Cold Star Research Articles

Modeling nuclear “pasta” and the transition to uniform nuclear matter with the 3D Skyrme-Hartree-Fock method at finite temperature: Core-collapse supernovae

The first results of a new three-dimensional, finite temperature Skyrme-Hartree-Fock$+$BCS study of the properties of inhomogeneous nuclear matter at densities and temperatures leading to the transition to uniform nuclear matter are presented. Calculations are carried out in a cubic box representing a unit cell of the locally periodic structure of the matter. A constraint is placed on the two independent components of the quadrupole moment of the neutron density to investigate the dependence of the total energy density of matter on the geometry of the nuclear structure in the unit cell. This approach allows self-consistent modeling of effects such as (i) neutron drip, resulting in a neutron gas external to the nuclear structure; (ii) shell effects of bound and unbound nucleons; (iii) the variety of exotic nuclear shapes that emerge, collectively termed nuclear pasta; and (iv) the dissolution of these structures into uniform nuclear matter as density and/or temperature increase. In Part I of this work the calculation of the properties of inhomogeneous nuclear matter in the core collapse of massive stars is reported. Emphasis is on exploring the effects of the numerical method on the results obtained; notably, the influence of the finite cell size on the nuclear shapes and energy-density obtained. Results for nuclear matter in $\ensuremath{\beta}$ equilibrium in cold neutrons stars are the subject of Part II. The calculation of the band structure of unbound neutrons in neutron star matter, yielding thermal conductivity, specific heat, and entrainment parameters, is outlined in Part III. Calculations are performed at baryon number densities of ${n}_{b}=0.04\text{\ensuremath{-}}0.12 {\mathrm{fm}}^{\ensuremath{-}3}$, a proton fraction of ${y}_{p}=0.3$ and temperatures in the range 0--7.5 MeV. A wide variety of nuclear shapes are shown to emerge. It is suggested that thermodynamical properties change smoothly in the pasta regime up to the transition to uniform matter; at that transition, thermodynamic properties of the matter vary discontinuously, indicating a phase transition of first or second order. The calculations are carried out using the ${\mathrm{SkM}}^{*}$ Skyrme parametrization; a comparison with calculations using Sly4 at ${n}_{b}=0.08 {\mathrm{fm}}^{\ensuremath{-}3}$, $T=0$ MeV is made.

QCD vacuum and hypothesis of the appearance of quark matter in the interior of stars

The problem of whether “cold” (T ≤ 10 MeV) subhadronic matter may appear in the central domain of massive neutron stars is discussed. It is shown that nontrivial properties of the QCD vacuum—namely, the existence of a quark-gluon condensate in the hadronic phase and its destruction in the transition to the quark phase—oppose so efficiently the stability of the two-phase structure of a cold star (quark center and hadronic periphery) that the probability of this configuration is very low. The loss of stability at the stage of the phase transition is expected to result in the heating of the central domain up to very high temperatures (T ≥ 100 MeV). Some seemingly enigmatic astrophysical observations and global processes that control the evolution of compact heavy stars may originate from this mechanism.

QUANTUM CONDENSATES IN EXTREME GRAVITY: IMPLICATIONS FOR COLD STARS AND DARK MATTER

Stable end-point stars currently fall into two distinct classes — white dwarfs and neutron stars — differing enormously in central density and radial size. No stable cold dead stars are thought to span the intervening densities or have masses beyond ~2–3 solar masses. I show, however, that the general-relativistic condition of hydrostatic equilibrium augmented by the equation of state of a neutron condensate at 0 K generates stable sequences of cold stars that span the density gap and can have masses well beyond prevailing limits. The radial sizes and mass limit of each sequence are determined by the mass and scattering length of the composite bosons. Solutions for hypothetical bosons of ultrasmall mass and large scattering length yield huge self-gravitating systems of low density, resembling galactic dark matter halos.

Measuring stellar temperatures: an astrophysical laboratory for undergraduate students

While astrophysics is a fascinating subject, it hardly lends itself to laboratory experiences accessible to undergraduate students. In this paper, we describe a feasible astrophysical laboratory experience in which the students are guided to take several stellar spectra, using a telescope, a spectrograph and a CCD camera, and perform a full data analysis using the equivalent widths method on some spectral lines. This allows them to derive stellar temperatures after a suitable calibration and fitting procedure. On average, the estimated star's temperatures agree with the known results within an error margin of 5–10% for cold stars, and 20–25% for hot stars.

Phase diagram of neutron star quark matter in nonlocal chiral models

We analyze the phase diagram of two-flavor quark matter under neutron star constraints for a nonlocal covariant quark model within the mean-field approximation. Applications to cold compact stars are discussed.

Intragroup Gas in Differently Evolved Compact Groups of Galaxies

AbstractCompact groups are highly isolated, dense galaxy systems, and therefore tidal interactions are expected to be continuous and dynamically important. For this reason they represent unique laboratories for studying interactions in extreme environments and tidally induced star formation as well as morphological and dynamical evolution of galaxies in general. This study is part of a broader investigation of the evolutionary status of HCGs. In order to explain the HI deficiency found in compact groups of galaxies we have proposed a broad evolutionary scenario in which the amount of HI decreases with the secular evolutionary state of a group. The fate of this gas is however not clear. In order to investigate the physical mechanisms responsible for these changes in the gas content of galaxies we have started a study to look for an intragroup medium (IGM). We have considered this IGM to be neutral (see e.g. reports of HI clouds within Virgo cluster (Oosterloo & Van Gorkom 2005), or hot gas (a higher X-ray detection rate of HI deficient groups is found relative to groups with a normal HI content, Verdes-Montenegro et al. 2001). Enhanced star formation activity might also be contributing to exhausting the gas supply. We have observed 25 HCGs with diverse HI content and distribution with the Green Bank radiotelescope, and performed VLA-HI as well as X-ray (XMM or Chandra) observations of the most HI deficient groups HCG 7, 15, 30, 37, 40, 44 and 97. We have also analysed FIR data from Verdes-Montenegro et al. (1998).Our results are: (a) HI deficient groups show a larger amount of diffuse neutral and X-ray gas when compared to non-deficient ones, consistent with our evolutionary picture; (b) Most groups are still deficient after including diffuse gas. (c) A hot IGM is not detected in most of these systems, but the detection rate is higher for the high velocity dispersion groups, and in these cases ram pressure stripping could potentially cause the observed HI-deficiency. (d) In several of these groups we detect intragroup radio continuum emission, which is surprising since similar features were seen previously only in massive clusters (known as “relic” sources). (e) Star formation activity traced by FIR luminosity is not enhanced, contrary to the general expectation of tidally interacting systems.Hence neither the presence of a hot or cold IGM nor star formation activity seem to be able to uniquely explain the HI deficiency found in compact groups of galaxies. We plan to explore the possibility that earlier starbursts could have contributed to the present HI deficiency.

The importance of tides for the Local Group dwarf spheroidals

There are two main tidal effects that can act on the Local Group dwarf spheroidals (dSphs): tidal stripping and tidal shocking. Using N-body simulations, we show that tidal stripping always leads to flat or rising projected velocity dispersions beyond a critical radius; it is ∼5 times more likely, when averaging over all possible projection angles, that the cylindrically averaged projected dispersion will rise, rather than be flat. In contrast, the Local Group dSphs, as a class, show flat or falling projected velocity dispersions interior to ∼1 kpc. This argues for tidal stripping being unimportant interior to ∼1 kpc for most of the Local Group dSphs observed so far. We show that tidal shocking may still be important, however, even when tidal stripping is not. This could explain the observed correlation for the Local Group dSphs between central surface brightness and distance from the nearest large galaxy. These results have important implications for the formation of the dSphs and for cosmology. As a result of the existence of cold stars at large radii in several dSphs, a tidal origin for the formation of these Local Group dSphs (in which they contain no dark matter) is strongly disfavoured. In the cosmological context, a naive solution to the missing satellites problem is to allow only the most massive substructure dark matter haloes around the Milky Way to form stars. It is possible for dSphs to reside within these haloes (∼10 10 M� ) and have their velocity dispersions lowered through the action of tidal shocks, but only if they have a central density core in their dark matter, rather than a cusp. A central density cusp persists even after unrealistically extreme tidal shocking and leads to central velocity dispersions which are too high to be consistent with data from the Local Group dSphs. dSphs can reside within cuspy dark matter haloes if their haloes are less massive (∼10 9 M� ) and therefore have smaller

Scalar dark matter and cold stars

In a medium composed of scalar particles with non-zero mass, the range of Van-der-Waals-type scalar mediated interactions among nucleons becomes infinite when the medium makes a transition to a Bose–Einstein condensed phase. We explore this phenomenon in an astrophysical context. Namely, we study the effect of a scalar dark matter background on the equilibrium of degenerate stars. In particular we focus on white dwarfs and the changes induced in their masses and in their radii.

The pulsed X-ray light curves of the isolated neutron star RBS1223

We present a multi-epoch spectral and timing analysis of the isolated neutron star RBS1223. New XMM-Newton data obtained in January 2004 confirm the spin period to be twice as long as previously thought, P_spin = 10.31 s. The combined ROSAT, Chandra, XMM-Newton data (6 epochs) give, contrary to earlier findings, no clear indication of a spin evolution of the neutron star. The X-ray light curves are double-humped with pronounced hardness ratio variations suggesting an inhomogeneous surface temperature with two spots separated by about ~160 degr. The sharpness of the two humps suggests a mildly relativistic star with a ratio between R_ns, the neutron star radius at source, and r_S, the Schwarzschild-radius, of R_ns/r_S > 3.5. Assuming Planckian energy distributions as local radiation sources, light curves were synthesized which were found to be in overall qualitative agreement with observed light curves in two different energy bands. The temperature distribution used was based on the crustal field models by Geppert et al. (2004) for a central temperature of T_c = 10^8 K and an external dipolar field of B ~ 10^{13} G. This gives a mean atmospheric temperature of 55 eV. A much simpler model with two homogeneous spots with T_\infty = 92 eV, and 84 eV, and a cold rest star, T_star,\infty < 45 eV, invisible at X-ray wavelengths, was found to be similarly successful. The new temperature determination and the new dot(P)_{spin} suggest that the star is older than previously thought, T \simeq 10^(5..6) yrs. The model-dependent distance to RBS1223 is estimated between 76 pc and 380 pc (for R_ns = 12 km).

Cold gas in dark matter halos and the formation of late-type galaxies

We use recent observations of the HI mass function to constrain the amount of cold gas in dark matter halos. It is found that the cold gas mass in a halo decreases rapidly with decreasing halo mass for low-mass halos with M< 10 12 h −1 M� . This result is in conflict with the standard model, in which most of the gas in a low-mass halo is assumed to settle into a gaseous disk, and the cold gas is depleted by star formation and supernova-driven outflow until the disk becomes gravitationally stable. Heating by the UV background can reduce the amount of cold gas in halos with masses < 10 10 h −1 M� , but is insufficient to explain the observational result. A consistent model can be found if low-mass halos are embedded in a preheated medium, with a specific gas entropy ∼ 10 keV cm 2 . Such a model can also explain why the faint-end slope of the galaxy luminosity function is flat. We propose a preheating model in which the medium around low-mass halos is preheated by gravitational pancaking. Since large-scale tidal fields tend to suppress the formation of low-mass virialised halos while promoting the formation of pancakes, the formation of massive pancakes may precede that of low-mass dark halos at the present time. 'Previrialisation' of such pancakes can heat the intergalactic medium. The preheated gas has a cooling time longer than the age of the Universe at z ∼ 2, and has a specific entropy comparable to that required to reduce the amount of cold gas and star formation activity in galaxy halos.

Tip of the Red Giant Branch distances to galaxies with composite stellar populations

The accurate determination of galaxy relative distances is extremely important for the empirical calibration of the uncertain metallicity dependence of some standard candles like Cepheid stars, or for studying the galaxy space distribution and peculiar velocities. Here we have investigated the reliability of the widely used I-band Tip of the Red Giant Branch (TRGB) relative distances for a sample of Local Group galaxies with complex star formation histories (SFHs) and age–metallicity relationships (AMRs) namely the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC) and LGS3. The use of the K band is also discussed. By employing theoretical stellar population synthesis techniques, we find that using actual determinations of SFH and AMR of the LMC and SMC, their RGB is populated by stars much younger (by ∼9 Gyr) than the Galactic globular cluster counterparts, on which the I-band (and K-band) TRGB absolute magnitude is calibrated. This age difference induces a bias in both the photometric metallicity estimates based on the comparison of RGB colours with globular cluster ones, and the TRGB distances. The extent of the distance bias – which is not influenced by the actual value of the TRGB absolute magnitude zero-point – is strongly dependent on the specific TRGB technique applied, and on the assumed I-band bolometric correction (BCI) scale adopted; the correction to apply to the SMC–LMC distance modulus ranges from 0 up to +0.10 mag. LGS3 is an example of a galaxy populated mainly by old stars, so that photometric metallicity and distance estimates using globular cluster calibrations are reliable. However, the relative distance moduli between the Magellanic Clouds and LGS3 are affected by the population effects discussed for the LMC and SMC. The correction to apply to the LGS3–LMC distance modulus ranges between −0.05 and +0.14 mag, whereas in the case of the LGS3–SMC distance modulus it goes from −0.07 to +0.04 mag. In the case of all three relative distances discussed before, the correction to apply to the K-band TRGB distances are larger than the I-band case. Our results clearly show that the presence of a well-developed RGB in the colour – magnitude diagram of a stellar system with a complex SFH does not guarantee that it is populated by globular cluster-like red giants, and therefore the TRGB method for distance determination has to be applied with caution. A definitive assessment of the appropriate corrections for population effects on TRGB distances has, however, to wait for a substantial reduction in the uncertainties on the BCI scale for cold stars.

Blood pressure reactivity to psychological stress predicts hypertension in the CARDIA study.

A longstanding but controversial hypothesis is that individuals who exhibit frequent, large increases in blood pressure (BP) during psychological stress are at risk for developing essential hypertension. We tested whether BP changes during psychological stress predict incident hypertension in young adults. We used survival analysis to predict hypertensive status during 13 years of follow-up in a sample of >4100 normotensive black and white men and women (age at entry, 18 to 30 years) enrolled in the CARDIA study. BP responses to 3 psychological challenges--cold pressor, star tracing, and video game tasks--were measured. Hypertensive status was defined as use of antihypertensive medication or measured BP > or =140/90 mm Hg. After adjustment for race, gender, covariates (education, body mass index, age, and resting pressure), and their significant interactions, the larger the BP responses were to each of the 3 tasks, the earlier hypertension occurred (P<0.0001 to <0.01). The systolic BP effect for the cold pressor task was apparent for women and for whites in race- and gender-specific models, whereas the diastolic BP effect for the video game was apparent for men. Young adults who show a large BP response to psychological stress may be at risk for hypertension as they approach midlife.

Abundance difference between components of wide binaries

We present iron abundance analysis for 23 wide binaries with main sequence components in the temperture range 4900–6300 K, taken from the sample of the pairs currently included in the radial velocity planet search on going at the Telescopio Nazionale Galileo (TNG) using the high resolution spectrograph SARG. The use of a line-by-line differential analysis technique between the components of each pair allows us to reach errors of about 0.02 dex in the iron content difference. Most of the pairs have abundance differences lower than 0.02 dex and there are no pairs with differences larger than 0.07 dex. The four cases of differences larger than 0.02 dex may be spurious because of the larger error bars affecting pairs with large temperature difference, cold stars and rotating stars. The pair HD 219542, previously reported by us to have a different composition, here is shown to be normal. For non-rotating stars warmer than 5500 K, characterized by a thinner convective envelope and for which our analyis appears to be of higher accuracy, we are able to exclude in most cases the consumption of more than 1 Earth Mass of iron (about 5 Earth masses of meteoritic material) during the main sequence lifetime of the stars, placing more stringent limits (about 0.4 Earth masses of iron) in five cases of warm stars. This latter limit is similar to the estimates of rocky material accreted by the Sun during its main sequence lifetime. Combining the results of the present analysis with those for the Hyades and Pleiades, we conclude that the hypothesis that pollution by planetary material is the only mechanism responsible for the highest metallicity of the stars with planets may be rejected at more than 99% level of confidence if the incidence of planets in these samples is as high as 8% and similar to the field stars included in current radial velocity surveys. However, the significance of this result drops considerably if the incidence of planets around stars in binary systems and clusters is less than a half of that around normal field stars.

Molecular hydrogen as dark mass in dwarf galaxies

We report here an example of a series of computer simulations of the formation of disk galaxies, including a standard cold dark matter component, gas, and star formation, which result in objects which closely resemble observed giant galaxies, with a population of long-lived dwarf satellites which contain little or no non–baryonic dark matter.

Some aspects of strangeness in astrophysics and cosmology

It has been suggested that strange quark matter may constitute the absolute ground state of Quantum Chromodynamics. Weak interaction plays a very important role in this scenario. For physical processes where the time scales are long enough to accomodate weak interaction, strange quark matter may indeed be formed. Such situations occur in the interior of cold stars as well as in the phase transition separating the quark-gluon and the hadronic phases in the primordial universe. We show that the occurrence of primordial strange quark matter can explain the cosmological cold dark matter in a natural way, without having to introduce any new physics beyond the standard model. Evidence for such matter may already have been found in astrophysical observations.

RADIATING SPHERICAL COLLAPSE WITH HEAT FLOW

We present here a simple model of radiative gravitational collapse with radial heat flux which describes qualitatively the stages close to the formation of a superdense cold star. Starting with a static general solution for a cold star, the model can generate solutions for the earlier evolutionary stages. The temporal evolution of the model is specified by solving the junction conditions appropriate for radiating gravitational collapse. The results will be useful in constructing models for the evolution of X-ray pulsars, like Her X-1.

Formation of an Evanescent Proto–Neutron Star Binary and the Origin of Pulsar Kicks

If core collapse leads to the formation of a rapidly rotating, bar-unstable proto-neutron star surrounded by fallback material, then we might expect it to cool and fragment to form a double (proto-)neutron star binary in a superclose orbit. The lighter star should survive for awhile, until tidal mass loss propels it toward the minimum stable mass of a (proto-)neutron star, whereupon it explodes. Imshennik & Popov have shown that the explosion of the unstable, cold star can result in a large recoil velocity of the remaining neutron star. Here, we consider several factors that mitigate the effect and broaden the range of final recoil speeds, in particular the finite velocity and gravitational deflection of the ejecta, a range of original masses for the low-mass companion and its cooling history, rotational phase averaging of the momentum impulse from noninstantaneous mass loss, and the possibility of a common envelope phase. In spite of these mitigating factors, we argue that this mechanism can still lead to substantial neutron star recoil speeds, close to, or even above, 1000 km s-1.

Cold Bose stars: Self-gravitating Bose-Einstein condensates

Using the Gross-Pitaevskii equation and the Thomas-Fermi approximation, we study a cold star composed of a dilute Bose-Einstein condensate. The exact solution of the mass distribution of a nonrotating star is obtained. For a slowly rotating star, the second-order perturbative solutions are obtained. It is found that one solution is stable and the other is metastable.

Research Note Radial velocities of giant M stars near the ecliptic

A search dedicated to the identification of new, dwarf and cold stars close to the Sun has allowed us to measure the radial velocities of a set of stars of late spectral type. The stars selected were M-type stars present in the Hipparcos Input Catalogue located in a strip of ± 20° around the Ecliptic and observables from La Silla. Our final set contains 22 M-type stars whose radial velocities V r have been measured with an accuracy of ~0.5 km s -1 during a single observing run. From their velocities, parallaxes and magnitudes we deduce that all the stars observed are red giants, with space velocities relative to the Sun mostly lower than 100 km s -1 .

Self-gravitating bosons at nonzero temperature

A system of charged bosons at finite temperature and chemical potential is studied in a general-relativistic framework. We assume that the boson fields interact only gravitationally. At sufficiently low temperature the system exists in two phases: the gas and the condensate. By studying the condensation process numerically we determine the critical temperature $T_c$ at which the condensate emerges. As the temperature decreases, the system eventually settles down in the ground state of a cold boson star.