Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userA Far-Ultraviolet Survey of 47 Tucanae. I. Imaging
160
0
0.0
(
0
)
Authors
Christian Knigge
Ask ChatGPT about the research
No Arabic abstract
We present results from the imaging portion of a far-ultraviolet (FUV) survey of the core of 47 Tucanae. We have detected 767 FUV sources, 527 of which have optical counterparts in archival HST/WFPC2 images of the same field. Most of our FUV sources are main-sequence (MS) turn-off stars near the detection limit of our survey. However, the FUV/optical color-magnitude diagram (CMD) also reveals 19 blue stragglers (BSs), 17 white dwarfs (WDs) and 16 cataclysmic variable (CV) candidates. The BSs lie on the extended cluster MS, and four of them are variable in the FUV data. The WDs occupy the top of the cluster cooling sequence, down to an effective temperature of T_{eff} simeq 20,000 K. Our FUV source catalog probably contains many additional, cooler WDs without optical counterparts. Finally, the CV candidates are objects between the WD cooling track and the extended cluster MS. Four of the CV candidates are previously known or suspected cataclysmics. All of these are bright and variable in the FUV. Another CV candidate is associated with the semi-detached binary system V36 that was recently found by Albrow et al. (2001). V36 has an orbital period of 0.4 or 0.8 days, blue optical colors and is located within 1 arcsec of a Chandra x-ray source. A few of the remaining CV candidates may represent chance superpositions or SMC interlopers, but at least half are expected to be real cluster members with peculiar colors. However, only a few of these CV candidates are possible counterparts to Chandra x-ray sources. Thus it is not yet clear which, if any, of them are true CVs, rather than non-interacting MS/WD binaries or Helium WDs.
rate research
Read More
We have used the Ultraviolet Imaging Telescope to obtain deep far-UV (1620 Angstrom), 40 diameter images of the prototypical metal-rich globular cluster 47 Tucanae. We find a population of about 20 hot (Teff > 9000 K) objects near or above the predicted UV luminosity of the hot horizontal branch (HB) and lying within two half-light radii of the cluster center. We believe these are normal hot HB or post-HB objects rather than interacting binaries or blue stragglers. IUE spectra of two are consistent with post-HB phases. These observations, and recent HST photometry of two other metal-rich clusters, demonstrate that populations with rich, cool HBs can nonetheless produce hot HB and post-HB stars. The cluster center also contains an unusual diffuse far-UV source which is more extended than its V-band light. It is possible that this is associated with an intracluster medium, for which there was earlier infrared and X-ray evidence, and is produced by C IV emission or scattered light from grains.
Multi-epoch observations with ACS and WFC3 on HST provide a unique and comprehensive probe of stellar dynamics within 47 Tucanae. We confront analytic models of the globular cluster with the observed stellar proper motions that probe along the main sequence from just above 0.8 to 0.1M$_odot$ as well as white dwarfs younger than one gigayear. One field lies just beyond the half-light radius where dynamical models (eg lowered Maxwellian distributions) make robust predictions for the stellar proper motions. The observed proper motions in this outer field show evidence for anisotropy in the velocity distribution as well as skewness; the latter is evidence of rotation. The measured velocity dispersions and surface brightness distributions agree in detail with a rotating, anisotropic model of the stellar distribution function with mild dependence of the proper-motion dispersion on mass. However, the best fitting models under-predict the rotation and skewness of the stellar velocities. In the second field, centered on the core of the cluster, the mass segregation in proper motion is much stronger. Nevertheless the model developed in the outer field can be extended inward by taking this mass segregation into account in a heuristic fashion. The proper motions of the main-sequence stars yield a mass estimate of the cluster of $1.31 pm 0.02 times 10^6 mathrm{M}_odot$ at a distance of 4.7 kpc. By comparing the proper motions of a sample of giant and sub-giant stars with the observed radial velocities we estimate the distance to the cluster kinematically to be $4.29 pm 0.47$ kpc.
We present linear polarization in the V band for 77 stars in the field of the globular cluster 47 Tucanae (NGC 104), and for 14 bright-star-free regions, located along an elliptical isophotal contour of the cluster, as well as UBVRI measurements for the cluster nucleus. The observations show variable foreground polarization that, once removed, leaves marginally significant polarization residuals for the non-variable bright red giants. Although these residuals are small there is, however, a systematic trend in the sense that the larger ones are seen towards the south of the cluster (in a direction opposite to that of the cluster proper motion). In contrast, most of the variable stars do show significant intrinsic polarization. The behavior of the star-free regions is similar to that of the non-variable stars and sets an upper limit to the possible existence of a global pattern of scattered (and polarized) intra-cluster light in the V band. In turn, the multicolor observations of the cluster nucleus cannot be fitted with a Serkowski law and exhibit a polarization excess both in U and B. This polarization could be explained as a combination of the foreground interstellar component and another one arising in dust located in the nucleus and illuminated by a bright blue post-AGB star (at 48 from the cluster center). An inspection of a set of archive HST WFPC2 images reveals the presence of a number of dark patches in the innermost regions of the cluster. A prominent patch (some 5 by 3 in size) located at 12 from the cluster center and with a position angle (N to E) of 120 degrees, has a slightly different polarization, compared to that of the cluster nucleus, and appears as a good candidate to be identified as a dust globule within the cluster.
Using images from the Hubble Space Telescope Wide-Field Camera 3, we measure the rate of diffusion of stars through the core of the globular cluster 47 Tucanae using a sample of young white dwarfs identified in these observations. This is the first direct measurement of diffusion due to gravitational relaxation. We find that the diffusion rate $kappaapprox 10-13$ arcsecond$^2$ Myr$^{-1}$ is consistent with theoretical estimates of the relaxation time in the core of 47 Tucanae of about 70 Myr.
The far-ultraviolet (FUV) spectrum of the Bright Star (B8 III) in 47 Tuc (NGC 104) shows a remarkable pattern: it is well fit by LTE models at wavelengths longer than Lyman beta, but at shorter wavelengths it is fainter than the models by a factor of two. A spectrum of this star obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) shows broad absorption troughs with sharp edges at 995 and 1010 A and a deep absorption feature at 1072 A, none of which are predicted by the models. We find that these features are caused by resonances in the photoionization cross sections of the first and second excited states of atomic nitrogen (2s$^2$ 2p$^3$ $^2$D$^0$ and $^2$P$^0$). Using cross sections from the Opacity Project, we can reproduce these features, but only if we use the cross sections at their full resolution, rather than the resonance-averaged cross sections usually employed to model stellar atmospheres. These resonances are strongest in stellar atmospheres with enhanced nitrogen and depleted carbon abundances, a pattern typical of post-AGB stars.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
