arXiv subjects

Gregory G. Fahlman

Publications and source records attributed to Gregory G. Fahlman.

At least 19 recordsLinked to original sources

arXiv2013

Comparing the White Dwarf Cooling Sequences in 47 Tuc and NGC 6397

Using deep Hubble Space Telescope imaging, color-magnitude diagrams are constructed for the globular clusters 47 Tuc and NGC 6397. As expected, because of its lower metal abundance, the main sequence of NGC 6397 lies well to the blue of that of 47 Tuc. A comparison of the white dwarf cooling sequences of the two clusters, however, demonstrates that these sequences are indistinguishable over most of their loci - a consequence of the settling out of heavy elements in the dense white dwarf atmosphere and the near equality of their masses. Lower quality data on M4 continues this trend to a third cluster whose metallicity is intermediate between these two. While the path of the white dwarfs in the color-magnitude diagram is nearly identical in 47 Tuc and NGC 6397, the numbers of white dwarfs along the path are not. This results from the relatively rapid relaxation in NGC 6397 compared to 47 Tuc and provides a cautionary note that simply counting objects in star clusters in random locations as a method of testing stellar evolutionary theory is likely dangerous unless dynamical considerations are included.

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arXiv2013

A Dynamical Signature of Multiple Stellar Populations in 47 Tucanae

Based on the width of its main sequence, and an actual observed split when viewed through particular filters, it is widely accepted that 47 Tucanae contains multiple stellar populations. In this contribution, we divide the main-sequence of 47 Tuc into four color groups, which presumably represent stars of various chemical compositions. The kinematic properties of each of these groups is explored via proper-motions, and a strong signal emerges of differing proper-motion anisotropies with differing main-sequence color; the bluest main-sequence stars exhibit the largest proper-motion anisotropy which becomes undetectable for the reddest stars. In addition, the bluest stars are also the most centrally concentrated. A simiilar analysis for SMC stars, which are located in the background of 47 Tuc on our frames, yields none of the anisotropy exhibited by the 47 Tuc stars. We discuss implications of these results for possible formation scenarios of the various populations.

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arXiv2012

Ultra-Deep Hubble Space Telescope Imaging of the Small Magellanic Cloud: The Initial Mass Function of Stars with M <~ 1 Msun

We present a new measurement of the stellar initial mass function (IMF) based on ultra-deep, high-resolution photometry of >5,000 stars in the outskirts of the Small Magellanic Cloud (SMC) galaxy. The Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) observations reveal this rich, co-spatial population behind the foreground globular cluster 47 Tuc, which we targeted for 121 HST orbits. The stellar main sequence of the SMC is measured in the F606W, F814W color-magnitude diagram (CMD) down to ~30th magnitude, and is cleanly separated from the foreground star cluster population using proper motions. We simulate the SMC population by extracting stellar masses (single and unresolved binaries) from specific IMFs, and converting those masses to luminosities in our bandpasses. The corresponding photometry for these simulated stars is drawn directly from a rich cloud of 4 million artificial stars, thereby accounting for the real photometric scatter and completeness of the data. Over a continuous and well populated mass range of M = 0.37 - 0.93 Msun (i.e., down to a ~75% completeness limit at F606W = 28.7), we demonstrate that the IMF is well represented by a single power-law form with slope α= -1.90 (^{+0.15}_{-0.10}) (3 sigma error) (i.e., dN/dM \propto M^α). This is shallower than the Salpeter slope of α= -2.35, which agrees with the observed stellar luminosity function at higher masses. Our results indicate that the IMF does {\it not} turn over to a more shallow power-law form within this mass range. We discuss implications of this result for the theory of star formation, the inferred masses of galaxies, and the (lack of a) variation of the IMF with metallicity.

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arXiv2011

A Deep, Wide-Field, and Panchromatic View of 47 Tuc and the SMC with HST: Observations and Data Analysis Methods

In HST Cycle 17, we imaged the well known globular star cluster 47 Tucanae for 121 orbits using ACS and both the UVIS and IR channels of WFC3 (GO-11677, PI - H. Richer). This unique data set was obtained to address many scientific questions that demand a very deep, panchromatic, and panoramic view of the cluster's stellar populations. In total, the program obtained over 0.75 Ms of imaging exposure time with the three HST cameras, over a time span of 9 months in 2010. The primary ACS field was imaged in the two broadband filters F606W and F814W filters, at 13 orientations, for all 121 orbits. The parallel WFC3 imaging provides a panchromatic (0.4 - 1.7 micron) and contiguous imaging swath over a 250 degree azimuthal range at impact radii of 6.5 -- 17.9 pc in 47 Tuc. This imaging totals over 60 arcmin^2 in area and utilizes the F390W and F606W broadband filters on WFC3/UVIS and the F110W and F160W broadband filters on WFC3/IR. In this paper, we describe the observational design of the new survey and one of the methods used to analyze all of the imaging data. This analysis combines over 700 full-frame images taken with the three HST cameras into a handful of ultra-deep, well-sampled combined images in each of the six filters. The results reveal unprecedented color-magnitude diagrams (CMDs) of the cluster extending to >30th magnitude in the optical, 29th magnitude in the UV, and 27th magnitude in the IR. The data set provides a characterization of the complete stellar populations of 47 Tuc, extending from the faintest hydrogen burning dwarfs through the main-sequence and giant branches, down to very cool white dwarf remnants in the cluster. The imaging also provides the deepest probe of the stellar populations of the background Small Magellanic Cloud (SMC) galaxy, resolving low mass main-sequence dwarfs with M < 0.2 Msun.

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arXiv2008

A Deep HST Study of the Globular Cluster NGC 6397: Reduction Methods

We describe here the reduction methods that we developed to study the faintest red dwarfs and white dwarfs in an outer field of NGC6397, which was observed by \hst for 126 orbits in 2005. The particular challenge of this data set is that the faintest stars are not readily visible in individual exposures, so special care must be taken to combine the information in all the exposures in order to identify and measure them. Unfortunately, it is hard to find the faintest stars without also finding a large number of faint galaxies, so we developed specialized tools to distinguish between the point-like stars and the barely resolved galaxies. We found that artificial-star tests, while obviously necessary for completeness determination, can also play an important role in helping us optimize our finding and measuring algorithms. Although this paper focuses on this data set specifically, many of the techniques are new and might find application in other work, particularly when a large number of images is available for a single field.

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arXiv2008

Deep ACS Imaging in the Globular Cluster NGC6397: Dynamical Models

We present N-body models to complement deep imaging of the metal-poor core-collapsed cluster NGC6397 obtained with the Hubble Space Telescope. All simulations include stellar and binary evolution in-step with the stellar dynamics and account for the tidal field of the Galaxy. We focus on the results of a simulation that began with 100000 objects (stars and binaries), 5% primordial binaries and Population II metallicity. After 16 Gyr of evolution the model cluster has about 20% of the stars remaining and has reached core-collapse. We compare the color-magnitude diagrams of the model at this age for the central region and an outer region corresponding to the observed field of NGC6397 (about 2-3 half-light radii from the cluster centre). This demonstrates that the white dwarf population in the outer region has suffered little modification from dynamical processes - contamination of the luminosity function by binaries and white dwarfs with non-standard evolution histories is minimal and should not significantly affect measurement of the cluster age. We also show that the binary fraction of main-sequence stars observed in the NGC6397 field can be taken as representative of the primordial binary fraction of the cluster. For the mass function of the main-sequence stars we find that although this has been altered significantly by dynamics over the cluster lifetime, especially in the central and outer regions, that the position of the observed field is close to optimal for recovering the initial mass function of the cluster stars (below the current turn-off mass). More generally we look at how the mass function changes with radius in a dynamically evolved stellar cluster and suggest where the best radial position to observe the initial mass function is for clusters of any age.

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arXiv2007

Deep ACS Imaging in the Globular Cluster NGC 6397: The Cluster Color Magnitude Diagram and Luminosity Function

We present the CMD from deep HST imaging in the globular cluster NGC 6397. The ACS was used for 126 orbits to image a single field in two colors (F814W, F606W) 5 arcmin SE of the cluster center. The field observed overlaps that of archival WFPC2 data from 1994 and 1997 which were used to proper motion (PM) clean the data. Applying the PM corrections produces a remarkably clean CMD which reveals a number of features never seen before in a globular cluster CMD. In our field, the main sequence stars appeared to terminate close to the location in the CMD of the hydrogen-burning limit predicted by two independent sets of stellar evolution models. The faintest observed main sequence stars are about a magnitude fainter than the least luminous metal-poor field halo stars known, suggesting that the lowest luminosity halo stars still await discovery. At the bright end the data extend beyond the main sequence turnoff to well up the giant branch. A populous white dwarf cooling sequence is also seen in the cluster CMD. The most dramatic features of the cooling sequence are its turn to the blue at faint magnitudes as well as an apparent truncation near F814W = 28. The cluster luminosity and mass functions were derived, stretching from the turn off down to the hydrogen-burning limit. It was well modeled with either a very flat power-law or a lognormal function. In order to interpret these fits more fully we compared them with similar functions in the cluster core and with a full N-body model of NGC 6397 finding satisfactory agreement between the model predictions and the data. This exercise demonstrates the important role and the effect that dynamics has played in altering the cluster IMF.

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arXiv2007

Probing the Faintest Stars in a Globular Star Cluster

NGC 6397 is the second closest globular star cluster to the Sun. Using 5 days of time on the Hubble Space Telescope, we have constructed the deepest ever color-magnitude diagram for this cluster. We see a clear truncation in each of its two major stellar sequences. Faint red main sequence stars run out well above our observational limit and near to the theoretical prediction for the lowest mass stars capable of stable hydrogen-burning in their cores. We also see a truncation in the number counts of faint blue stars, namely white dwarfs. This reflects the limit to which the bulk of the white dwarfs can cool over the lifetime of the cluster. There is also a turn towards bluer colors in the least luminous of these objects. This was predicted for the very coolest white dwarfs with hydrogen-rich atmospheres as the formation of H2 causes their atmospheres to become largely opaque to infrared radiation due to collision-induced absorption.

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arXiv2007

The Space Motion of the Globular Cluster NGC 6397

As a by-product of high-precision, ultra-deep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly point-like. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find mu_alpha = 3.56 +/- 0.04 mas/yr and mu_delta = -17.34 +/- 0.04 mas/yr. We note that the formal statistical errors quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397's UVW space velocity and its orbit around the Milky Way, and find that the cluster has made frequent passages through the Galactic disk.

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arXiv2004

The Initial-Final Mass Relationship: Spectroscopy of White Dwarfs in NGC 2099 (M37)

We present new observations of very faint white dwarfs (WDs) in the rich open star cluster NGC 2099 (M37). Following deep, wide field imaging of the cluster using CFHT, we have now obtained spectroscopic observations of candidate WDs using both GMOS on Gemini and LRIS on Keck. Of our 24 WD candidates (all fainter than V = 22.4), 21 are spectroscopically confirmed to be bona fide WDs, 4-5 of which are most likely field objects. Fitting 18 of the 21 WD spectra with model atmospheres, we find that most WDs in this cluster are quite massive (0.7-0.9 Msun), as expected given the cluster's young age (650 Myr) and, hence, high turnoff mass (~2.4 Msun). We determine a new initial-final mass relationship and almost double the number of existing data points from previous studies. The results indicate that stars with initial masses between 2.8 and 3.4 Msun lose 70-75% of their mass through stellar evolution. For the first time, we find some evidence of a metallicity dependence on the initial-final mass relationship.

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arXiv2004

Mass Segregation and Tidal Tails of the Globular Cluster NGC 7492

We present a wide field CCD photometric study of Galactic globular cluster NGC 7492. The derived VR color-magnitude diagram extends down to about 3.5 mag below the cluster main sequence turn-off. The field covers 42' X 42' about 3 times larger than the known tidal radius of this cluster. The sample of cluster member candidates obtained by CMD-mask process has been used to construct luminosity and mass functions and surface density map. NGC 7492 has a very flat mass function with very little variation in the slope with the distance from the cluster center. However, there is a clear evidence for the increase of the slope of the mass function from inner to outer regions, indicating a mass segregation of the cluster. The surface density map of NGC 7492 shows extensions toward the Galactic anticenter (northeast) and northwest from the cluster center. A comparison of luminosity function for stars in the tails with that for stars within the tidal radius suggests that the extensions shown in the surface density map could be a real feature. The overall shape of NGC 7492 is significantly flattened. If the flattened shape of the NGC 7492 is caused by its rotation, Galactic tidal field must have given important influences, since the initial rotation would have been almost completely removed by dynamical relaxation.

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arXiv2003

The CFHT Open Star Cluster Survey. IV. Two Rich, Young Open Star Clusters: NGC 2168 (M35) and NGC 2323 (M50)

We continue our study of rich Galactic clusters by presenting deep CCD observations of both NGC 2168 (M35) and NGC 2323 (M50). Both clusters are found to be rich (NGC 2168 contains at least 1000 stars brighter than V = 22 and NGC 2323 contains approximately 2100 stars brighter than our photometric limit of V = 23) and young (age of NGC 2168 = 180 Myrs, age of NGC 2323 = 130 Myrs). The color-magnitude diagrams for the clusters exhibit clear main sequences stretching over 14 magnitudes in the V, B-V plane. Comparing these long main sequences with those of earlier clusters in the survey, as well as with the Hyades, has allowed for accurate distances to be established for each cluster (dist. of NGC 2168 = 912 +/- 70/65 pc, dist. of NGC 2323 = 1000 +/- 81/75 pc). Analysis of the luminosity and mass functions suggest that despite their young ages, both clusters are somewhat dynamically relaxed exhibiting signs of mass-segregation. This is especially interesting in the case of NGC 2323, which has an age of only 1.3 times the dynamical relaxation time. The present photometry is also deep enough to detect all of the white dwarfs in both clusters. We discuss some interesting candidates which may be the remnants of quite massive (M > 5 Mo) progenitor stars. The white dwarf cooling age of NGC 2168 is found to be in good agreement with the main-sequence turn-off age. These objects are potentially very important for setting constraints on the white dwarf initial-final mass relationship and upper mass limit for white dwarf production.

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arXiv2003

The Galactic Inner Halo: Searching for White Dwarfs and Measuring the Fundamental Galactic Constant, Vo/Ro

We establish an extragalactic, zero-motion frame of reference within the deepest optical image of a globular star cluster, an HST 123-orbit exposure of M4 (GO 8679, cycle 9). The line of sight beyond M4 (l,b (deg) = 351,16) intersects the inner halo (spheroid) of our Galaxy at a tangent-point distance of 7.6 kpc (for Ro = 8 kpc). We isolate these spheroid stars from the cluster based on their proper motions over the 6-year baseline between these and previous epoch HST data (GO 5461, cycle 4). Distant background galaxies are also found on the same sight line using image-morphology techniques. This fixed reference frame allows us to independently determine the fundamental Galactic constant, Vo/Ro = 25.3 +/- 2.6 km/s/kpc, thus providing a velocity of the Local Standard of Rest, v = 202.7 +/- 24.7 km/s for Ro = 8.0 +/- 0.5 kpc. Secondly, the galaxies allow a direct measurement of M4's absolute proper motion, mu_total = 22.57 +/- 0.76 mas/yr, in excellent agreement with recent studies. The clear separation of galaxies from stars in these deep data also allow us to search for inner-halo white dwarfs. We model the conventional Galactic contributions of white dwarfs along our line of sight and predict 7.9 (thin disk), 6.3 (thick disk) and 2.2 (spheroid) objects to the limiting magnitude at which we can clearly delineate stars from galaxies (V = 29). An additional 2.5 objects are expected from a 20% white dwarf dark halo consisting of 0.5 Mo objects, 70% of which are of the DA type. After considering the kinematics and morphology of the objects in our data set, we find the number of white dwarfs to be consistent with the predictions for each of the conventional populations. However, we do not find any evidence for dark halo white dwarfs.

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arXiv2003

The Pulsar-White Dwarf-Planet System in Messier 4: Improved Astrometry

A young and undermassive white dwarf has been identified as the possible companion to the millisecond pulsar PSR B1620-26 in Messier 4. This association is important as it then helps constrain the mass of the third body in the system to be of order a few times that of Jupiter. The presence of this planet in M4 has critical implications for planetary formation mechanisms in metal-poor environments such as globular clusters and the early Universe. The identification of the white dwarf is purely via the agreement in position between it and the pulsar and was limited by the accuracy of the pointing of HST which is +/-0.7 arcsec. We have redetermined the position of the white dwarf using ground-based data tied to USNOB-1.0 and find that the pulsar and white dwarf are now coincident to within 0.12 +/- 0.13 arcsec further strengthening the case for association between the two. We have also attempted to improve the proper motion measurement of the white dwarf by a maximum likelihood analysis of the stellar positions measured over a baseline of 5 years. While the errors are reduced by almost a factor of 6 from our previous work, we still have not resolved the cluster's intrinsic dispersion in proper motion. Thus the proper motion of the white dwarf with respect to the cluster itself is still not known although it is very small and is within 2-sigma of that of the cluster internal dispersion.

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arXiv2002

Wide field CCD photometry of the globular cluster M92

We present wide field CCD photometry of a galactic globular cluster M92 obtained in the V and I bands with the CFH12K mosaic CCD at the Canada-France-Hawaii Telescope. A well-defined color-magnitude diagram is derived down to 5 magnitudes fainter than the cluster main sequence turn-off. After removing the background contribution, we obtain luminosity and mass functions, surface density profiles, and the surface number density maps of the stars belonging to the cluster. The surface density profile of all stars shows that the cluster's halo extends at least out to ~30' from the cluster center in agreement with previous study, but the profile of faint stars at the very outer region of the cluster shows a different gradient compared with that of bright stars. For a mass function of the cluster, we find that the inner region of the cluster has x = 1.2+-0.2, whereas the outer region has x = 1.8 +-0.3, clearly indicating a mass segregation of the cluster. An estimate of the photometric mass of the cluster implies that the remnant populations (white dwarfs and neutron stars) contribute at least 25% of the total cluster mass. The surface density map of M92 shows some evidence that the tidal tail of M92 may be oriented perpendicular to the direction toward the Galactic center.

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arXiv2002

The Lower Main Sequence and Mass Function of the Globular Cluster Messier 4

The deepest optical image ever in a globular star cluster, a Hubble Space Telescope (HST) 123 orbit exposure in a single field of Messier 4, was obtained in 2 filters (F606W, F814W) over a 10 week period in early 2001. A somewhat shallower image obtained in 1995 allowed us to select out cluster and field objects via their proper motion displacement resulting in remarkably clean color-magnitude diagrams that reach to V = 30, I = 28. The cluster main sequence luminosity function contains very few stars fainter than MV = 15.0, MI = 11.8 which, in both filters, is more than 2 magnitudes brighter than our limit. This is about the faintest luminosity seen among field Population II subdwarfs of the same metallicity. However, there remains a sprinkling of potential cluster stars to lower luminosity all the way down to our limiting magnitudes. These latter objects are significantly redder than any known metal-poor field subdwarf. Comparison with the current generation of theoretical stellar models implies that the masses of the lowest luminosity cluster stars observed are near 0.09 Msun. We derive the mass function of the cluster in our field and find that it is very slowly rising towards the lowest masses with no convincing evidence of a turnover even below 0.1 Msun. The formal slope between 0.65 and 0.09 Msun is alpha = 0.75 (Salpeter of 2.35) with a 99% confidence interval 0.55 - 1.05. A consistency check between these slopes and the number of observed cluster white dwarfs yields a range of possible conclusions, one of which is that we have indeed seen the termination of the white dwarf cooling sequence in M4.

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arXiv2001

The CFHT Open Star Cluster Survey. III. The White Dwarf Cooling Age of the Rich Open Star Cluster NGC 2099 (M37)

We present deep CCD photometry of the very rich, intermediate aged (similar to the Hyades) open star cluster NGC 2099 (M37). The V, B-V color-magnitude diagram (CMD) for the cluster shows an extremely well populated and very tightly constrained main-sequence extending over 12 magnitudes from the turn-off. The CFH12K photometry for this cluster is faint enough (V ~ 24.5) to detect the remnants of the most massive progenitor cluster stars under the Type I SNe limit. Therefore, the CMD of the cluster also exhibits a well defined white dwarf `clump' caused by the decreased rate of cooling of these stars as they age, and a subsequent gap with very few objects. The termination point (Mv = 11.95 +/- 0.30) of the white dwarf luminosity function gives a white dwarf cooling age of 566 +/- 154/176 Myrs which is in excellent agreement with the main-sequence turn-off isochrone age (520 Myrs). By carefully accounting for errors, we show that the cooling age confirms that models including convective core overshooting are preferred for young-intermediate aged clusters. We also derive the reddening (E(B-V) = 0.21 +/- 0.03) and distance ((m-M)v = 11.55 +/- 0.13) to NGC 2099 by matching main-sequence features in the cluster to a new fiducial main-sequence for the Hyades. As a continuing part of the goals of the CFHT Open Star Cluster Survey to better understand dynamical processes of open clusters, we also fit a King model to the cluster density distribution and investigate the cluster main-sequence luminosity and mass functions in increasing concentric annuli. We find some evidence for mass segregation within the boundary of NGC 2099 as expected given the cluster's age relative to the dynamical age. The present global mass function for the cluster is found to be shallower than a Salpeter IMF.

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arXiv2001

The CFHT Open Star Cluster Survey I -- Cluster Selection and Data Reduction

We present this paper in conjuction with the following as the first results in the CFHT Open Star Cluster Survey. This survey is a large BVR imaging data set of 19 open star clusters in our Galaxy. This data set was taken with the CFH12K mosaic CCD (42' X 28') and the majority of the clusters were imaged under excellent photometric, sub-arcsecond seeing conditions. The combination of multiple exposures extending to deep (V ~ 25) magnitudes with short (< 10 second) frames allows for many different studies. The primary aim of this survey is to catalogue the white dwarf stars in these clusters and establish observational constraints on the initial-final mass relationship for these stars and the upper mass limit to white dwarf production. Additionally, we hope to better determine the properties of the clusters, such as age and distance, and also test evolution and dynamical theories by analyzing luminosity and mass functions. In order to more easily incorporate this data in further studies, we have produced a catalogue of positions, magnitudes, colors, and stellarity confidence for all stars in each cluster of the survey. This paper is intended both as a source for the astronomical community to obtain information on the clusters in the survey and as a detailed reference of reduction procedures for further publications of individual clusters. We discuss the methods employed to reduce the data and compute the photometric catalogue. The scientific results for each individual cluster and global results from the study of the entire survey will be presented in further publications (results for the rich old open cluster NGC 6819 immediately follow this paper).

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