Distances and Stellar Population properties using the SBF method

We present some results on the study of stellar population properties and distances of galaxies using the SBF technique. The applications summarized here show that the Surface Brightness Fluctuations (SBF) method is able to i) provide accurate distances of resolved and unresolved stellar systems from ~10 Kpc to ~150 Mpc, and ii) to reliably constrain the physical properties (e.g. age and metallicity) of unresolved stellar systems.Comment: 4 page

Angular Momentum Transport via Internal Gravity Waves in Evolving Stars

Recent asteroseismic advances have allowed for direct measurements of the internal rotation rates of many sub-giant and red giant stars. Unlike the nearly rigidly rotating Sun, these evolved stars contain radiative cores that spin faster than their overlying convective envelopes, but slower than they would in the absence of internal angular momentum transport. We investigate the role of internal gravity waves in angular momentum transport in evolving low mass stars. In agreement with previous results, we find that convectively excited gravity waves can prevent the development of strong differential rotation in the radiative cores of Sun-like stars. As stars evolve into sub-giants, however, low frequency gravity waves become strongly attenuated and cannot propagate below the hydrogen burning shell, allowing the spin of the core to decouple from the convective envelope. This decoupling occurs at the base of the sub-giant branch when stars have surface temperatures of roughly 5500 K. However, gravity waves can still spin down the upper radiative region, implying that the observed differential rotation is likely confined to the deep core near the hydrogen burning shell. The torque on the upper radiative region may also prevent the core from accreting high-angular momentum material and slow the rate of core spin-up. The observed spin-down of cores on the red giant branch cannot be totally attributed to gravity waves, but the waves may enhance shear within the radiative region and thus increase the efficacy of viscous/magnetic torques.Comment: 13 pages, 5 figures, accepted to Ap

Surface Brightness Fluctuations: a theoretical point of view

We present new theoretical evaluations of optical and near-IR Surface Brightness Fluctuations (SBF) magnitudes for single-burst stellar populations in the age range t=5-15 Gyr and metallicity from Z_{\sun}/200 to 2Z_{\sun}. Our theoretical predictions can be successfully used to derive reliable distance evaluations. They also appear to be a new and valuable tool to trace the properties of unresolved stellar populations.Comment: 2 pages, incl. 1 figure, uses newpasp.sty, to be published in ``New Horizons in Globular Cluster Astronomy'', ASP Conference Series, 2002; Eds.: G. Piotto, G. Meylan, G. Djorgowski and M. Riello, in pres

Asteroseismic signatures of evolving internal stellar magnetic fields

Recent asteroseismic analyses have revealed the presence of strong (B $\gtrsim 10^5$ G) magnetic fields in the cores of many red giant stars. Here, we examine the implications of these results for the evolution of stellar magnetic fields, and we make predictions for future observations. Those stars with suppressed dipole modes indicative of strong core fields should exhibit moderate but detectable quadrupole mode suppression. The long magnetic diffusion times within stellar cores ensure that dynamo-generated fields are confined to mass coordinates within the main sequence convective core, and the observed sharp increase in dipole mode suppression rates above $1.5 \, M_\odot$ may be explained by the larger convective core masses and faster rotation of these more massive stars. In clump stars, core fields of $\sim10^5 \, {\rm G}$ can suppress dipole modes, whose visibility should be equal to or less than the visibility of suppressed modes in ascending red giants. High dipole mode suppression rates in low-mass ($M \lesssim 2 \, M_\odot$) clump stars would indicate that magnetic fields generated during the main sequence can withstand subsequent convective phases and survive into the compact remnant phase. Finally, we discuss implications for observed magnetic fields in white dwarfs and neutron stars, as well as the effects of magnetic fields in various types of pulsating stars.Comment: Submitted to ApJ. The Authorea version of the paper, including the data shown in Fig.1, can be found at https://www.authorea.com/3821