Lars Bildsten

Rutgers, The State University of New Jersey, New Brunswick, New Jersey, United States

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Publications (248)1274.58 Total impact

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    ABSTRACT: Type Iax supernovae are stellar explosions that are spectroscopically similar to some type Ia supernovae at the time of maximum light emission, except with lower ejecta velocities. They are also distinguished by lower luminosities. At late times, their spectroscopic properties diverge from those of other supernovae, but their composition (dominated by iron-group and intermediate-mass elements) suggests a physical connection to normal type Ia supernovae. Supernovae of type Iax are not rare; they occur at a rate between 5 and 30 per cent of the normal type Ia rate. The leading models for type Iax supernovae are thermonuclear explosions of accreting carbon-oxygen white dwarfs that do not completely unbind the star, implying that they are 'less successful' versions of normal type Ia supernovae, where complete stellar disruption is observed. Here we report the detection of the luminous, blue progenitor system of the type Iax SN 2012Z in deep pre-explosion imaging. The progenitor system's luminosity, colours, environment and similarity to the progenitor of the Galactic helium nova V445 Puppis suggest that SN 2012Z was the explosion of a white dwarf accreting material from a helium-star companion. Observations over the next few years, after SN 2012Z has faded, will either confirm this hypothesis or perhaps show that this supernova was actually the explosive death of a massive star.
    Nature 08/2014; 512(7512):54-6. · 38.60 Impact Factor
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    ABSTRACT: Type Iax supernovae (SNe Iax) are thermonuclear explosions that are related to SNe Ia, but are physically distinct. The most important differences are that SNe Iax have significantly lower luminosity (1% - 50% that of typical SNe Ia), lower ejecta mass (~0.1 - 0.5 M_sun), and may leave a bound remnant. The most extreme SN Iax is SN 2008ha, which peaked at M_V = -14.2 mag, about 5 mag below that of typical SNe Ia. Here, we present Hubble Space Telescope (HST) images of UGC 12682, the host galaxy of SN 2008ha, taken 4.1 years after the peak brightness of SN 2008ha. In these deep, high-resolution images, we detect a source coincident (0.86 HST pixels; 0.043"; 1.1 sigma) with the position of SN 2008ha with M_F814W = -5.4 mag. We determine that this source is unlikely to be a chance coincidence, but that scenario cannot be completely ruled out. If this source is directly related to SN 2008ha, it is either the luminous bound remnant of the progenitor white dwarf or its companion star. The source is consistent with being an evolved >3 M_sun initial mass star, and is significantly redder than the SN Iax 2012Z progenitor system, the first detected progenitor system for a thermonuclear SN. If this source is the companion star for SN 2008ha, there is a diversity in SN Iax progenitor systems, perhaps related to the diversity in SN Iax explosions. If the source is the bound remnant of the white dwarf, it must have expanded significantly. Regardless of the nature of this source, we constrain the progenitor system of SN 2008ha to have an age of <80 Myr.
    The Astrophysical Journal 08/2014; 792(1). · 6.73 Impact Factor
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    ABSTRACT: We report the optical identification of the companion to the {\it Fermi} black widow millisecond pulsar PSR J1544+4937. We find a highly variable source on Keck LRIS images at the nominal pulsar position, with 2 magnitude variations over orbital period in the B, g, R, and I bands. The nearly achromatic light curves are difficult to explain with a simply irradiated hemisphere model, and suggest that the optical emission is dominated by a nearly isothermal hot patch on the surface of the companion facing the pulsar. We roughly constrain the distance to PSR J1544+4937 to be between 2 and 5 kpc. A more reliable distance measurement is needed in order to constrain the composition of the companion.
    The Astrophysical Journal Letters 06/2014; 791(1). · 6.35 Impact Factor
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    ABSTRACT: Asteroseismology of 1.0-2.0 M ☉ red giants by the Kepler satellite has enabled the first definitive measurements of interior rotation in both first ascent red giant branch (RGB) stars and those on the helium burning clump. The inferred rotation rates are 10-30 days for the 0.2 M ☉ He degenerate cores on the RGB and 30-100 days for the He burning core in a clump star. Using the Modules for Experiments in Stellar Evolution code, we calculate state-of-the-art stellar evolution models of low mass rotating stars from the zero-age main sequence to the cooling white dwarf (WD) stage. We include transport of angular momentum due to rotationally induced instabilities and circulations, as well as magnetic fields in radiative zones (generated by the Tayler-Spruit dynamo). We find that all models fail to predict core rotation as slow as observed on the RGB and during core He burning, implying that an unmodeled angular momentum transport process must be operating on the early RGB of low mass stars. Later evolution of the star from the He burning clump to the cooling WD phase appears to be at nearly constant core angular momentum. We also incorporate the adiabatic pulsation code, ADIPLS, to explicitly highlight this shortfall when applied to a specific Kepler asteroseismic target, KIC8366239.
    The Astrophysical Journal 05/2014; 788(1):93. · 6.73 Impact Factor
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    ABSTRACT: Asteroseismology of 1.0-2.0 Msun red giants by the Kepler satellite has enabled the first definitive measurements of interior rotation in both first ascent red giant branch (RGB) stars and those on the Helium burning clump. The inferred rotation rates are 10-30 days for the ~0.2Msun He degenerate cores on the RGB and 30-100 days for the He burning core in a clump star. Using the MESA code we calculate state-of-the-art stellar evolution models of low mass rotating stars from the zero-age main sequence to the cooling white dwarf (WD) stage. We include transport of angular momentum due to rotationally induced instabilities and circulations, as well as magnetic fields in radiative zones (generated by the Tayler-Spruit dynamo). We find that all models fail to predict core rotation as slow as observed on the RGB and during core He burning, implying that an unmodeled angular momentum transport process must be operating on the early RGB of low mass stars. Later evolution of the star from the He burning clump to the cooling WD phase appears to be at nearly constant core angular momentum. We also incorporate the adiabatic pulsation code, ADIPLS, to explicitly highlight this shortfall when applied to a specific Kepler asteroseismic target, KIC8366239. The MESA inlist adopted to calculate the models in this paper can be found at \url{https://authorea.com/1608/} (bottom of the document).
    04/2014;
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    ABSTRACT: We search for the Fe K${\alpha}$ line in spectra of Ultra Compact X-ray Binaries (UCXBs). For this purpose we have analyzed XMM-Newton observations of five confirmed UCXBs. We find that the object 2S 0918-549 - whose optical spectrum bears tentative signatures of a C/O accretion disk - is devoid of any emission features in the 6-7 keV range, with an upper limit of less than 10 eV for the equivalent width (EW) of the iron line. 4U 1916-05 - whose optical spectrum is consistent with reflection from a He-rich accretion disk - exhibits a bright broad iron emission line. This behavior is in agreement with the theoretical predictions presented in Koliopanos, Gilfanov and Bildsten (2013). Namely, we expect strong suppression of the Fe K${\alpha}$ emission line in spectra originating in moderately bright (LogLx less than $\approx$ 37.5) UCXBs with C/O or O/Ne/Mg-rich donors. On the other hand the EW of the iron line in spectra from UCXBs with He-rich donors is expected to retain its nominal value of $\approx$ 100 eV. Our analysis also reveals a strong Fe K${\alpha}$ line in the spectrum of 4U 0614+091. This detection points towards a He-rich donor and seems to be at odds with the source's classification as C/O-rich. Nevertheless, a He-rich donor would explain the bursting activity reported for this system. Lastly, based on our theoretical predictions, we attribute the lack of a strong iron emission line - in the two remaining UCXB sources in our sample (XTE J1807-294, 4U 0513-40) - as an indication of a C/O or O/Ne/Mg white dwarf donor. From the upper limits of the Fe K${\alpha}$ line EW in 4U 0513-40, 2S 0918-549 and XTE J1807-294 we obtain a lower limit on the oxygen-to-iron ratio,O/Fe$\ge$ 10$\times$[O/Fe]$_{\odot}$.
    04/2014;
  • 01/2014; 782(2).
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    ABSTRACT: The iPTF detection of the most recent outburst of the recurrent nova system RX J0045.4+4154 in the Andromeda Galaxy has enabled the unprecedented study of a massive (M>1.3 $M_\odot$) accreting white dwarf (WD). We detected this nova as part of the near daily iPTF monitoring of M31 to a depth of R$\approx$21 and triggered optical photometry, spectroscopy and soft X-ray monitoring of the outburst. Peaking at an absolute magnitude of M_R$\approx$-6.6 mag, and with a decay time of 1 mag per day, it is a faint and very fast novae. It shows optical emission lines of He/N and expansion velocities of 1900 to 2600 km/s 1-4 days after the optical peak. The Swift monitoring of the X-ray evolution revealed a supersoft source (SSS) with $kT_{eff}$ $\approx$ 90-110 eV that appeared within 5 days after the optical peak, and lasted only 12 days. Most remarkably, this is not the first event from this system, rather it is a recurrent nova with a time between outbursts of approximately 1 year, the shortest known. Recurrent X-ray emission from this binary was detected by ROSAT in 1992 and 1993, and the source was well characterized as a M>1.3 $M_\odot$ WD SSS. Both the duration and effective temperature of the SS phase imply M>1.32 $M_\odot$, while MESA models of accreting WDs allow us to constrain the accretion rate to $\dot{M}>1.7\times10^{-7}$ $M_\odot$/yr and WD mass >1.32 $M_\odot$. If the WD keeps 30% of the accreted material, it will take less than a Myr to reach core densities high enough for carbon ignition (if made of C/O) or electron capture (if made of O/Ne) to end the binary evolution.
    The Astrophysical Journal 01/2014; 786(1). · 6.73 Impact Factor
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    ABSTRACT: We present high-quality ULTRACAM photometry of the eclipsing detached double-white dwarf binary NLTT 11748. This system consists of a carbon/oxygen white dwarf and an extremely-low mass (< 0.2 Msun) helium-core white dwarf in a 5.6 hr orbit. To date such extremely-low mass WDs, which can have thin, stably-burning outer layers, have been modeled via poorly-constrained atmosphere and cooling calculations where uncertainties in the detailed structure can strongly influence the eventual fates of these systems when mass-transfer begins. With precise (individual precision ~1%) high-cadence (~2 s) multi-color photometry of multiple primary and secondary eclipses spanning >1.5 yr, we constrain the masses and radii of both objects in the NLTT 11748 system to a statistical uncertainty of a few percent. However, we find that overall uncertainty in the thickness of the envelope of the secondary carbon/oxygen white dwarf leads to a larger (~13%) systematic uncertainty in the primary He WD's mass. Over the full range of possible envelope thicknesses we find that our primary mass (0.136-0.162 Msun) and surface gravity (log(g)=6.32-6.38; radii are 0.0423-0.0433 Rsun) constraints do not agree with previous spectroscopic determinations. We use precise eclipse timing to detect the Romer delay at 7 sigma significance, providing an additional weak constraint on the masses and limiting the eccentricity to e*cos(omega)= -4e-5 +/- 5e-5. Finally, we use multi-color data to constrain the secondary's effective temperature (7600+/-120 K) and cooling age (1.6-1.7 Gyr).
    The Astrophysical Journal 11/2013; 780(2). · 6.73 Impact Factor
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    ABSTRACT: We present our observations of SN 2010mb, a Type Ic SN lacking spectroscopic signatures of H and He. SN 2010mb has a slowly-declining light curve ($\sim600\,$days) that cannot be powered by $^{56}$Ni/$^{56}$Co radioactivity, the common energy source for Type Ic SNe. We detect signatures of interaction with hydrogen-free CSM including a blue quasi-continuum and, uniquely, narrow oxygen emission lines that require high densities ($\sim10^9$cm$^{-3}$). From the observed spectra and light curve we estimate that the amount of material involved in the interaction was $\sim3$M$_{\odot}$. Our observations are in agreement with models of pulsational pair-instability SNe described in the literature.
    09/2013; 785(1).
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    ABSTRACT: We examine the properties of white dwarfs (WDs) accreting hydrogen-rich matter in and near the stable burning regime of accretion rates as modeled by time-dependent calculations done with Modules for Experiments in Stellar Astrophysics (MESA). We report the stability boundary for WDs of masses between 0.51 solar masses and 1.34 solar masses as found via time-dependent calculations. We also examine recurrent novae that are accreting at rates close to, but below, the stable burning limit and report their recurrence times and ignition masses. Our dense grid in accretion rates finds the expected minimum possible recurrence times as a function of the WD mass. This enables inferences to be made about the minimum WD mass possible to reach a specific recurrence time. We compare our computational models of post-outburst novae to the stably burning WDs and explicitly calculate the duration and effective temperature (Teff) of the post-novae WD in the supersoft phase. We agree with the measured turnoff time - Teff relation in M31 by Henze and collaborators, infer WD masses in the 1.0-1.3 solar masses range, and predict ejection masses consistent with those observed. We close by commenting on the importance of the hot helium layer generated by stable or unstable hydrogen burning for the short- and long-term evolution of accreting white dwarfs.
    The Astrophysical Journal 09/2013; 777(2). · 6.73 Impact Factor
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    Kevin Moore, Dean Townsley, Lars Bildsten
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    ABSTRACT: Accreted helium layers on white dwarfs have been highlighted for many decades as a possible site for a detonation triggered by a thermonuclear runaway. In this paper, we find the minimum helium layer thickness that will sustain a steady laterally propagating detonation and show that it depends on the density and composition of the helium layer, specifically C12 and O16. Detonations in these thin helium layers have speeds slower than the Chapman-Jouget (CJ) speed from complete helium burning, v_cj=1.5e9 cm/s. Though gravitationally unbound, the ashes still have unburned helium (~ 80% in the thinnest cases) and only reach up to heavy elements such as Ca40, Ti44, Cr48, and Fe52. It is rare for these thin shells to generate large amounts of Ni56. We also find a new set of solutions that can propagate in even thinner helium layers when O16 is present at a minimum mass fraction of ~0.07. Driven by energy release from alpha captures on O16 and subsequent elements, these slow detonations only create ashes up to Si28 in the outer detonated He shell. We close by discussing how the unbound helium burning ashes may create faint and fast ".Ia" supernovae as well as events with virtually no radioactivity, and speculate on how the slower helium detonation velocities impact the off-center ignition of a carbon detonation that could cause a Type Ia supernova in the double detonation scenario.
    The Astrophysical Journal 08/2013; 776(2). · 6.73 Impact Factor
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    ABSTRACT: Non-solar composition of the donor star in ultra-compact X-ray binaries (UCXBs) may have a pronounced effect on the fluorescent lines appearing in their spectra due to reprocessing of primary radiation by the accretion disc and the white dwarf surface. We show that the most dramatic and easily observable consequence of the anomalous C/O abundance is the significant, by more than an order of magnitude, attenuation of the Kα line of iron. It is caused by screening of the presence of iron by oxygen - in the C/O-dominated material the main interaction process for an E ≈ 7 keV photon is absorption by oxygen rather than by iron, contrary to the solar composition case. Ionization of oxygen at high mass accretion rates adds a luminosity dependence to this behaviour - the iron line is significantly suppressed only at low luminosity, log (LX) ≲ 37-37.5, and should recover its nominal strength at higher luminosity. The increase of the equivalent width of the Kα lines of carbon and oxygen, on the other hand, saturates at rather moderate values. Screening by He is less important, due to its low ionization threshold and because in the accretion disc it is mostly ionized. Consequently, in the case of the He-rich donor, the iron line strength remains close to its nominal value, determined by the iron abundance in the accretion disc. This opens the possibility of constraining the nature of donor stars in UCXBs by means of X-ray spectroscopy with moderate energy resolution.
    Monthly Notices of the Royal Astronomical Society 06/2013; 432(2):1264-1273. · 5.52 Impact Factor
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    Ken J. Shen, Lars Bildsten
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    ABSTRACT: The progenitor channel responsible for the majority of Type Ia supernovae is still uncertain. One emergent scenario involves the detonation of a He-rich layer surrounding a C/O white dwarf, which sends a shock wave into the core. The quasi-spherical shock wave converges and strengthens at an off-center location, forming a second, C-burning, detonation that disrupts the whole star. In this paper, we examine this second detonation of the double detonation scenario using a combination of analytic and numeric techniques. We perform a spatially resolved study of the imploding shock wave and outgoing detonation and calculate the critical imploding shock strengths needed to achieve a core C detonation. We find that He detonations in recent two-dimensional simulations yield converging shock waves that are strong enough to ignite C detonations in C/O cores, with the caveat that a truly robust answer requires multi-dimensional detonation initiation calculations. We also find that, due to the greatly increased difficulty of igniting O-burning, convergence-driven detonations in O/Ne cores are far harder to achieve and are perhaps unrealized in standard binary evolution.
    The Astrophysical Journal 05/2013; · 6.73 Impact Factor
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    ABSTRACT: We report the discovery of R Coronae Borealis (RCB) stars in the Andromeda galaxy (M31) using the Palomar Transient Factory (PTF). RCB stars are rare hydrogen-deficient, carbon-rich supergiant variables, most likely the merger products of two white dwarfs. These new RCBs, including two confirmed ones and two candidates, are the first to be found beyond the Milky Way and the Magellanic Clouds. All of M31 RCBs showed >1.5 mag irregular declines over timescales of weeks to months. Due to the limiting magnitude of our data (R~21-22 mag), these RCB stars have R~19.5 to 20.5 mag at maximum light, corresponding to M_R= -4 to -5, making them some of the most luminous RCBs known. Spectra of two objects show that they are warm RCBs, similar to the Milky Way RCBs RY Sgr and V854 Cen. We consider these results, derived from a pilot study of M31 variables, as an important proof-of-concept for the study of rare bright variables in nearby galaxies with the PTF or other synoptic surveys.
    The Astrophysical Journal Letters 03/2013; 767(2). · 6.35 Impact Factor
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    ABSTRACT: Some observations suggest that very massive stars experience extreme mass-loss episodes shortly before they explode as supernovae as do several models. Establishing a causal connection between these mass-loss episodes and the final explosion would provide a novel way to study pre-supernova massive-star evolution. Here we report observations of a mass-loss event detected 40 days before the explosion of the type IIn supernova SN 2010mc (also known as PTF 10tel). (1 data file).
    VizieR Online Data Catalog. 03/2013;
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    ABSTRACT: We present optical and infrared monitoring data of SN 2012hn collected by the Public ESO Spectroscopic Survey for Transient Objects (PESSTO). We show that SN 2012hn has a faint peak magnitude (MR ~ -15.7) and shows no hydrogen and no clear evidence for helium in its spectral evolution. Instead, we detect prominent Ca II lines at all epochs, which relates this transient to previously described 'Ca-rich' or 'gap' transients. However, the photospheric spectra (from -3 to +32 d with respect to peak) of SN 2012hn show a series of absorption lines which are unique, and a red continuum that is likely intrinsic rather than due to extinction. Lines of Ti II and Cr II are visible. This may be a temperature effect, which could also explain the red photospheric colour. A nebular spectrum at +150d shows prominent CaII, OI, CI and possibly MgI lines which appear similar in strength to those displayed by core-collapse SNe. To add to the puzzle, SN 2012hn is located at a projected distance of 6 kpc from an E/S0 host and is not close to any obvious starforming region. Overall SN 2012hn resembles a group of faint H-poor SNe that have been discovered recently and for which a convincing and consistent physical explanation is still missing. They all appear to explode preferentially in remote locations offset from a massive host galaxy with deep limits on any dwarf host galaxies, favouring old progenitor systems. SN 2012hn adds heterogeneity to this sample of objects. We discuss potential explosion channels including He-shell detonations and double detonations of white dwarfs as well as peculiar core-collapse SNe.
    Monthly Notices of the Royal Astronomical Society 02/2013; · 5.52 Impact Factor
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    ABSTRACT: Some observations suggest that very massive stars experience extreme mass-loss episodes shortly before they explode as supernovae, as do several models. Establishing a causal connection between these mass-loss episodes and the final explosion would provide a novel way to study pre-supernova massive-star evolution. Here we report observations of a mass-loss event detected 40 days before the explosion of the type IIn supernova SN 2010mc (also known as PTF 10tel). Our photometric and spectroscopic data suggest that this event is a result of an energetic outburst, radiating at least 6 × 10(47) erg of energy and releasing about 10(-2) solar masses of material at typical velocities of 2,000 km s(-1). The temporal proximity of the mass-loss outburst and the supernova explosion implies a causal connection between them. Moreover, we find that the outburst luminosity and velocity are consistent with the predictions of the wave-driven pulsation model, and disfavour alternative suggestions.
    Nature 02/2013; 494(7435):65-7. · 38.60 Impact Factor
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    ABSTRACT: Of the more than 150000 targets followed by the Kepler Mission, about 10% were selected as red giants. Due to their high scientific value, in particular for Galaxy population studies and stellar structure and evolution, their Kepler light curves were made public in late 2011. More than 13000 (over 85%) of these stars show intrinsic flux variability caused by solar-like oscillations making them ideal for large scale asteroseismic investigations. We automatically extracted individual frequencies and measured the period spacings of the dipole modes in nearly every red giant. These measurements naturally classify the stars into various populations, such as the red giant branch, the low-mass (M/Msol < 1.8) helium-core-burning red clump, and the higher-mass (M/Msol > 1.8) secondary clump. The period spacings also reveal that a large fraction of the stars show rotationally induced frequency splittings. This sample of stars will undoubtedly provide an extremely valuable source for studying the stellar population in the direction of the Kepler field, in particular when combined with complementary spectroscopic surveys.
    The Astrophysical Journal Letters 02/2013; 765(2). · 6.35 Impact Factor
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    ABSTRACT: We substantially update the capabilities of the open source software package Modules for Experiments in Stellar Astrophysics (MESA), and its one-dimensional stellar evolution module, MESA Star. Improvements in MESA Star's ability to model the evolution of giant planets now extends its applicability down to masses as low as one-tenth that of Jupiter. The dramatic improvement in asteroseismology enabled by the space-based Kepler and CoRoT missions motivates our full coupling of the ADIPLS adiabatic pulsation code with MESA Star. This also motivates a numerical recasting of the Ledoux criterion that is more easily implemented when many nuclei are present at non-negligible abundances. This impacts the way in which MESA Star calculates semi-convective and thermohaline mixing. We exhibit the evolution of 3-8 Msun stars through the end of core He burning, the onset of He thermal pulses, and arrival on the white dwarf cooling sequence. We implement diffusion of angular momentum and chemical abundances that enable calculations of rotating-star models, which we compare thoroughly with earlier work. We introduce a new treatment of radiation-dominated envelopes that allows the uninterrupted evolution of massive stars to core collapse. This enables the generation of new sets of supernovae, long gamma-ray burst, and pair-instability progenitor models. We substantially modify the way in which MESA Star solves the fully coupled stellar structure and composition equations, and we show how this has improved MESA's performance scaling on multi-core processors. Updates to the modules for equation of state, opacity, nuclear reaction rates, and atmospheric boundary conditions are also provided. We describe the MESA Software Development Kit (SDK) that packages all the required components needed to form a unified and maintained build environment for MESA. [Abridged]
    The Astrophysical Journal Supplement Series 01/2013; 208(1). · 16.24 Impact Factor

Publication Stats

6k Citations
1,274.58 Total Impact Points

Institutions

  • 2014
    • Rutgers, The State University of New Jersey
      • Department Physics and Astronomy
      New Brunswick, New Jersey, United States
  • 1999–2014
    • University of California, Santa Barbara
      • • Department of Physics
      • • Kavli Institute for Theoretical Physics
      Santa Barbara, California, United States
  • 1997–2014
    • California Institute of Technology
      • Division of Physics, Mathematics, and Astronomy
      Pasadena, California, United States
    • Universities Space Research Association
      Houston, Texas, United States
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, Massachusetts, United States
  • 2013
    • Aarhus University
      • Department of Physics and Astronomy
      Aarhus, Central Jutland, Denmark
  • 2010–2013
    • CSU Mentor
      Long Beach, California, United States
    • James Cook University
      Townsville, Queensland, Australia
    • Weizmann Institute of Science
      • Department of Particle Physics and Astrophysics
      Israel
  • 2012
    • Las Cumbres Observatory Global Telescope Network
      Goleta, California, United States
    • University of Notre Dame
      • Department of Physics
      South Bend, Indiana, United States
  • 2011
    • National Institute of Astrophysics
      Roma, Latium, Italy
  • 1995–2011
    • University of California, Berkeley
      • • Theoretical Astrophysics Center
      • • Department of Physics
      Berkeley, California, United States
  • 2009
    • Arizona State University
      • School of Earth and Space Exploration
      Phoenix, Arizona, United States
  • 2008
    • Harvard-Smithsonian Center for Astrophysics
      Cambridge, Massachusetts, United States
  • 2005
    • University of Chicago
      • Department of Astronomy and Astrophysics
      Chicago, Illinois, United States
    • University of Alberta
      • Theoretical Physics Institute
      Edmonton, Alberta, Canada
  • 2001
    • University of Colorado at Boulder
      Boulder, Colorado, United States
    • Michigan State University
      • Department of Physics and Astronomy
      East Lansing, MI, United States
  • 1998
    • Max Planck Institute for Extraterrestrial Physics
      Arching, Bavaria, Germany
  • 1992
    • Northwestern University
      Evanston, Illinois, United States