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Register a new userStar formation history, double degenerates and type Ia supernovae in the thin disc
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We investigate the relation between the star formation history and the evolution of the double-degenerate (DD) population in the thin disc of the Galaxy, which we assume to have formed 10 Gyr before the present. We introduce the use of star-formation contribution functions as a device for evaluating the birth rates, total number and merger rates of DDs. These contribution functions help to demonstrate the relation between star-formation history and the current DD population and, in particular, show how the numbers of different types of DD are sensitive to different epochs of star formation. We have compared the impact of different star-formation models on the rates and numbers of DDs and on the rates of type Ia (SNIa) and core-collapse supernovae (ccSN). In addition to a quasi-exponential decline model, we considered an instantaneous (or initial starburst) model, a constant-rate model, and an enhanced-rate model. All were normalised to produce the present observed star density in the local thin disc. The evolution of the rates and numbers of both DDs and SNIa are different in all four models, but are most markedly different in the instantaneous star-formation model, which produces a much higher rate than the other three models in the past, primarily as a consequence of the normalisation. Predictions of the current SNIa rate range from ~2 to 5times10^{-4} yr^{-1} in the four models, and are slightly below the observed rate because we only consider the DD merger channel. The predicted ccSN rate ranges from 1.5 to 3 century^{-1}, and is consistent with observations.
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The expected gravitational wave (GW) signal due to double degenerates (DDs) in the thin Galactic disc is calculated using a Monte Carlo simulation. The number of young close DDs that will contribute observable discrete signals in the frequency range $1.58 - 15.8$ mHz is estimated by comparison with the sensitivity of proposed GW observatories. The present-day DD population is examined as a function of Galactic star-formation history alone. It is shown that the frequency distribution, in particular, is a sensitive function of the Galactic star formation history and could be used to measure the time since the last major star-formation epoch.
We report on systematic radial velocity surveys for white dwarf - white dwarf binaries (double degenerates - DDs) including SPY (ESO Supernovae Ia progenitor survey) recently carried out at the VLT. A large sample of DD will allow us to put strong constrains on the phases of close binary evolution of the progenitor systems and to perform an observational test of the DD scenario for supernovae of type Ia. We explain how parameters of the binaries can be derived from various methods. Results for a sample of DDs are presented and discussed.
Using a sample of nearby spiral galaxies hosting 185 supernovae (SNe) Ia, we perform a comparative analysis of the locations and light curve decline rates $(Delta m_{15})$ of normal and peculiar SNe Ia in the star formation deserts (SFDs) and beyond. To accomplish this, we present a simple visual classification approach based on the UV/H$alpha$ images of the discs of host galaxies. We demonstrate that, from the perspective of the dynamical timescale of the SFD, where the star formation (SF) is suppressed by the bar evolution, the $Delta m_{15}$ of SN Ia and progenitor age can be related. The SFD phenomenon gives an excellent possibility to separate a subpopulation of SN Ia progenitors with the ages older than a few Gyr. We show, for the first time, that the SFDs contain mostly faster declining SNe Ia $(Delta m_{15} > 1.25)$. For the galaxies without SFDs, the region within the bar radius, and outer disc contain mostly slower declining SNe Ia. To better constrain the delay times of SNe Ia, we encourage new studies (e.g. integral field observations) using the SFD phenomenon on larger and more robust datasets of SNe Ia and their host galaxies.
The outer regions of disc galaxies are becoming increasingly recognized as key testing sites for models of disc assembly and evolution. Important issues are the epoch at which the bulk of the stars in these regions formed and how discs grow radially over time. To address these issues, we use Hubble Space Telescope Advanced Camera for Surveys imaging to study the star formation history (SFH) of two fields at 9.1 and 11.6 kpc along M33s northern major axis. These fields lie at ~ 4 and 5 V-band disc scale-lengths and straddle the break in M33s surface brightness profile. The colour-magnitude diagrams (CMDs) reach the ancient main sequence turnoff with a signal-to-noise ratio of ~ 5. From detailed modelling of the CMDs, we find that the majority of stars in both fields combined formed at z < 1. The mean age in the inner field, S1, is ~ 3 +/- 1 Gyr and the mean metallicity is [M/H] ~ -0.5 +/- 0.2 dex. The star formation history of S1 unambiguously reveals how the inside-out growth previously measured for M33s inner disc out to ~ 6 kpc extends out to the disc edge at ~ 9 kpc. In comparison, the outer field, S2, is older (mean age ~ 7 +/- 2 Gyr), more metal-poor (mean [M/H] ~ -0.8 +/- 0.3 dex), and contains ~ 30 times less stellar mass. These results provide the most compelling evidence yet that M33s age gradient reverses at large radii near the disc break and that this reversal is accompanied by a break in stellar mass surface density. We discuss several possible interpretations of this behaviour including radial stellar mixing, warping of the gaseous disc, a change in star formation efficiency, and a transition to another structural component. These results offer one of the most detailed views yet of the peripheral regions of any disc galaxy and provide a much-needed observational constraint on the last major epoch of star formation in the outer disc.
By means of 3D hydrodynamic simulations, we study how Type Ia supernovae (SNe) explosions affect the star formation history and the chemical properties of second generation (SG) stars in globular clusters (GC). SG stars are assumed to form once first generation asymptotic giant branch (AGB) stars start releasing their ejecta; during this phase, external gas is accreted by the system and SNe Ia begin exploding, carving hot and tenuous bubbles. Given the large uncertainty on SNe Ia explosion times, we test two different values for the delay time. We run two different models for the external gas density: in the low-density scenario with short delay time, the explosions start at the beginning of the SG star formation, halting it in its earliest phases. The external gas hardly penetrates the system, therefore most SG stars present extreme helium abundances (Y > 0.33). The low-density model with delayed SN explosions has a more extended SG star formation epoch and includes SG stars with modest helium enrichment. On the contrary, the high-density model is weakly affected by SN explosions, with a final SG mass similar to the one obtained without SNe Ia. Most of the stars form from a mix of AGB ejecta and pristine gas and have a modest helium enrichment. We show that gas from SNe Ia may produce an iron spread of $sim 0.14$ dex, consistent with the spread found in about 20% of Galactic GCs, suggesting that SNe Ia might have played a key role in the formation of this sub-sample of GCs.
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