Main-sequence Turnoff Mass Research Articles

Transitory Tidal Heating and Its Impact on Cluster Isochrones

The kinetic energy in tidal flows, when converted into heat, can affect the internal structure of a star and shift its location on a color–magnitude diagram from that of standard models. In this paper we explore the impact of injecting heat into stars with masses near the main-sequence turnoff mass (1.26 M ⊙) of the open cluster M67. The heating rate is obtained from the tidal shear energy dissipation rate, which is calculated from first principles by simultaneously solving the equations that describe orbital motion and the response of a star’s layers to the gravitational, Coriolis, centrifugal, gas pressure, and viscous forces. The stellar structure models are computed with MESA. We focus on the effects of injecting heat in pulses lasting 0.01 Gyr, a time frame consistent with the synchronization timescale in binary systems. We find that the location of the tidally perturbed stars in the M67 color–magnitude diagram is shifted to significantly higher luminosities and effective temperatures than predicted by the standard model isochrone and includes locations corresponding to some of the blue straggler stars. Because tidal heating takes energy from the orbit, causing it to shrink, blue straggler stars could be merger or mass transfer progenitors, as well as products of these processes.

On the black hole content and initial mass function of 47 Tuc

ABSTRACT The globular cluster (GC) 47 Tuc has recently been proposed to host an intermediate-mass black hole (IMBH) or a population of stellar mass black holes (BHs). To shed light on its dark content, we present an application of self-consistent multimass models with a varying mass function and content of stellar remnants, which we fit to various observational constraints. Our best-fitting model successfully matches the observables and correctly predicts the radial distribution of millisecond pulsars and their gravitational accelerations inferred from long-term timing observations. The data favours a population of BHs with a total mass of $430^{+386}_{-301}$ M⊙, but the most likely model has very few BHs. Since our models do not include a central IMBH and accurately reproduce the observations, we conclude that there is currently no need to invoke the presence of an IMBH in 47 Tuc. The global present-day mass function inferred is significantly depleted in low-mass stars (power-law slope $\alpha =-0.52^{+0.17}_{-0.16}$). Given the orbit and predicted mass-loss history of this massive GC, the dearth of low-mass stars is difficult to explain with a standard initial mass function (IMF) followed by long-term preferential escape of low-mass stars driven by two-body relaxation, and instead suggests that 47 Tuc may have formed with a bottom-light IMF. We discuss alternative evolutionary origins for the flat mass function and ways to reconcile this with the low BH retention fraction. Finally, by capturing the effect of dark remnants, our method offers a new way to probe the IMF in a GC above the current main-sequence turn-off mass, for which we find a slope of −2.49 ± 0.08.

HUBBLE SPACE TELESCOPE PROPER MOTION (HSTPROMO) CATALOGS OF GALACTIC GLOBULAR CLUSTERS. IV. KINEMATIC PROFILES AND AVERAGE MASSES OF BLUE STRAGGLER STARS∗

ABSTRACT We make use of the Hubble Space Telescope proper-motion catalogs derived by Bellini et al. to produce the first radial velocity dispersion profiles for blue straggler stars (BSSs) in Galactic globular clusters (GCs), as well as the first dynamical estimates for the average mass of the entire BSS population. We show that BSSs typically have lower velocity dispersions than stars with mass equal to the main-sequence turnoff mass, as one would expect for a more massive population of stars. Since GCs are expected to experience some degree of energy equipartition, we use the relation , where η is related to the degree of energy equipartition, along with our velocity dispersion profiles to estimate BSS masses. We estimate η as a function of cluster relaxation from recent Monte Carlo cluster simulations by Bianchini et al. and then derive an average mass ratio and an average mass M ⊙ from 598 BSSs across 19 GCs. The final error bars include any systematic errors that are random between different clusters, but not any potential biases inherent to our methodology. Our results are in good agreement with the average mass of M ⊙ for the 35 BSSs in Galactic GCs in the literature with properties that have allowed individual mass determination.

WEIGHING STARS: THE IDENTIFICATION OF AN EVOLVED BLUE STRAGGLER STAR IN THE GLOBULAR CLUSTER 47 TUCANAE*

ABSTRACT Globular clusters are known to host peculiar objects named blue straggler stars (BSSs), significantly heavier than the normal stellar population. While these stars can be easily identified during their core hydrogen-burning phase, they are photometrically indistinguishable from their low-mass sisters in advanced stages of the subsequent evolution. A clear-cut identification of these objects would require the direct measurement of the stellar mass. We used the detailed comparison between chemical abundances derived from neutral and from ionized spectral lines as a powerful stellar “weighing device” to measure stellar mass and to identify an evolved BSS in 47 Tucanae. In particular, high-resolution spectra of three bright stars, located slightly above the level of the “canonical” horizontal branch (HB) sequence in the color–magnitude diagram of 47 Tucanae, have been obtained with the UVES spectrograph. The measurements of iron and titanium abundances performed separately from neutral and ionized lines reveal that two targets have stellar parameters fully consistent with those expected for low-mass post-HB objects, while for the other target the elemental ionization balance is obtained only by assuming a mass of ∼ 1.4 M ⊙ ?> , which is significantly larger than the main sequence turn-off mass of the cluster ( ∼ 0.85 M ⊙ ?> ). The comparison with theoretical stellar tracks suggests that this is a BSS descendant possibly experiencing its core helium-burning phase. The large applicability of the proposed method to most of the globular clusters in our Galaxy opens the possibility to initiate systematic searches for evolved BSSs, thus giving access to still unexplored phases of their evolution.

RECOVERY OF THE HISTORICAL SN1957D IN X-RAYS WITHCHANDRA

SN1957D, located in one of the spiral arms of M83, is one of the small number of extragalactic supernovae that has remained detectable at radio and optical wavelengths during the decades after its explosion. Here we report the first detection of SN1957D in X-rays, as part of a 729 ks observation of M83 with \chandra. The X-ray luminosity (0.3 - 8 keV) is 1.7 (+2.4,-0.3) 10**37 ergs/s. The spectrum is hard and highly self-absorbed compared to most sources in M83 and to other young supernova remnants, suggesting that the system is dominated at X-ray wavelengths by an energetic pulsar and its pulsar wind nebula. The high column density may be due to absorption within the SN ejecta. HST WFC3 images resolve the supernova remnant from the surrounding emission and the local star field. Photometry of stars around SN1957D, using WFC3 images, indicates an age of less than 10**7 years and a main sequence turnoff mass more than 17 solar masses. New spectra obtained with Gemini-South show that the optical spectrum continues to be dominated by broad [O III] emission lines, the signature of fast-moving SN ejecta. The width of the broad lines has remained about 2700 km/s (FWHM). The [O III] flux dropped precipitously between 1989 and 1991, but continued monitoring shows the flux has been almost constant since. In contrast, radio observations over the period 1990-2011 show a decline rate inf the flux proportional to t**-4, far steeper than the rate observed earlier, suggesting that the primary shock has overrun the edge of a pre-SN wind.

KECK OBSERVATIONS OF THE YOUNG METAL-POOR HOST GALAXY OF THE SUPER-CHANDRASEKHAR-MASS TYPE Ia SUPERNOVA SN 2007if

We present Keck LRIS spectroscopy and $g$-band photometry of the metal-poor, low-luminosity host galaxy of the super-Chandrasekhar mass Type Ia supernova SN 2007if. Deep imaging of the host reveals its apparent magnitude to be $m_g=23.15\pm0.06$, which at the spectroscopically-measured redshift of $z_{helio}=0.07450\pm0.00015$ corresponds to an absolute magnitude of $M_g=-14.45\pm0.06$. Galaxy $g-r$ color constrains the mass-to-light ratio, giving a host stellar mass estimate of $\log(M_*/M_\odot)=7.32\pm0.17$. Balmer absorption in the stellar continuum, along with the strength of the 4000\AA\ break, constrain the age of the dominant starburst in the galaxy to be $t_\mathrm{burst}=123^{+165}_{-77}$ Myr, corresponding to a main-sequence turn-off mass of $M/M_\odot=4.6^{+2.6}_{-1.4}$. Using the R$_{23}$ method of calculating metallicity from the fluxes of strong emission lines, we determine the host oxygen abundance to be $12+\log(O/H)_\mathrm{KK04}=8.01\pm0.09$, significantly lower than any previously reported spectroscopically-measured Type Ia supernova host galaxy metallicity. Our data show that SN 2007if is very likely to have originated from a young, metal-poor progenitor.

THE NGC 300 TRANSIENT: AN ALTERNATIVE METHOD FOR MEASURING PROGENITOR MASSES

We present an alternative technique for measuring the precursor masses of transient events in stars undergoing late stage stellar evolution. We use the well-established techniques of stellar population modeling to age-date the stars surrounding the site of the recent transient event in NGC 300 (NGC 300 OT2008-1). The surrounding stars must share a common turnoff mass with the transient, since almost all stars form in stellar clusters that remain physically associated for periods longer than the lifetime of the most massive stars. We find that the precursor of NGC 300 OT2008-1 is surrounded by stars that formed in a single burst between 8-13 Myr ago, with 70% confidence. The transient was therefore likely to be due to a progenitor whose mass falls between the main sequence turnoff mass (12 Msun) and the maximum stellar mass (25 Msun) found for isochrones bounding this age range. We characterize the general applicability of this technique in identifying precursor masses of historic and future transients and supernovae (SNe), noting that it requires neither precursor imaging nor sub-arcsecond accuracy in the position of the transient. It is also based on the well-understood physics of the main sequence, and thus may be a more reliable source of precursor masses than fitting evolutionary tracks to precursor magnitudes. We speculate that if the progenitor mass is close to 17 Msun, there may be a connection between optical transients such as NGC 300 OT2008-1 and the missing type II-P SNe, known as the red supergiant problem.

Planetary nebula candidates in extragalactic young star clusters

Context. During an analysis of optical spectra of 80 young star clusters in several nearby spiral galaxies, [Oiii] and [Nii] emission lines were noted in some cases. Three of these emission line sources are associated with clusters older than 30 Myr, and are identified as likely planetary nebula (PN) candidates. Aims. These objects may represent a rare opportunity to study PNe whose progenitor stars are known to be of intermediate masses, although detailed analysis is challenging because of the underlying strong continuum from the cluster stars. This paper presents and discusses basic properties of the PN candidates and their host clusters. Methods. Based on the observed emission line fluxes, the excitation parameters and luminosities of the nebulae are derived. This allows a crude placement of the central stars in two of the objects on the H-R diagram. Cluster ages and masses are estimated from broad-band colours and by fitting model SSP spectra to the observed spectra. Results. The two PN candidates where central star luminosities and temperatures can be estimated are found to be consistent with post-AGB model tracks for a central star mass of about 0.60 M . One of the host clusters has an age of 32–65 Myr, corresponding to a main sequence turn-off mass of MTO = 6.6–9.0 M . For the other cluster the age is 282–407 Myr, corresponding to MTO = 3.2–3.6 M . By estimating the number of stars evolving off the main sequence per year, a total of 6 PNe are expected in the full sample of 80 clusters for a PN lifetime of 104 years. The factor of two disagreement with the actual observed number may be due, among other things, to uncertainties in PN lifetimes. It is interesting to note that all three PN candidates are associated with clusters which are more diffuse than average. Conclusions. While PNe have previously been found in some old globular clusters, the candidates identified here are among the first identified in young star clusters.

Hubble Space Telescope Proper Motions and Stellar Dynamics in the Core of the Globular Cluster 47 Tucanae

We have used HST imaging of the central regions of the globular cluster 47 Tucanae (=NGC 104), taken with the WFPC2 and ACS instruments between 1995 and 2002, to derive proper motions and U- and V-band magnitudes for 14,366 stars within 100'' (about 5 core radii) of the cluster center. This represents the largest set of member velocities collected for any globular cluster. The stars involved range in brightness from just fainter than the horizontal branch of the cluster to more than 2.5 mag below the main-sequence turnoff. In the course of obtaining these kinematic data, we also use a recent set of ACS images to define a list of astrometrically calibrated positions (and F475W magnitudes) for nearly 130,000 stars in a larger, 3' × 3' central area. We describe our data reduction procedures in some detail and provide the full position, photometric, and velocity data in the form of downloadable electronic tables. We have used the star counts to obtain a new estimate for the position of the cluster center and to define the density profile of main-sequence turnoff and giant branch stars in to essentially zero radius, thus constraining the global spatial structure of the cluster better than before. A single-mass, isotropic King model fit to it is then used as a rough point of reference against which to compare the gross characteristics of our proper-motion data. We search in particular for any evidence of very fast-moving stars, in significantly greater numbers than expected for the extreme tails of the velocity distribution in a sample of our size. We find that likely fewer than 0.1%, and no more than about 0.3%, of stars with measured proper motions have total speeds above the nominal central escape velocity of the cluster. At lower speeds, the proper-motion velocity distribution very closely matches that of a regular King model (which is itself nearly Gaussian given the high stellar density) at all observed radii. Considerations of only the velocity dispersion then lead to a number of results: (1) Blue stragglers in the core of 47 Tuc have a velocity dispersion σμ smaller than that of the cluster giants by a factor of , consistent with the former being on average twice as massive as normal, main-sequence turnoff stars. (2) The velocity distribution in the inner 5 core radii of the cluster is essentially isotropic, and the detailed dependence of σμ on R for the brighter stars suggests that heavy remnants contribute only a fraction of a percent to the total cluster mass. Both of these results are in keeping with earlier, more realistic multimass and anisotropic models of 47 Tuc. (3) Using a sample of 419 line-of-sight velocities measured for bright giants within R ≤ 105'', we obtain a kinematic distance to the cluster, D = 4.0 ± 0.35 kpc, formally some 10%-20% lower than recent estimates based on standard color-magnitude diagram fitting, and more consistent with the value implied by fitting to the white dwarf cooling sequence. And (4) by fitting simple models of isotropic, single-mass stellar clusters with central point masses to our observed σμ(R) profile, we infer a 1 σ upper limit of M• 1000-1500 M☉ for any intermediate-mass black hole in 47 Tuc. The formal best-fit hole mass ranges from zero, if only the kinematics of stars near the main-sequence turnoff mass are modeled, to ~700-800 M☉, if fainter, less massive stars are also used. We can neither confirm nor refute the hypothesis that 47 Tuc might lie on an extension of the M•-σ relation observed for galaxy bulges.

A Spectroscopic Analysis of Blue Stragglers, Horizontal Branch Stars, and Turnoff Stars in Four Globular Clusters

We present a spectroscopic analysis of HST STIS and FOS low- and intermediate-resolution spectroscopy of 55 stars in four globular clusters (47 Tucanae, M3, NGC 6752, and NGC 6397). Stars hotter than Teff = 5750 K and with a signal-to-noise ratio larger than 15 were analyzed with non-local thermodynamic equilibrium model atmospheres, and values for their effective temperatures and gravities were obtained. Using photometric fluxes, we also obtained radii, luminosities, and spectroscopic masses. Twenty-four stars in our sample are blue stragglers (BSs). Their photometric colors and magnitudes place these BSs above and redward of the clusters' zero-age main sequence: this is consistent with the gravities we find for these stars, which are lower than zero-age main-sequence gravities. A comparison with stellar evolutionary tracks shows that almost all of our BSs are in the Hertzsprung gap. This is contrary to theory, because of the short timescale expected for stars in this evolutionary phase. The mean BS mass is 1.04 M☉ for 14 nonvariable stars, or 1.07 M☉ counting all 24 BSs in our sample. For the nonvariable stars the mean BS masses for individual clusters are 1.73, 1.01, 0.95, and 0.72 M☉ for NGC 6397, NGC 6752, 47 Tuc, and M3, respectively. Adding the variable stars (which improves the statistics but increases the uncertainty), the mean masses become 1.27, 1.05, 0.99, and 0.99 M☉, respectively. Although there is considerable scatter, the BS spectroscopic masses correlate with both effective temperature and brightness of the stars, as expected. The mean nonvariable turnoff star mass (0.58 M☉) is significantly below the values determined for the BSs and below the main-sequence turnoff mass. The mean nonvariable horizontal-branch (HB) star mass is higher than expected (0.79 M☉). In particular, several HB stars have masses well above the main-sequence turnoff mass. Some of these HB stars are suspected of actually being BSs, since most of them reside at ambiguous locations on the CMD, making them prone to misclassification. Values and limits to the stellar rotation rates (v sin i) are imposed by fitting weak metal lines, the Ca II K line wings, or the helium lines for the hotter stars. Five BSs with reasonably constrained rotations show average and median v sin i values of 109 and 100 km s-1, respectively, suggesting v ~ 160 km s-1. At least some GC BSs are very rapid rotators, but this information cannot yet constrain their origin as stellar collision or binary mergers because of the lack of clear theoretical predictions. Six extreme HB stars have rotation rates v sin i between 50 and 200 km s-1, which are high for these stars and might indicate a binary origin. De Marco et al. found that four BSs and two HB stars in our sample have Balmer jumps that are too large for the effective temperatures implied by the slopes of their Paschen continua. Two additional HB stars are now identified in the current study as having the same feature. For these stars, the presence of a disk of partly ionized material is suspected, although high stellar rotation rates could also partly explain the data.

A new mass-ratio for the X-ray binary X2127+119 in M 15?

The luminous low-mass X-ray binary X2127+119 in the core of the globular cluster M 15 (NGC 7078), which has an orbital period of 17 hours, has long been assumed to contain a donor star evolving off the main sequence, with a mass of 0.8 M ○. (the main-sequence turn-off mass for M 15). We present orbital-phase-resolved spectroscopy of X2127+119 in the Ha and Me I λ6678 spectral region, obtained with the Hubble Space Telescope. We show that these data are incompatible with the assumed masses of X2127+119's component stars. The continuum eclipse is too shallow, indicating that much of the accretion disc remains visible during eclipse, and therefore that the size of the donor star relative to the disc is much smaller in this high-inclination system than the assumed mass-ratio allows. Furthermore, the flux on X2127+119's He I λ6678 emission, which has a velocity that implies an association with the stream-disc impact region, remains unchanged through eclipse, implying that material from the impact region is always visible. This should not be possible if the previously-assumed mass ratio is correct. In addition, we do not detect any spectral features from the donor star, which is unexpected for a 0.8-M ○. sub-giant in a system with a 17-h period.

Stellar collisions during binary-binary and binary-single star interactions

Physical collisions between stars occur frequently in dense star clusters, either via close encounters between two single stars, or during strong dynamical interactions involving binary stars. Here we study stellar collisions that occur during binary-single and binary-binary interactions, by performing numerical scattering experiments. Our results include cross sections, branching ratios, and sample distributions of parameters for various outcomes. For interactions of hard binaries containing main-sequence stars, we find that the normalized cross section for at least one collision to occur (between any two of the four stars involved) is essentially unity, and that the probability of collisions involving more than two stars is significant. Hydrodynamic calculations have shown that the effective radius of a collision product can be 2-30 times larger than the normal main-sequence radius for a star of the same total mass. We study the effect of this expansion, and find that it increases the probability of further collisions considerably. We discuss these results in the context of recent observations of blue stragglers in globular clusters with masses exceeding twice the main-sequence turnoff mass. We also present Fewbody, a new, freely available numerical toolkit for simulating small-N gravitational dynamics that is particularly suited to performing scattering experiments.

PRIMEVAL WINDS WITHIN GLOBULAR CLUSTERS

ABSTRACT Early stellar mass-loss rates within Galactic globular clusters were likely to have been much larger than at present. Within the most tightly bound globular clusters a substantial intracluster medium could have been acquired during the epoch when the main sequence turn-off mass was around 5 solar mass. By contrast, the stellar ejecta are likely to have been lost entirely from clusters having small binding energies, either stochastically through the expansion of discrete mass-loss shells, or via outflow within a continuous intracluster wind. Criteria for the establishment of continuous winds within globular clusters are obtained from a consideration of the cluster escape velocities and the expected rate of stellar mass ejection events. If mass loss from intermediate-mass stars was characterized by ejection speeds of ~20 km s-1, it is concluded that a number of Galactic globular clusters may have been the sites of primeval winds. It is found that the accretion of material from such a wind, even if it were pure nitrogen, would be insufficient to account for the nitrogen enhancements observed among some CN-strong cluster giants. In particular, the CN enhancements observed among some stars in low-mass clusters such as Palomar 5 are unlikely to have been produced by accretion, unless such clusters were initially much more massive than at present.

Morphology, near-infrared luminosity, and mass of the Galactic bulge from COBE DIRBE observations

Near-infrared images of the Galactic bulge at 1.25, 2.2, 3.5, and 4.9 microns obtained by the Diffuse Infrared Background Experiment (DIRBE) onboard the Cosmic Background Explorer (COBE) satellite are used to characterize its morphology and to determine its infrared luminosity and mass. Earlier analysis of the DIRBE observations (Weiland et al. 1994) provided supporting evidence for the claim made by Blitz &amp; Spergel (1991) that the bulge is bar-shaped with its near end in the first Galactic quadrant. Adopting various triaxial analytical functions to represent the volume emissivity of the source, we confirm the barlike nature of the bulge and show that triaxial Gaussian-type functions provide a better fit to the data than other classes of functions, including an axisymmetric spheroid. The introduction of a `boxy' geometry, such as the one used by Kent, Dame, &amp; Fazio (1991) improves the fit to the data. Our results show that the bar is rotated in the plane with its near side in the first Galactic quadrant creating an angle of 20 deg +/- 10 deg between its major axis and the line of sight to the Galactic center. Typical axis ratios of the bar are (1:0.33 +/- 0.11:0.23 +/- 0.08), resembling the geometry of prolate spheroids. There is no statistically significant evidence for an out-of-plane tilt of the bar at 2.2 microns, and marginal evidence for a tilt of approximately equal 2 deg at 4.9 microns. The introduction of a roll around the intrinsic major axis of the bulge improves the `boxy' appearance of some functions. A simple integration of the observed projected intensity of the bulge gives a bulge luminosity of 1.2 x 10<SUP>9</SUP>, 4.1 x 10<SUP>8</SUP>, 2.3 x 10<SUP>8</SUP>, and 4.3 x 10<SUP>7</SUP> solar luminosity, respectively, at 1.25, 2.2, 3.5, and 4.9 microns wavelength for a Galactocentric distance of 8.5 kpc. The 2.2 microns luminosity function of the bulge population in the direction of Baade's window yields a bolometric luminosity of L<SUB>bol</SUB> = 5.3 x 10<SUP>9</SUP> solar luminosity. Stellar evolutionary models relate this luminosity to the number of main-sequence progenitor stars that currently populate the red giant branch. Combined with the recent determination of the main-sequence turnoff mass for the bulge by the Hubble Space Telescope (Holtzman et al. 1993) we derive a photometrically determined bulge mass of approximately equal to 1.3 x 10<SUP>10</SUP> solar mass for a Salpeter initial mass function extended down to 0.1 solar mass.