Turn-off Point Research Articles

A multicolour CCD photometric and mass function study of the distant southern open star clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253

We derive cluster parameters and mass functions from new UBVRI CCD photometric observations of ~3500 stars reaching down to V ~20mag for the distant southern open star clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253. For NGC 3105 and Hogg 15, CCD data are presented for the first time. The reddening is non-uniform across the face of the young (age < 300Myr) clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15 and Pismis 20, with average values of E(B-V)=1.06, 1.44, 0.52, 1.15 and 1.20mag respectively, while it is uniform with average values of E(B-V)=0.72 and 0.20mag for the older clusters NGC 4815 and 6253 respectively. The values of colour excess ratios indicate the presence of normal interstellar reddening across the cluster regions studied here. Well-defined main sequences can be seen in all the clusters. However, main-sequence turn-off points and subgiant branches are well defined only in the older clusters NGC 4815 and 6253. The distances to the clusters NGC 3105, NGC 3603, Melotte 105, Hogg 15, NGC 4815, Pismis 20 and NGC 6253 are 9.5±1.5, 7.2±1.2, 2.3±0.2, 3.0±0.3, 2.75±0.2, 3.55±0.35 and 1.8±0.12kpc respectively, while the corresponding ages derived using theoretical convective core overshooting stellar evolutionary isochrones are 25±10, 3±2, 250±30, 6±2, 400±50, 6±2 and 2500±600Myr respectively. In the mass range ~1-75M⊙, the mass functions of all clusters except for NGC 6253 have been studied. The slopes of their mass spectra agree within errors with the Salpeter value (1.35). The slope of the mass function for stars more massive than 10M⊙ is almost the same as for the lower mass stars. The mass function slopes of clusters younger than 500Myr seem to have no dependence on Galactic longitude, Galactocentric distance and cluster age. As the inherent uncertainties in the mass function determinations of young Galactic star clusters can produce internal scatter that is larger than the external scatter, we conclude that, above 1M⊙, the initial mass function is universal with a slope of Salpeter value.

Age as the Second Parameter in NGC 288/NGC 362? I. Turnoff Ages: A Purely Differential Comparison

We present deep V, I photometry of the globular clusters NGC 288, 362, and 1851 obtained during a single observational run under strictly homogeneous conditions. We use the bimodal horizontal branch (HB) of NGC 1851 as a "bridge" to obtain the optimum relative match between the HBs of NGC 288 and NGC 362. In this way we can effectively remove the uncertainties associated with distance, reddening, and inhomogeneities in the absolute calibration, thus obtaining a very robust, purely differential estimate of the age difference between these two clusters. According to the bridge test, NGC 288 is found to be older than NGC 362 by 2 ± 1 Gyr. This result is fully confirmed also by all classical differential age diagnostics, either based on the luminosity (ΔV) or color [δ(V-I)@2.5] of the main-sequence turnoff point.

The IMF of open star clusters with Tycho-2

We studied the fields of nine nearby open star clusters based on the Tycho-2 catalogue. The clusters are: Blanco 1, Stock 2, the alpha Per cluster, the Pleiades, NGC 2451, IC 2391, Praesepe, IC 2602, and NGC 7092. We determined membership probabilities for the stars in the cluster fields from the stellar proper motions and used the Tycho-2 photometry to compute the initial mass function (IMF) of the clusters from the main sequence turn-off point down to approx. 1 M_sun. We found IMF slopes ranging from Gamma=-0.69 down to Gamma=-2.27 (when the Salpeter (1955) value would be Gamma=-1.35). We also studied the membership of individual stars of special astrophysical interest. In some cases previous results had to be revised. As a by-product, we investigated some general properties of the Tycho-2 catalogue; we confirmed that the Tycho-2 proper motions show only marginal deviations from the Hipparcos catalogue. On the other hand, in some regions the completeness of the catalogue seems to decrease at magnitudes brighter than claimed by Hog et al. (2000).

W UMa-Type Binary Stars in Globular Clusters

A sample of 86 contact binary systems in 14 globular clusters with available color index data in B-V or in V-I has been analyzed. A large fraction of all systems (at least one-third) are numerous foreground Galactic disk projections over long lines of sight to the clusters. Since the selection of the cluster members has been based on the MV(log P,color) calibrations, the matter of a metallicity correction required particular attention with the result that such a correction is apparently not needed at the present level of accuracy. Analysis of the color-magnitude and period-color relations shows that globular cluster members have different properties from the Galactic disk contact systems: They are underluminous mainly because of the smaller sizes and, consequently, have shorter orbital periods; the color-index effect of the diminished blanketing is relatively less important, especially for V-I. Among the class 1 members (deviations in MV smaller than 0.5 mag), the most common are blue straggler (BS) systems. The variability amplitudes for the BS systems show a significantly different distribution from that for systems below the turn-off point (TOP): The BS systems in the sample have only small amplitudes (which may be an indication of small mass ratios), while the distribution for the systems below the TOP is peculiar in containing only large-amplitude systems. This difference may be linked to the relatively small number of the detected main-sequence contact systems below the TOP as resulting from an observational selection effect due to the rapidly increasing measurement difficulties below the TOP. As a consequence, efforts at determining the frequency of occurrence of the contact systems below the TOP have been judged to be premature. The frequency among the BS stars could be moderately well established at about 45 ± 10 BS stars per one contact BS binary; thus, contact binaries are about 3 times more common among the BS stars than among the disk population dwarfs. Contact binary systems with periods longer than 0.6 days are absent in the sample, possibly because the more massive stars have left the contact binary domain.

The Age of the Sculptor Dwarf Spheroidal Galaxy from Imaging with WFPC211Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5‐26555.

ABSTRACTFrom images taken with the Hubble Space Telescope's WFPC2, we have obtained photometry of a field in the Sculptor dwarf spheroidal galaxy to 3 mag below the main‐sequence turnoff. We determine an age equal to that of the earliest globular clusters for the bulk of the stars in our field of view. We attempt to constrain the star formation history of the Sculptor dwarf by examining the main‐sequence luminosity function. The presence of a half‐dozen blue straggler candidates blueward of the turnoff points to a possibly complex star formation history. However, the contribution of any intermediate‐age population is difficult to measure conclusively, because of the uncertain origin of blue stragglers and the sparseness of the photometric sample.

The Luminosity Function of M3

We present a high precision, large sample luminosity function (LF) for the Galactic globular cluster M3. With a combination of ground based and Hubble Space Telescope data we cover the entire radial extent of the cluster. The observed LF is well fit by canonical standard stellar models from the red giant branch (RGB) tip to below the main sequence turnoff point. Specifically, neither the RGB LF-bump nor subgiant branch LF indicate any breakdown in the standard models. On the main sequence we find evidence for a flat initial mass function and for mass segregation due to the dynamical evolution of the cluster.

RR Lyrae Luminosity Differences between Oosterhoff Group I and II Cluster Systems and the Origin of the Oosterhoff Dichotomy

We present a comparative study of the Oosterhoff II cluster M2 and the Oosterhoff I cluster M3. Both have similar metallicities, [Fe/H] = -1.62 for M2 and -1.66 for M3, but very different horizontal-branch (HB) morphologies (B-R)/(B + V + R) = 0.92 for M2 and 0.08 for M3. A period shift analysis and main-sequence fitting show that RRab variables in M2 are about 0.2 mag brighter than those in M3. Comparisons of the M2 period shift with Oosterhoff I clusters NGC 3201 and NGC 7006 also yield similar results, while a comparison between M2 and the Oosterhoff II cluster NGC 5986 reveals that the RR Lyrae luminosities are very similar. The luminosity difference is thought to be due to the evolutionary effect described in 1990 by Lee, Demarque, & Zinn: the M2 RRab variables have evolved away from the zero-age horizontal branch (ZAHB), while most M3 RRab variables lie near the ZAHB. A comparison of the mean period change rates of two clusters supports this hypothesis. Our relative age estimation using the difference in color between the base of giant branch and turn-off point shows that M2 is about 2 Gyr older than M3. Our result strongly suggests that the Oosterhoff dichotomy is due to age differences between Oosterhoff group I and II. This is consistent with the idea that the global second parameter is age. We discuss the kinematic differences between Oosterhoff group I and II clusters. Our result shows that the Oosterhoff group I clusters have zero or retrograde rotation with vrot = -68 ± 56 km s-1 and σlos = 131 ± 28 km s-1, while the Oosterhoff group II clusters have prograde rotation with vrot = +94 ± 47 km s-1 and σlos = 115 ± 29 km s-1, confirming a similar conclusion of van den Bergh. The difference in kinematics and ages between Oosterhoff group I and II clusters suggests that they may have different origins: The Oosterhoff II clusters were formed very early in the proto-Galaxy while the Oosterhoff I clusters were formed at different locations and at a later time, and were probably merger events. The period distributions of an unbiased sample of field RRab variables with | Z | ≤ 3 kpc and | Z | ≥ 5 kpc indicate that they may belong to different populations, with peak periods of 0.65 and 0.55 days, respectively. If the hypothesis that the Oosterhoff dichotomy is due to evolution is correct, then this period shift among the field RR Lyrae variables suggests that the RRab population with | Z | ≤ 3 kpc is somewhat older than the RRab population with | Z | ≥ 5 kpc. This also suggests different formation histories. In an appendix, we discuss that the frequently used Gaussian HB mass-dispersion rate (i.e., the mass-loss rate at the red giant branch [RGB] tip) in synthetic HB model calculations cannot fully explain the extended blue HB population and the pulsational properties of RR Lyrae variables in M2. Comparisons with synthetic HB models strongly suggests that an enhanced mass loss is required that extends the HB toward lower HB masses. We also discuss statistical effects on the metallicity estimate using P0,min for field RRab variables reported by Castellani, Maceroni, & Tosi in 1983. Our calculations suggest that the statistical effect is sufficient to explain the apparent gradient in P0,min without introducing a metallicity gradient.

Variable stars in the globular cluster NGC 6362

ABSTRA C T We report the results of a CCD search for short-period variables in the field of a southern globular cluster NGC 6362. We identified 19 new candidate variables, five of which are cluster RR Lyraes, four are probable SX Phe-type stars and eight are eclipsing binaries. Of the discovered binaries, three are EA-type systems, two of which are located on the cluster CMD in the turn-off region and the third 1 mag above the turn-off point, in the yellow straggler region. Five other systems are of W UMa type, three of which are foreground objects and two are likely cluster members. The remaining two candidates exhibit a modulation of the brightness level with an amplitude of 0.1‐0.2 mag; further observations are needed to reveal the nature of their variability. Phased VI light curves for 20 variables (18 RR Lyrae stars and two blue stragglers) located in the central region of the cluster are presented.

Deriving star formation histories: inverting Hertzsprung--Russell diagrams through a variational calculus maximum likelihood method

We introduce a new method for solving maximum likelihood problems through variational calculus, and apply it to the case of recovering an unknown star formation history, SFR(t), from the resulting Hertzsprung--Russell (HR) diagram. This approach allows a totally non-parametric solution, which has the advantage of requiring no initial assumptions about SFR(t). As a full maximum likelihood statistical model is used, and we take advantage of all the information available in the HR diagram, rather than concentrating on particular features such as turn-off points or luminosity functions. We test the method using a series of synthetic HR diagrams produced from known SFR(t), and find it to be quite successful under noise conditions comparable to those present in current observations. At this point we restrict the analysis to situations in which the metallicity of the system is known, as is the case with the resolved population of dwarf spheroidal companions to the Milky Way or the solar neighbourhood Hipparcos data. We also include tests to quantify the way uncertainties in the assumed metallicity, binary fraction and initial mass function (IMF) affect our inferences.

The Lithium-rich supergiant HD172365

The F-type supergiant HD172365 was discovered by Luck (1982) to be a high Li abundance star. We find that the lithium abundance in this star is log A(Li) = 2.9 from a careful spectroscopic study. This value is larger than the abundance derived by Luck. We discuss the properties of this supergiant and tentatively conclude that HD172365 may, in fact, be a post-blue straggler star with a mass ∼2 times greater than the “turn-off point” mass for the open cluster IC 4756, of which HD172365 is a member. The star is formed by a merger of a close binary system. This conclusion is supported by, for example, its large projected rotational velocity, solar carbon abundance and high lithium content.

Multicolor CCD Photometry of the Poorly Studied Globular Cluster M80

We present results of the first multicolor (UBVI) CCD photometry of 4899 stars in a wide (4' × 7') field of the poorly studied globular cluster M80. Our results feature both a wide magnitude range (11.5 < V < 21.5) and a relatively large number of stars belonging to brighter sequences of the color-magnitude diagram (CMD). A comparison between the V-(B-V) CMDs of M80 and M79 (NGC 1904) shows a notable similarity in morphology and similar slope of the red giant branches (RGBs) of both clusters. Moreover, the V magnitude of the bump in the luminosity function of the RGB compared with the position of the horizontal branch (HB) and of the turnoff point corresponds to a most probable metallicity value of [Fe/H] = 1.70 ± 0.15 for M80. A comparison of the CMDs and of the HB luminosity functions of the two clusters demonstrates that the two HBs span an almost identical range in V and have a similar uneven (fragmented) distribution of stars along this branch. However, we find a difference (earlier noted for NGC 6541 and M79) between M79 and M80 in the position of the HB relative to the RGB, both in the U-(B-V) CMD and in the two-color (U-B) - (B-V) diagram. Our photometry of M80 yields a number of very probable UV-bright star and blue straggler candidates. The latter can be grouped, by photometric characteristics and position in the cluster, into two distinct classes of apparently different nature. The difference in V magnitude between the HB and the turnoff point of M80 is ΔV = 3.60 ± 0.09. The helium abundance estimated from the R-method is Y = 0.21 ± 0.02. The age of the cluster, determined by comparing the difference in color between the turnoff point and the lower RGB in the V-(B-V) diagram with that for the oxygen-enhanced isochrones, is 17 ± 2 Gyr. However, similar differences in other colors do not lead to a single age value.

NTT CCD photometry of the globular cluster M 79 = NGC 1904 in UBV

We have obtained new CCD photometry (with short exposures) of the Milky Way globular cluster M 79 using the NTT under excellent seeing. The main-sequence turnoff point is at , ; the horizontal-branch level at the blue edge of the instability strip is . The cluster metallicity, estimated from the positions of the bump” on the red giant branch, is [Fe/H] . We discuss the luminosity functions of the horizontal branch and the asymptotic giant branch, in particular, the fragmentary structure of the horizontal branch in M 79 and in other clusters. Our isochrone analysis shows that M 79 belongs to the oldest Milky Way globular clusters. Its age, in the scale of Bergbusch & VandenBerg (1992) oxygen-enhanced isochrones, is Gyr, and in the scale of VandenBerg & Bell (1985) isochrones it is Gyr. The cluster has an extremely blue horizontal branch at intermediate metallicity and may be considered a good example of the second parameter” interpreted as age. The apparent distance modulus of the cluster from Bergbusch and VandenBerg isochrones is in good agreement with the value derived from the position of the horizontal branch ().

The Color‐Magnitude Diagram in Baade's Window Revisited

We have reanalyzed the OGLE V and I photometry of ~500,000 stars in Baade's Window, and we confirm the extinction map published by Stanek. However, we find that the interpretation of the OGLE color-magnitude diagram for the disk stars proposed by Paczynski et al. was incorrect: the dominant disk population in Baade's Window is old, and we find no evidence for a large hole in the inner Galactic disk. We find evidence for a small systematic error in the OGLE photometry for stars below the clump: the (V-I) color indices of the faint stars with V > 18 are too red by ~0.1 mag. We find tentative evidence from the OGLE and HST photometry that the bulge main-sequence turnoff point is brighter by ~0.5 mag than it is in either 47 Tuc or NGC 6791, indicating that the dominant population of the Galactic bulge is considerably younger than those clusters.

Collisions of Main-Sequence Stars and the Formation of Blue Stragglers in Globular Clusters

We report the results of new SPH calculations of parabolic collisions between two main-sequence stars in a globular cluster. Such collisions are directly relevant to the formation of blue stragglers. In particular, we consider parent stars of mass M/MTO = 0.2, 0.5, 0.75, and 1, where MTO is the cluster turnoff mass (typically about 0.8M⊙). Low-mass stars (with M = 0.2MTO or 0.5MTO) are assumed to be fully convective and are therefore modeled as n = 1.5 polytropes. Stars at the turnoff (with M = MTO) are assumed to be mostly radiative and are modeled as n = 3 polytropes. Intermediate-mass stars (with M = 0.75MTO) are modeled as composite polytropes consisting of a radiative core with polytropic index n = 3 and a convective envelope with n = 1.5. We focus our study on the question of hydrodynamic mixing of helium and hydrogen, which plays a crucial role in determining the observable characteristics of blue stragglers. In all cases we find that there is negligible hydrodynamic mixing of helium into the outer envelope of the merger remnant. The amount of hydrogen mixed into the core of the merger depends strongly on the entropy profiles of the two colliding stars. For two stars with nearly equal masses (and hence entropy profiles) very little hydrodynamic mixing occurs at all, especially if they are close to the turnoff point. This is because the hydrogen-rich material from both stars maintains, on average, a higher specific entropy than the helium-rich material. If the two parent stars are close to turnoff, very little hydrogen is present at the center of the merger remnant and the main-sequence lifetime of the blue straggler could be very short. In contrast, during a collision between two stars of sufficiently different masses (mass ratio q ∼< 0.5), the hydrogen-rich material Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853. Email: lombardi@spacenet.tn.cornell.edu. Department of Astronomy, Cornell University. Department of Physics, MIT 6-201, Cambridge, MA 02139. Email: rasio@mit.edu. Center for Astrophysics & Relativity, 326 Siena Drive, Ithaca, NY 14850

Main-Sequence Stars and the Star Formation History of the Outer Disk in the Large Magellanic Cloud

Using the Wide Field Planetary Camera 2 on the Hubble Space Telescope, we have obtained a deep color-magnitude diagram in V- and I-band equivalents for more than 2000 stars in a patch of the outer disk of the Large Magellanic Cloud LMC). Aperture photometry is feasible from these data with good signal-to-noise ratio for stars with V ≤ 25, which allows us for the first time to construct a color magnitude diagram for LMC disk stars on the lower main sequence, extending beyond the oldest main sequence turnoff point. We analyze the structure of the main-sequence band and overall morphology of the color-magnitude diagram to obtain a star formation history for the region. A comparison between the distribution of stars across the main-sequence band for M_v ≤ 4 and a stellar population model constrains historical star formation rates within the past 3 Gyr. The stellar populations in this region sample the outer LMC disk for stars with ages of 1 Gyr or older that have had time to spatially mix. The structure of the main-sequence band requires that star formation occurred at a roughly constant rate during most of the past ≈ 3 Gyr. However, the distribution of subgiant stars indicate that a pronounced peak in the star formation rate likely occurred about 2 Gyr ago, prior to which the star formation rate had not been enhanced for several Gyr. Studies over timescales of more than 3 Gyr require a separation of the effects of star formation history and the chemical evolution on the LMC color-magnitude diagrams, which is difficult to achieve without additional constraints. If lower main-sequence stars in the LMC have moderate metallicities, then the age for most LMC disk stars is less than about 8 Gyr.

Flare Production of 6Li in Population II Stars

Motivated by the recent report of a $^6$Li detection in the atmosphere of HD~84937, we couple stellar evolution calculations with light isotope production via stellar flares. We find that as a consequence of their small convective envelope mass near the turn-off point, low-metallicity dwarfs and subgiants may possess observable amounts of $^6$Li arising from such flare activity. We point out an observational test which could discriminate between flare produced $^6$Li and protostellar $^6$Li. In the $T_{\rm eff}$ range $6000-6600$~K, the $^7$Li/$^6$Li ratio on the subgiant branch should increase as a function of $T_{\rm eff}$ if flare production is important, whereas the same ratio should be constant if a protostellar origin is the source of the observed lithium. The absence of a flare produced variation in the $^7$Li/$^6$Li ratio would allow for a more reliable inference of the -- cosmologically important -- atmospheric depletion mechanisms in stars.

On blue straggler formation by direct collisions of main sequence stars

We report the results of new SPH calculations of parabolic collisions between main-sequence (MS) stars. The stars are assumed to be close to the MS turn-off point in a globular cluster and are therefore modeled as $n=3$, $\Gamma=5/3$ polytropes. We find that the high degree of central mass concentration in these stars has a profound effect on the hydrodynamics. In particular, very little hydrodynamic mixing occurs between the dense, helium-rich inner cores and the outer envelopes. As a result, and in contrast to what has been assumed in previous studies, blue stragglers formed by direct stellar collisions are not necessarily expected to have anomalously high helium abundances in their envelopes or to have their cores replenished with fresh hydrogen fuel.

Galactic clusters with associated Cepheid variables. 4: C2128+488 (Anon. Platais) and V1726 Cygni

view Abstract Citations (20) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Galactic Clusters With Associated Cepheid Variables. IV. C2128+488 (ANON. PLATAIS) And V1726 Cygni Turner, David G. ; Mandushev, Georgi I. ; Forbes, Douglas Abstract New photoelectric UBV photometry of 32 stars, photographic UBV photometry of 120 stars, and spectroscopic observations of 8 stars are presented for C 2128+488 (Anon. Platais), the cluster associated with the 4d.237 s-Cepheid V1726 Cygni. The resulting photometry, spectral classifications, and radial velocities are used with spectroscopic and proper motion data previously published by Platais and others for the Cepheid and other stars in this field in a detailed cluster analysis. The newly obtained cluster distance modulus is Vzero - MV = 10.98 +/- 0.02s.e. (d = 1568 +/- 13 pc), and a value of R = AV/EV-V = 3.07 +/- 0.27 s.e. is found to describe the dust extinction in the field. A space reddening of EB-V = 0.43 +/- 0.02 is derived for V1726 Cyg from two late B-type companions which bracket the Cepheid, as well as from the reddenings of other stars in the field. Its resulting luminosity as a probable cluster member is (MV) = -3.42 +/- 0.07. A cluster radial velocity of -15.4 +/-0.2 km/sec and a cluster turnoff point at spectral type B7 are both consistent with the likely membership of V1726 Cyg, as are the available proper motion data. The small amplitude, sinusoidal light curve, and location of V1726 Cyg on the blue edge of the Cepheid instability strip are consistent with the properties expected for overtone pulsation, although such a possibility appears to imply an unrealistically small value (beta = 0.5 +/- 0.2) for the color coefficient in the PLC relation. A more reasonable value of beta = 2.1 +/- 0.2 is derived for the color term applicable to galactic cluster Cepheids in this program when V1726 Cyg is assumed to be pulsating in the fundamental mode. Publication: The Astronomical Journal Pub Date: May 1994 DOI: 10.1086/116988 Bibcode: 1994AJ....107.1796T Keywords: Astronomical Photometry; Cepheid Variables; Classifications; Color-Color Diagram; Color-Magnitude Diagram; Cygnus Constellation; Galactic Clusters; Spectroscopic Analysis; Astronomical Observatories; Electrophotometers; Optical Filters; Photographic Plates; Photographs; Photomultiplier Tubes; Radial Velocity; Telescopes; Astronomy; CEPHEIDS full text sources ADS | data products SIMBAD (160) GCPD (1) Related Materials (6) Part 1: 1986AJ.....92..111T Part 2: 1992AJ....104.1132T Part 3: 1992AJ....104.1865T Part 5: 1997AJ....113.2104T Part 6: 1998AJ....115.1958T Part 7: 2008MNRAS.388..444T

Discovery of 17 variable stars in the old open cluster NGC 6791

We report results of photometric monitoring of the NGC 6791 field in V and B on four nights. We discovered eight contact binaries and nine other variables, with the majority of the latter (which are probably also binaries) observed in the region of the turn-off point of the cluster. The contact binaries seem to exhibit various stages of evolution, but two of them are blue stragglers. One of the variables is probably a binary composed of a red dwarf and a hot subdwarf

White dwarfs in the halo of the Hyades Cluster - The case of the missing white dwarfs

On subtracting the cooling age of the dimmest white dwarf in the Hyades cluster (3×10 8 yr) from the age of the cluster (7×10 8 yr), we estimate the mass of the progenitor of this white dwarf to be 3.0 M ○ ., compared with the mass 2.25 M ○ . of a main-sequence star near the turnoff point in the cluster. Adopting a conventional mass function and normalizing to the 24 brightest main-sequence stars in the Hyades, we infer that there should be at least 21 white dwarfs dimmer than the 7 confirmed white dwarf cluster members which presumably derive from stars of initial mass in the range 2.5-3 M ○ .