arXiv subjects

Jan E. Staff

Publications and source records attributed to Jan E. Staff.

At least 19 recordsLinked to original sources

arXiv2024

Hydrodynamic simulations of WD-WD mergers and the origin of RCB stars

We study the properties of double white dwarf (DWD) mergers by performing hydrodynamic simulations using the new and improved adaptive mesh refinement code Octo-Tiger. We follow the orbital evolution of DWD systems of mass ratio q=0.7 for tens of orbits until and after the merger to investigate them as a possible origin for R Coronae Borealis (RCB) type stars. We reproduce previous results, finding that during the merger, the Helium WD donor star is tidally disrupted within 20-80 minutes since the beginning of the simulation onto the accretor Carbon-Oxygen WD, creating a high temperature shell around the accretor. We investigate the possible Helium burning in this shell and the merged object's general structure. Specifically, we are interested in the amount of Oxygen-16 dredged-up to the hot shell and the amount of Oxygen-18 produced. This is critical as the discovery of very low Oxygen-16 to Oxygen-18 ratios in RCB stars pointed out the merger scenario as a favorable explanation for their origin. A small amount of hydrogen in the donor may help keep the Oxygen-16 to Oxygen-18 ratios within observational bounds, even if moderate dredge-up from the accretor occurs. In addition, we perform a resolution study to reconcile the difference found in the amount of Oxygen-16 dredge-up between smoothed-particle hydrodynamics and grid-based simulations.

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arXiv2024

Disk Wind Feedback from High-mass Protostars. III. Synthetic CO Line Emission

To test theoretical models of massive star formation it is important to compare their predictions with observed systems. To this end, we conduct CO molecular line radiative transfer post-processing of 3D magneto-hydrodynamic (MHD) simulations of various stages in the evolutionary sequence of a massive protostellar core, including its infall envelope and disk wind outflow. Synthetic position-position-velocity (PPV) cubes of various transitions of CO, 13CO, and C18O emission are generated. We also carry out simulated Atacama Large Millimeter/submillimeter Array (ALMA) observations of this emission. We compare the mass, momentum and kinetic energy estimates obtained from molecular lines to the true values, finding that the mass and momentum estimates can have uncertainties of up to a factor of four. However, the kinetic energy estimated from molecular lines is more significantly underestimated. Additionally, we compare the mass outflow rate and momentum outflow rate obtained from the synthetic spectra with the true values. Finally, we compare the synthetic spectra with real examples of ALMA-observed protostars and determine the best fitting protostellar masses and outflow inclination angles. We then calculate the mass outflow rate and momentum outflow rate for these sources, finding that both rates agree with theoretical protostellar evolutionary tracks.

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arXiv2023

Disk Wind Feedback from High-mass Protostars. IV. Shock-Ionized Jets

Massive protostars launch accretion-powered, magnetically-collimated outflows, which play crucial roles in the dynamics and diagnostics of the star formation process. Here we calculate the shock heating and resulting free-free radio emission in numerical models of outflows of massive star formation within the framework of the Turbulent Core Accretion model. We post-process 3D magneto-hydrodynamic simulation snapshots of a protostellar disk wind interacts with an infalling core envelope, and calculate shock temperatures, ionization fractions, and radio free-free emission. We find heating up to ~10^7 K and near complete ionization in shocks at the interface between the outflow cavity and infalling envelope. However, line-of-sight averaged ionization fractions peak around ~10%, in agreement with values reported from observations of massive protostar G35.20-0.74N. By calculating radio continuum fluxes and spectra, we compare our models with observed samples of massive protostars. We find our fiducial models produce radio luminosities similar to those seen from low and intermediate-mass protostars that are thought to be powered by shock ionization. Comparing to more massive protostars, we find our model radio luminosities are ~10 to 100 times less luminous. We discuss how this apparent discrepancy either reflects aspects of our modeling related to the treatment of cooling of the post-shock gas or a dominant contribution in the observed systems from photoionization. Finally, our models exhibit 10-year radio flux variability of ~5%, especially in the inner 1000 au region, comparable to observed levels in some hyper-compact HII regions.

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arXiv2023

Disk Wind Feedback from High-mass Protostars. II. The Evolutionary Sequence

Star formation is ubiquitously associated with the ejection of accretion-powered outflows that carve bipolar cavities through the infalling envelope. This feedback is expected to be important for regulating the efficiency of star formation from a natal pre-stellar core. These low-extinction outflow cavities greatly affect the appearance of a protostar by allowing the escape of shorter wavelength photons. Doppler-shifted CO line emission from outflows is also often the most prominent manifestation of deeply embedded early-stage star formation. Here, we present 3D magneto-hydrodynamic simulations of a disk wind outflow from a protostar forming from an initially $60\:M_\odot$ core embedded in a high pressure environment typical of massive star-forming regions. We simulate the growth of the protostar from $m_*=1\:M_\odot$ to $26\:M_\odot$ over a period of $\sim$100,000 years. The outflow quickly excavates a cavity with half opening angle of $\sim10^\circ$ through the core. This angle remains relatively constant until the star reaches $4\:M_\odot$. It then grows steadily in time, reaching a value of $\sim 50^\circ$ by the end of the simulation. We estimate a lower limit to the star formation efficiency (SFE) of 0.43. However, accounting for continued accretion from a massive disk and residual infall envelope, we estimate that the final SFE may be as high as $\sim0.7$. We examine observable properties of the outflow, especially the evolution of the cavity opening angle, total mass and momentum flux, and velocity distributions of the outflowing gas, and compare with the massive protostars G35.20-0.74N and G339.88-1.26 observed by ALMA, yielding constraints on their intrinsic properties.

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arXiv2023

The SOFIA Massive (SOMA) Star Formation Survey. IV. Isolated Protostars

We present $\sim10-40\,μ$m SOFIA-FORCAST images of 11 isolated protostars as part of the SOFIA Massive (SOMA) Star Formation Survey, with this morphological classification based on 37 $μ$m imaging. We develop an automated method to define source aperture size using the gradient of its background-subtracted enclosed flux and apply this to build spectral energy distributions (SEDs). We fit the SEDs with radiative transfer models, developed within the framework of turbulent core accretion (TCA) theory, to estimate key protostellar properties. Here, we release the sedcreator python package that carries out these methods. The SEDs are generally well fitted by the TCA models, from which we infer initial core masses $M_c$ ranging from $20-430\:M_\odot$, clump mass surface densities $Σ_{\rm cl}\sim0.3-1.7\:{\rm{g\:cm}}^{-2}$ and current protostellar masses $m_*\sim3-50\:M_\odot$. From a uniform analysis of the 40 sources in the full SOMA survey to date, we find that massive protostars form across a wide range of clump mass surface density environments, placing constraints on theories that predict a minimum threshold $Σ_{\rm cl}$ for massive star formation. However, the upper end of the $m_*-Σ_{\rm cl}$ distribution follows trends predicted by models of internal protostellar feedback that find greater star formation efficiency in higher $Σ_{\rm cl}$ conditions. We also investigate protostellar far-IR variability by comparison with IRAS data, finding no significant variation over an $\sim$40 year baseline.

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arXiv2020

The SOFIA Massive (SOMA) Star Formation Survey. III. From Intermediate- to High-Mass Protostars

We present $\sim10-40\,μ$m SOFIA-FORCAST images of 14 intermediate-mass protostar candidates as part of the SOFIA Massive (SOMA) Star Formation Survey. We build spectral energy distributions (SEDs), also utilizing archival Spitzer, Herschel and IRAS data. We then fit the SEDs with radiative transfer (RT) models of Zhang & Tan (2018), based on Turbulent Core Accretion theory, to estimate key protostellar properties. With the addition of these intermediate-mass sources, SOMA protostars span luminosities from $\sim10^{2}-10^{6}\:L_{\odot}$, current protostellar masses from $\sim0.5-30\:M_{\odot}$ and ambient clump mass surface densities, $Σ_{\rm cl}$ from $0.1-3\:{\rm{g\:cm}^{-2}}$. A wide range of evolutionary states of the individual protostars and of the protocluster environments are also probed. We have also considered about 50 protostars identified in Infrared Dark Clouds and expected to be at the earliest stages of their evolution. With this global sample, most of the evolutionary stages of high- and intermediate-mass protostars are probed. From the best fitting models, there is no evidence of a threshold value of protocluster clump mass surface density being needed to form protostars up to $\sim25\:M_\odot$. However, to form more massive protostars, there is tentative evidence that $Σ_{\rm{cl}}$ needs to be $\gtrsim1\:{\rm{g\,cm}}^{-2}$. We discuss how this is consistent with expectations from core accretion models that include internal feedback from the forming massive star.

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arXiv2019

Disk wind feedback from high-mass protostars

We perform a sequence of 3D magnetohydrodynamic (MHD) simulations of the outflow-core interaction for a massive protostar forming via collapse of an initial cloud core of $60~{M_\odot}$. This allows us to characterize the properties of disk wind driven outflows from massive protostars, which can allow testing of different massive star formation theories. It also enables us to assess quantitatively the impact of outflow feedback on protostellar core morphology and overall star formation efficiency. We find that the opening angle of the flow increases with increasing protostellar mass, in agreement with a simple semi-analytic model. Once the protostar reaches $\sim24~{M_\odot}$ the outflow's opening angle is so wide that it has blown away most of the envelope, thereby nearly ending its own accretion. We thus find an overall star formation efficiency of $\sim50\%$, similar to that expected from low-mass protostellar cores. Our simulation results therefore indicate that the MHD disk wind outflow is the dominant feedback mechanism for helping to shape the stellar initial mass function from a given prestellar core mass function.

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arXiv2019

The SOFIA Massive (SOMA) Star Formation Survey. II. High Luminosity Protostars

We present multi-wavelength images observed with SOFIA-FORCAST from $\sim$10 to 40 $μ$m of seven high luminosity massive protostars, as part of the SOFIA Massive (SOMA) Star Formation Survey. Source morphologies at these wavelengths appear to be influenced by outflow cavities and extinction from dense gas surrounding the protostars. Using these images, we build spectral energy distributions (SEDs) of the protostars, also including archival data from Spitzer, Herschel and other facilities. Radiative transfer (RT) models of Zhang & Tan (2018), based on Turbulent Core Accretion theory, are then fit to the SEDs to estimate key properties of the protostars. Considering the best five models fit to each source, the protostars have masses $m_{*} \sim 12-64 \: M_{\odot}$ accreting at rates of $\dot{m}_{*} \sim 10^{-4}-10^{-3} \: M_{\odot} \: \rm yr^{-1}$ inside cores of initial masses $M_{c} \sim 100-500 \: M_{\odot}$ embedded in clumps with mass surface densities $Σ_{\rm cl} \sim 0.1-3 \: \rm g \: cm^{-2}$ and span a luminosity range of $10^{4} -10^{6} \: L_{\odot}$. Compared with the first eight protostars in Paper I, the sources analyzed here are more luminous, and thus likely to be more massive protostars. They are often in a clustered environment or have a companion protostar relatively nearby. From the range of parameter space of the models, we do not see any evidence that $Σ_{\rm cl}$ needs to be high to form these massive stars. For most sources the RT models provide reasonable fits to the SEDs, though the cold clump material often influences the long wavelength fitting. However, for sources in very clustered environments, the model SEDs may not be such a good description of the data, indicating potential limitations of the models for these regions.

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arXiv2018

The role of dredge-up in double white dwarf mergers

We present the results of an investigation of the dredge-up and mixing during the merger of two white dwarfs with different chemical compositions by conducting hydrodynamic simulations of binary mergers for three representative mass ratios. In all the simulations, the total mass of the two white dwarfs is $\lesssim1.0~{\rm M_\odot}$. Mergers involving a CO and a He white dwarf have been suggested as a possible formation channel for R Coronae Borealis type stars, and we are interested in testing if such mergers lead to conditions and outcomes in agreement with observations. Even if the conditions during the merger and subsequent nucleosynthesis favor the production of $^{18}{\mathrm O}$, the merger must avoid dredging up large amounts of $^{16}{\mathrm O}$, or else it will be difficult to produce sufficient $^{18}{\mathrm O}$ to explain the oxygen ratio observed to be of order unity. We performed a total of 9 simulations using two different grid-based hydrodynamics codes using fixed and adaptive meshes, and one smooth particle hydrodynamics (SPH) code. We find that in most of the simulations, $>10^{-2}~{\rm M_\odot}$ of $^{16}{\mathrm O}$ is indeed dredged up during the merger. However, in SPH simulations where the accretor is a hybrid He/CO white dwarf with a $\sim 0.1~{\rm M_\odot}$ layer of helium on top, we find that no $^{16}{\mathrm O}$ is being dredged up, while in the $q=0.8$ simulation $<10^{-4}~{\rm M_\odot}$ of $^{16}{\mathrm O}$ has been brought up, making a WD binary consisting of a hybrid CO/He WD and a companion He WD an excellent candidate for the progenitor of RCB stars.

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arXiv2017

Outflow-Confined HII regions. II. The Early Break-Out Phase

In this series of papers, we model the formation and evolution of the photoionized region and its observational signatures during massive star formation. Here we focus on the early break out of the photoionized region into the outflow cavity. Using results of 3-D magnetohydrodynamic-outflow simulations and protostellar evolution calculations, we perform post-processing radiative-transfer. The photoionized region first appears at a protostellar mass of 10Msun in our fiducial model, and is confined to within 10-100AU by the dense inner outflow, similar to some observed very small hypercompact HII regions. Since the ionizing luminosity of the massive protostar increases dramatically as Kelvin-Helmholz (KH) contraction proceeds, the photoionized region breaks out to the entire outflow region in <10,000yr. Accordingly, the radio free-free emission brightens significantly in this stage. In our fiducial model, the radio luminosity at 10 GHz changes from 0.1 mJy kpc2 at m=11Msun to 100 mJy kpc2 at 16Msun, while the infrared luminosity increases by less than a factor of two. The radio spectral index also changes in the break-out phase from the optically thick value of 2 to the partially optically thin value of 0.6. Additionally, we demonstrate that short-timescale variation in free-free flux would be induced by an accretion burst. The outflow density is enhanced in the accretion burst phase, which leads to a smaller ionized region and weaker free-free emission. The radio luminosity may decrease by one order of magnitude during such bursts, while the infrared luminosity is much less affected, since internal protostellar luminosity dominates over accretion luminosity after KH contraction starts. Such variability may be observable on timescales as short 10-100 yr, if accretion bursts are driven by disk instabilities.

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arXiv2017

The SOFIA Massive (SOMA) Star Formation Survey. I. Overview and First Results

We present an overview and first results of the Stratospheric Observatory For Infrared Astronomy Massive (SOMA) Star Formation Survey, which is using the FORCAST instrument to image massive protostars from $\sim10$--$40\:\rmμ\rm{m}$. These wavelengths trace thermal emission from warm dust, which in Core Accretion models mainly emerges from the inner regions of protostellar outflow cavities. Dust in dense core envelopes also imprints characteristic extinction patterns at these wavelengths, causing intensity peaks to shift along the outflow axis and profiles to become more symmetric at longer wavelengths. We present observational results for the first eight protostars in the survey, i.e., multiwavelength images, including some ancillary ground-based MIR observations and archival {\it{Spitzer}} and {\it{Herschel}} data. These images generally show extended MIR/FIR emission along directions consistent with those of known outflows and with shorter wavelength peak flux positions displaced from the protostar along the blueshifted, near-facing sides, thus confirming qualitative predictions of Core Accretion models. We then compile spectral energy distributions and use these to derive protostellar properties by fitting theoretical radiative transfer models. Zhang and Tan models, based on the Turbulent Core Model of McKee and Tan, imply the sources have protostellar masses $m_*\sim10$--50$\:M_\odot$ accreting at $\sim10^{-4}$--$10^{-3}\:M_\odot\:{\rm{yr}}^{-1}$ inside cores of initial masses $M_c\sim30$--500$\:M_\odot$ embedded in clumps with mass surface densities $Σ_{\rm{cl}}\sim0.1$--3$\:{\rm{g\:cm}^{-2}}$. Fitting Robitaille et al. models typically leads to slightly higher protostellar masses, but with disk accretion rates $\sim100\times$ smaller. We discuss reasons for these differences and overall implications of these first survey results for massive star formation theories.

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arXiv2017

Common envelope light-curves - I. grid-code module calibration

The common envelope binary interaction occurs when a star transfers mass onto a companion that cannot fully accrete it. The interaction can lead to a merger of the two objects or to a close binary. The common envelope interaction is the gateway of all evolved compact binaries, all stellar mergers and likely many of the stellar transients witnessed to date. Common envelope simulations are needed to understand this interaction and to interpret stars and binaries thought to be the byproduct of this stage. At this time, simulations are unable to reproduce the few observational data available and several ideas have been put forward to address their shortcomings. The need for more definitive simulation validation is pressing, and is already being fulfilled by observations from time-domain surveys. In this article, we present an initial method and its implementation for post-processing grid-based common envelope simulations to produce the light-curve so as to compare simulations with upcoming observations. Here we implemented a zeroth order method to calculate the light emitted from common envelope hydrodynamic simulations carried out with the 3D hydrodynamic code Enzo used in uni-grid mode. The code implements an approach for the computation of luminosity in both optically thick and optically thin regimes and is tested using the first 135 days of the common envelope simulation of Passy et al. (2012), where a 0.8 solar masses red giant branch star interacts with a 0.6 solar masses companion. This code is used to highlight two large obstacles that need to be overcome before realistic light curves can be calculated. We explain the nature of these problems and the attempted solutions and approximations in full detail to enable the next step to be identified and implemented. We also discuss our simulation in relation to recent data of transients identified as common envelope interactions.

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arXiv2016

Hydrodynamic Simulations of the Interaction between Giant Stars and Planets

We present the results of hydrodynamic simulations of the interaction between a 10 Jupiter mass planet and a red or asymptotic giant branch stars, both with a zero-age main sequence mass of 3.5 $M_\odot$. Dynamic in-spiral timescales are of the order of few years and a few decades for the red and asymptotic giant branch stars, respectively. The planets will eventually be destroyed at a separation from the core of the giants smaller than the resolution of our simulations, either through evaporation or tidal disruption. As the planets in-spiral, the giant stars' envelopes are somewhat puffed up. Based on relatively long timescales and even considering the fact that further in-spiral should take place before the planets are destroyed, we predict that the merger would be difficult to observe, with only a relatively small, slow brightening. Very little mass is unbound in the process. These conclusions may change if the planet's orbit enhances the star's main pulsation modes. Based on the angular momentum transfer, we also suspect that this star-planet interaction may be unable to lead to large scale outflows via the rotation-mediated dynamo effect of Nordhaus and Blackman. Detectable pollution from the destroyed planets would only result for the lightest, lowest metallicity stars. We furthermore find that in both simulations the planets move through the outer stellar envelopes at Mach-3 to Mach-5, reaching Mach-1 towards the end of the simulations. The gravitational drag force decreases and the in-spiral slows down at the sonic transition, as predicted analytically.

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arXiv2016

Quark-Noave in binaries: Observational signatures and implications to astrophysics

The explosive transition of a massive neutron star to a quark star (the Quark-Nova, QN) releases in excess of ~ 10^52 erg in kinetic energy which can drastically impact the surrounding environment of the QN. A QN is triggered when a neutron star gains enough mass to reach the critical value for quark deconfinement to happen in the core. In binaries, a neutron star has access to mass reservoirs (e.g. accretion from a companion or from a Common Envelope, CE). We explain observed light-curves of hydrogen-poor superluminous Supernovae (SLSNe Ia) in the context of a QN occurring in the second CE phase of a massive binary. In particular this model gives good fits to light-curves of SLSNe with double-humped light-curves. Our model suggests the QN as a mechanism for CE ejection and that they be taken into account during binary evolution. In a short period binary with a white dwarf companion, the neutron star can quickly grow in mass and experience a QN event. Part of the QN ejecta collides with the white dwarf, shocking, compressing, and heating it to driving a thermonuclear runaway producing a SN Ia impostor (a QN-Ia). Unlike "normal" Type Ia supernovae where no compact remnant is formed, a QN-Ia produces a quark star undergoing rapid spin-down providing additional power along with the 56Ni decay energy. Type Ia SNe are used as standard candles and contamination of this data by QNe-Ia can infer an incorrect cosmology.

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arXiv2015

Hydrodynamic Simulations of the Interaction between an AGB Star and a Main Sequence Companion in Eccentric Orbits

The Rotten Egg Nebula has at its core a binary composed of a Mira star and an A-type companion at a separation >10 au. It has been hypothesized to have formed by strong binary interactions between the Mira and a companion in an eccentric orbit during periastron passage ~800 years ago. We have performed hydrodynamic simulations of an asymptotic giant branch star interacting with companions with a range of masses in orbits with a range of initial eccentricities and periastron separations. For reasonable values of the eccentricity, we find that Roche lobe overflow can take place only if the periods are <<100 years. Moreover, mass transfer causes the system to enter a common envelope phase within several orbits. Since the central star of the Rotten Egg nebula is an AGB star, we conclude that such a common envelope phase must have lead to a merger, so the observed companion must have been a tertiary companion of a binary that merged at the time of nebula ejection. Based on the mass and timescale of the simulated disc formed around the companion before the common envelope phase, we analytically estimate the properties of jets that could be launched. Allowing for super-Eddington accretion rates, we find that jets similar to those observed are plausible, provided that the putative lost companion was relatively massive.

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arXiv2015

The puzzling early detection of low velocity 56Ni decay lines in SN 2014J: Hints of a compact remnant

We show that the low-velocity 56Ni decay lines detected earlier than expected in the type Ia SN 2014J find an explanation in the Quark-Nova Ia model which involves the thermonuclear explosion of a tidally disrupted sub-Chandrasekhar White Dwarf in a tight Neutron-Star-White-Dwarf binary system. The explosion is triggered by impact from the Quark-Nova ejecta on the WD material; the Quark-Nova is the explosive transition of the Neutron star to a Quark star triggered by accretion from a CO torus (the circularized WD material). The presence of a compact remnant (the Quark Star) provides: (i) an additional energy source (spin-down power) which allows us to fit the observed light-curve including the steep early rise; (ii) a central gravitational potential which slows down some of the 56Ni produced to velocities of a few 1000 km/s. In our model, the 56Ni decay lines become optically visible at ~20 days from explosion time in agreement with observations. We list predictions that can provide important tests for our model.

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arXiv2014

Specific predictions for SN 2014J in the context of the Quark Nova Ia model

The exact mechanism behind Type Ia supernovae (SNe-Ia) and the nature of the progenitors is poorly understood, although several theories vie for supremacy. Their secured importance to the study of cosmology necessitates a resolution to this problem. Observations of nearby SNe-Ia are therefore critical to determine which theory, if any, is the correct one. SN 2014J discovered at a relatively close 3.5 Mpc in the galaxy M82 provides such an opportunity. In this paper we give specific predictions for SN 2014J in the context of the Quark Nova Ia (QN-Ia) model. Predictions include X-ray luminosities just prior and hundreds of days after the explosion, light curve "glitches", neutrino emission, heavy element creation, and gravitational signatures.

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arXiv2013

Quark-Novae Ia in the Hubble diagram: Implications For Dark Energy

The accelerated expansion of the Universe was proposed through the use of Type-Ia SNe as standard candles. The standardization depends on an empirical correlation between the stretch/color and peak luminosity of the light curves. The use of Type Ia SN as standard candles rests on the assumption that their properties (and this correlation) do not vary with red-shift. We consider the possibility that the majority of Type-Ia SNe are in fact caused by a Quark-Nova detonation in a tight neutron-star-CO-white-dwarf binary system; a Quark-Nova Ia. The spin-down energy injected by the Quark Nova remnant (the quark star) contributes to the post-peak light curve and neatly explains the observed correlation between peak luminosity and light curve shape. We demonstrate that the parameters describing Quark-Novae Ia are NOT constant in red-shift. Simulated Quark-Nova Ia light curves provide a test of the stretch/color correlation by comparing the true distance modulus with that determined using SN light curve fitters. We determine a correction between the true and fitted distance moduli which when applied to Type-Ia SNe in the Hubble diagram recovers the Omega_M = 1 cosmology. We conclude that Type-Ia SNe observations do not necessitate the need for an accelerating expansion of the Universe (if the observed SNe-Ia are dominated by QNe-Ia) and by association the need for Dark Energy.

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