Browsing by Subject "stellar evolution"

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HE 1327-2326, An Unevolved Star With Fe/H < -5.0. II. New 3D-1D Corrected Abundances From A Very Large Telescope UVES Spectrum
(2008-09) Frebel, Anna; Collet, Remo; Eriksson, Kiell; Christlieb, Norbert; Aoki, Wako; Frebel, Anna
We present a new abundance analysis of HE 1327-2326, which is currently the most iron-poor star, based on observational data obtained with the VLT Ultraviolet and Visual Echelle Spectrograph (UVES). We correct the one-dimensional (1D) LTE abundances for three-dimensional (3D) effects to provide an abundance pattern that supersedes previous works and should be used to observationally test current models of the chemical yields of the first-generation supernovae (SNe). Apart from confirming the 1D LTE abundances found in previous studies before accounting for 3D effects, we make use of a novel technique to apply the 3D 1D corrections for CNO which are a function of excitation potential and line strength for the molecular lines that comprise the observable CH, NH, and OH features. We find that the fit to the NH band at 33608 is greatly improved due to the application of the 3D-1D corrections. This may indicate that 3D effects are actually observable in this star. We also report the first detection of several weak Ni lines. The cosmologically important element Li is still not detected; the new Li upper limit is extremely low, A(Li) < 0: 62, and in stark contrast with results not only from the Wilkinson Microwave Anisotropy Probe (WMAP) but also from other metal-poor stars. We also discuss how the new corrected abundance pattern of HE 1327-2326 is being reproduced by individual and integrated yields of SNe.
Interview with a White Dwarf
(McDonald Observatory, 2006) University of Texas at Austin; McDonald Observatory
Origin of Lithium Enrichment in K Giants
(2011-03) Kumar, Yerra B.; Reddy, Bacham E.; Lambert, David L.; Lambert, David L.
In this Letter, we report on a low-resolution spectroscopic survey for Li-rich K giants among 2000 low-mass (M <= 3 M-circle dot) giants spanning the luminosity range from below to above the luminosity of the clump. Fifteen new Li-rich giants including four super Li-rich K giants (log epsilon(Li) >= 3.2) were discovered. A significant finding is that there is a concentration of Li-rich K giants at the luminosity of the clump or red horizontal branch. This new finding is partly a consequence of the fact that our low-resolution survey is the first large survey to include giants well below and above the red giant branch (RGB) bump and clump locations in the H-R diagram. Origin of the lithium enrichment may be plausibly attributed to the conversion of He-3 via Be-7 to Li-7 by the Cameron-Fowler mechanism but the location for the onset of the conversion is uncertain. Two possible opportunities to effect this conversion are discussed: the bump in the first ascent of the RGB and the He-core flash at the tip of the RGB. The finite luminosity spread of the Li-rich giants serves to reject the idea that Li enhancement is, in general, a consequence of a giant swallowing a large planet.
The Pan-Pacific Planet Search. II. Confirmation Of A Two-Planet System Around Hd 121056
(2015-02) Wittenmyer, Robert A.; Wang, Lifan; Liu, Fan; Horner, Jonathan; Endl, Michael; Johnson, John A.; Tinney, C. G.; Carter, B. D.; Endl, Michael
Precise radial velocities from the Anglo-Australian Telescope (AAT) confirm the presence of a rare short-period planet around the K0 giant HD 121056. An independent two-planet solution using the AAT data shows that the inner planet has P = 89.1 +/- 0.1 days, and m sin i = 1.35 +/- 0.17 M-Jup. These data also confirm the planetary nature of the outer companion, with m sin i = 3.9 +/- 0.6 M-Jup and a = 2.96 +/- 0.16 AU. HD 121056 is the most-evolved star to host a confirmed multiple-planet system, and is a valuable example of a giant star hosting both a short-period and a long-period planet.
Regular Frequency Patterns In The Young Delta Scuti Star HD 261711 Observed By The CoRoT And MOST Satellites
(2013-04) Zwintz, K.; Fossati, L.; Guenther, D. B.; Ryabchikova, T.; Baglin, A.; Themessl, N.; Barnes, T. G.; Matthews, J. M.; Auvergne, M.; Bohlender, D.; Chaintreuil, S.; Kuschnig, R.; Moffat, A. F. J.; Rowe, J. F.; Rucinski, S. M.; Sasselov, D.; Weiss, W. W.; Barnes, T. G.
Context. The internal structure of pre-main-sequence (PMS) stars is poorly constrained at present. This could change significantly through high-quality asteroseismological observations of a sample of such stars. Aims. We concentrate on an asteroseismological study of HD261711, a rather hot delta Scuti-type pulsating member of the young open cluster NGC 2264 located at the blue border of the instability region. HD261711 was discovered to be a PMS delta Scuti star using the time series photometry obtained by the MOST satellite in 2006. Methods. High-precision, time-series photometry of HD261711 was obtained by the MOST and CoRoT satellites in four separate new observing runs that are put into context with the star's fundamental atmospheric parameters obtained from spectroscopy. Frequency Analysis was performed using Period04. The spectral analysis was performed using equivalent widths and spectral synthesis. Results. With the new MOST data set from 2011/12 and the two CoRoT light curves from 2008 and 2011/12, the delta Scuti variability was confirmed and regular groups of frequencies were discovered. The two pulsation frequencies identified in the data from the first MOST observing run in 2006 are confirmed and 23 new delta Scuti-type frequencies were discovered using the CoRoT data. Weighted average frequencies for each group were determined and are related to l = 0 and l = 1 p-modes. Evidence for amplitude modulation of the frequencies in two groups is seen. The effective temperature (T-eff) was derived to be 8600 +/- 200K, log g is 4.1 +/- 0.2, and the projected rotational velocity (upsilon sin i) is 53 +/- 1 km s(-1). Using our T-eff value and the radius of 1.8 +/- 0.5 R-circle dot derived from spectral energy distribution (SED) fitting, we get a luminosity log L/L-circle dot of 1.20 +/- 0.14 which agrees well to the seismologically determined values of 1.65 R-circle dot and, hence, a log L/L-circle dot of 1.13. The radial velocity of 14 +/- 2 km s(-1) we derived for HD261711, confirms the star's membership to NGC 2264. Conclusions. Our asteroseismic models suggest that HD261711 is a delta Scuti-type star close to the zero-age main sequence (ZAMS) with a mass of 1.8 to 1.9 M-circle dot. With an age of about 10 million years derived from asteroseismology, the star is either a young ZAMS star or a late PMS star just before the onset of hydrogen-core burning. The observed splittings about the l = 0 and 1 parent modes may be an artifact of the Fourier derived spectrum of frequencies with varying amplitudes.
Regular Frequency Patterns In The Young Delta Scuti Star HD 261711 Observed By The CoRoT And MOST Satellites
(2013-04) Zwintz, K.; Fossati, L.; Guenther, D. B.; Ryabchikova, T.; Baglin, A.; Themessl, N.; Barnes, T. G.; Matthews, J. M.; Auvergne, M.; Bohlender, D.; Chaintreuil, S.; Kuschnig, R.; Moffat, A. F. J.; Rowe, J. F.; Rucinski, S. M.; Sasselov, D.; Weiss, W. W.; Barnes, T. G.
Context. The internal structure of pre-main-sequence (PMS) stars is poorly constrained at present. This could change significantly through high-quality asteroseismological observations of a sample of such stars. Aims. We concentrate on an asteroseismological study of HD261711, a rather hot delta Scuti-type pulsating member of the young open cluster NGC 2264 located at the blue border of the instability region. HD261711 was discovered to be a PMS delta Scuti star using the time series photometry obtained by the MOST satellite in 2006. Methods. High-precision, time-series photometry of HD261711 was obtained by the MOST and CoRoT satellites in four separate new observing runs that are put into context with the star's fundamental atmospheric parameters obtained from spectroscopy. Frequency Analysis was performed using Period04. The spectral analysis was performed using equivalent widths and spectral synthesis. Results. With the new MOST data set from 2011/12 and the two CoRoT light curves from 2008 and 2011/12, the delta Scuti variability was confirmed and regular groups of frequencies were discovered. The two pulsation frequencies identified in the data from the first MOST observing run in 2006 are confirmed and 23 new delta Scuti-type frequencies were discovered using the CoRoT data. Weighted average frequencies for each group were determined and are related to l = 0 and l = 1 p-modes. Evidence for amplitude modulation of the frequencies in two groups is seen. The effective temperature (T-eff) was derived to be 8600 +/- 200K, log g is 4.1 +/- 0.2, and the projected rotational velocity (upsilon sin i) is 53 +/- 1 km s(-1). Using our T-eff value and the radius of 1.8 +/- 0.5 R-circle dot derived from spectral energy distribution (SED) fitting, we get a luminosity log L/L-circle dot of 1.20 +/- 0.14 which agrees well to the seismologically determined values of 1.65 R-circle dot and, hence, a log L/L-circle dot of 1.13. The radial velocity of 14 +/- 2 km s(-1) we derived for HD261711, confirms the star's membership to NGC 2264. Conclusions. Our asteroseismic models suggest that HD261711 is a delta Scuti-type star close to the zero-age main sequence (ZAMS) with a mass of 1.8 to 1.9 M-circle dot. With an age of about 10 million years derived from asteroseismology, the star is either a young ZAMS star or a late PMS star just before the onset of hydrogen-core burning. The observed splittings about the l = 0 and 1 parent modes may be an artifact of the Fourier derived spectrum of frequencies with varying amplitudes.
Regular Frequency Patterns In The Young Delta Scuti Star HD 261711 Observed By The CoRoT And MOST Satellites
(2013-04) Zwintz, K.; Fossati, L.; Guenther, D. B.; Ryabchikova, T.; Baglin, A.; Themessl, N.; Barnes, T. G.; Matthews, J. M.; Auvergne, M.; Bohlender, D.; Chaintreuil, S.; Kuschnig, R.; Moffat, A. F. J.; Rowe, J. F.; Rucinski, S. M.; Sasselov, D.; Weiss, W. W.; Barnes, T. G.
Context. The internal structure of pre-main-sequence (PMS) stars is poorly constrained at present. This could change significantly through high-quality asteroseismological observations of a sample of such stars. Aims. We concentrate on an asteroseismological study of HD261711, a rather hot delta Scuti-type pulsating member of the young open cluster NGC 2264 located at the blue border of the instability region. HD261711 was discovered to be a PMS delta Scuti star using the time series photometry obtained by the MOST satellite in 2006. Methods. High-precision, time-series photometry of HD261711 was obtained by the MOST and CoRoT satellites in four separate new observing runs that are put into context with the star's fundamental atmospheric parameters obtained from spectroscopy. Frequency Analysis was performed using Period04. The spectral analysis was performed using equivalent widths and spectral synthesis. Results. With the new MOST data set from 2011/12 and the two CoRoT light curves from 2008 and 2011/12, the delta Scuti variability was confirmed and regular groups of frequencies were discovered. The two pulsation frequencies identified in the data from the first MOST observing run in 2006 are confirmed and 23 new delta Scuti-type frequencies were discovered using the CoRoT data. Weighted average frequencies for each group were determined and are related to l = 0 and l = 1 p-modes. Evidence for amplitude modulation of the frequencies in two groups is seen. The effective temperature (T-eff) was derived to be 8600 +/- 200K, log g is 4.1 +/- 0.2, and the projected rotational velocity (upsilon sin i) is 53 +/- 1 km s(-1). Using our T-eff value and the radius of 1.8 +/- 0.5 R-circle dot derived from spectral energy distribution (SED) fitting, we get a luminosity log L/L-circle dot of 1.20 +/- 0.14 which agrees well to the seismologically determined values of 1.65 R-circle dot and, hence, a log L/L-circle dot of 1.13. The radial velocity of 14 +/- 2 km s(-1) we derived for HD261711, confirms the star's membership to NGC 2264. Conclusions. Our asteroseismic models suggest that HD261711 is a delta Scuti-type star close to the zero-age main sequence (ZAMS) with a mass of 1.8 to 1.9 M-circle dot. With an age of about 10 million years derived from asteroseismology, the star is either a young ZAMS star or a late PMS star just before the onset of hydrogen-core burning. The observed splittings about the l = 0 and 1 parent modes may be an artifact of the Fourier derived spectrum of frequencies with varying amplitudes.
Simulations On A Moving Mesh: The Clustered Formation Of Population III Protostars
(2011-08) Greif, Thomas H.; Springel, Volker; White, Simon D. M.; Glover, Simon C. O.; Clark, Paul C.; Smith, Rowan J.; Klessen, Ralf S.; Bromm, Volker; Bromm, Volker
The cosmic dark ages ended a few hundred million years after the big bang, when the first stars began to fill the universe with new light. It has generally been argued that these stars formed in isolation and were extremely massive-perhaps 100 times as massive as the Sun. In a recent study, Clark and collaborators showed that this picture requires revision. They demonstrated that the accretion disks that build up around Population III stars are strongly susceptible to fragmentation and that the first stars should therefore form in clusters rather than in isolation. We here use a series of high-resolution hydrodynamical simulations performed with the moving mesh code AREPO to follow up on this proposal and to study the influence of environmental parameters on the level of fragmentation. We model the collapse of five independent minihalos from cosmological initial conditions, through the runaway condensation of their central gas clouds, to the formation of the first protostar, and beyond for a further 1000 years. During this latter accretion phase, we represent the optically thick regions of protostars by sink particles. Gas accumulates rapidly in the circumstellar disk around the first protostar, fragmenting vigorously to produce a small group of protostars. After an initial burst, gravitational instability recurs periodically, forming additional protostars with masses ranging from similar to 0.1 to 10 M-circle dot. Although the shape, multiplicity, and normalization of the protostellar mass function depend on the details of the sink-particle algorithm, fragmentation into protostars with diverse masses occurs in all cases, confirming earlier reports of Population III stars forming in clusters. Depending on the efficiency of later accretion and merging, Population III stars may enter the main sequence in clusters and with much more diverse masses than are commonly assumed.
VLT/FLAMES Spectroscopy Of Red Giant Branch Stars In The Fornax Dwarf Spheroidal Galaxy
(2014-12) Lemasle, B.; de Boer, T. J. L.; Hill, V.; Tolstoy, E.; Irwin, M. J.; Jablonka, P.; Venn, K.; Battaglia, G.; Starkenburg, E.; Shetrone, M.; Letarte, B.; Francois, P.; Helmi, A.; Primas, F.; Kaufer, A.; Szeifert, T.; Shetrone, Matthew D.
Context. Fornax is one of the most massive dwarf spheroidal galaxies in the Local Group. The Fornax field star population is dominated by intermediate age stars but star formation was going on over almost its entire history. It has been proposed that Fornax experienced a minor merger event. Aims. Despite recent progress, only the high metallicity end of Fornax field stars ([Fe/H] > -1.2 dex) has been sampled in larger number via high resolution spectroscopy. We want to better understand the full chemical evolution of this galaxy by better sampling the whole metallicity range, including more metal poor stars. Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode to determine the abundances of several a, iron-peak and neutron-capture elements in a sample of 47 individual red giant branch stars in the Fornax dwarf spheroidal galaxy. We combine these abundances with accurate age estimates derived from the age probability distribution from the colour-magnitude diagram of Fornax. Results. Similar to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are typically alpha-rich while the young metal-rich stars are a-poor. In the classical scenario of the time delay between Type II (SNe II) and Type Ia Supernovae (SNe la), we confirm that SNe la started to contribute to the chemical enrichment at [Fe/H] between -2.0 and -1.8 dex. We find that the onset of SNe Ia took place between 12-10 Gyr ago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia and AGB stars in the abundance pattern of the younger stellar population of Fornax. Conclusions. Our findings of low [alpha/Fe] and enhanced [Eu/Mg] are compatible with an initial mass function that lacks the most massive stars and with star formation that kept going on throughout the whole history of Fornax. We find that massive stars kept enriching the interstellar medium in alpha-elements, although they were not the main contributor to the iron enrichment.
VLT/FLAMES Spectroscopy Of Red Giant Branch Stars In The Fornax Dwarf Spheroidal Galaxy
(2014-12) Lemasle, B.; de Boer, T. J. L.; Hill, V.; Tolstoy, E.; Irwin, M. J.; Jablonka, P.; Venn, K.; Battaglia, G.; Starkenburg, E.; Shetrone, M.; Letarte, B.; Francois, P.; Helmi, A.; Primas, F.; Kaufer, A.; Szeifert, T.; Shetrone, Matthew D.
Context. Fornax is one of the most massive dwarf spheroidal galaxies in the Local Group. The Fornax field star population is dominated by intermediate age stars but star formation was going on over almost its entire history. It has been proposed that Fornax experienced a minor merger event. Aims. Despite recent progress, only the high metallicity end of Fornax field stars ([Fe/H] > -1.2 dex) has been sampled in larger number via high resolution spectroscopy. We want to better understand the full chemical evolution of this galaxy by better sampling the whole metallicity range, including more metal poor stars. Methods. We use the VLT-FLAMES multi-fibre spectrograph in high-resolution mode to determine the abundances of several a, iron-peak and neutron-capture elements in a sample of 47 individual red giant branch stars in the Fornax dwarf spheroidal galaxy. We combine these abundances with accurate age estimates derived from the age probability distribution from the colour-magnitude diagram of Fornax. Results. Similar to other dwarf spheroidal galaxies, the old, metal-poor stars of Fornax are typically alpha-rich while the young metal-rich stars are a-poor. In the classical scenario of the time delay between Type II (SNe II) and Type Ia Supernovae (SNe la), we confirm that SNe la started to contribute to the chemical enrichment at [Fe/H] between -2.0 and -1.8 dex. We find that the onset of SNe Ia took place between 12-10 Gyr ago. The high values of [Ba/Fe], [La/Fe] reflect the influence of SNe Ia and AGB stars in the abundance pattern of the younger stellar population of Fornax. Conclusions. Our findings of low [alpha/Fe] and enhanced [Eu/Mg] are compatible with an initial mass function that lacks the most massive stars and with star formation that kept going on throughout the whole history of Fornax. We find that massive stars kept enriching the interstellar medium in alpha-elements, although they were not the main contributor to the iron enrichment.
White Dwarfs In Ngc6397 And M4: Constraints On The Physics Of Crystallization
(2010-08) Winget, D. E.; Montgomery, M. H.; Kepler, S. O.; Campos, F.; Bergeron, P.; Winget, D. E.; Montgomery, M. H.
We explore the physics of crystallization in the dense Coulomb plasma of the deep interiors of white dwarf stars using the color-magnitude diagram and luminosity function constructed from Hubble Space Telescope photometry of the globular cluster M 4 and compare it with our results for proper motion cleaned Hubble Space Telescope photometry of the globular cluster NGC 6397. We demonstrate that the data are consistent with a binary mixture of carbon and oxygen crystallizing at a value of Gamma higher than the theoretical value for a One Component Plasma (OCP). We show that this result is in line with the latest Molecular Dynamics simulations for binary mixtures of C/O. We discuss implications for future work.