwarfs of mass sim 0.5 M_{odot}. The total mass loss from the present turnoff to the white dwarf sequence is 0.31 M_{odot} and the intrinsic dispersion in the mean mass appears to be < 0.05 M_{odot}. Both the location of the white dwarf cooling sequence in the cluster color-magnitude diagram and the cumulative luminosity function attest to the basic correctness and completeness of the physics in theoretical models for the upper three magnitudes of the observed white dwarf cooling sequence. To test the theory in globular clusters at cooling ages beyond sim 3 times 10^8 years will require deeper and more complete data.
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