arXiv subjects

Sean Matt

Publications and source records attributed to Sean Matt.

At least 19 recordsLinked to original sources

arXiv2023

Further evidence of the link between activity and metallicity using the flaring properties of stars in the Kepler field

The magnetic activity level of low-mass stars is known to vary as a function of the physical properties of the star. Many studies have shown that the stellar mass and rotation are both important parameters that determine magnetic activity levels. In contrast, the impact of a star's chemical composition on magnetic activity has received comparatively little attention. Data sets for traditional activity proxies, e.g. X-ray emission or calcium emission, are not large enough to search for metallicity trends in a statistically meaningful way. Recently, studies have used the photometric variability amplitude as a proxy for magnetic activity to investigate the role of metallicity because it can be relatively easily measured for large samples of stars. These studies find that magnetic activity and metallicity are positively correlated. In this work, we investigate the link between activity and metallicity further by studying the flaring properties of stars in the Kepler field. Similar to the photometric variability, we find that flaring activity is stronger in more metal-rich stars for a fixed mass and rotation period. This result adds to a growing body of evidence that magnetic field generation is correlated with metallicity.

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arXiv2015

A spectro-polarimetric study of the planet-hosting G dwarf, HD 147513

The results from a spectro-polarimetric study of the planet-hosting Sun-like star, HD 147513 (G5V), are presented here. Robust detections of Zeeman signatures at all observed epochs indicate a surface magnetic field, with longitudinal magnetic field strengths varying between 1.0-3.2 G. Radial velocity variations from night to night modulate on a similar timescale to the longitudinal magnetic field measurements. These variations are therefore likely due to the rotational modulation of stellar active regions rather than the much longer timescale of the planetary orbit (Porb=528 d). Both the longitudinal magnetic field measurements and radial velocity variations are consistent with a rotation period of 10 +/- 2 days, which are also consistent with the measured chromospheric activity level of the star (log R'(HK)=-4.64). Together, these quantities indicate a low inclination angle, i~18 degrees. We present preliminary magnetic field maps of the star based on the above period and find a simple poloidal large-scale field. Chemical analyses of the star have revealed that it is likely to have undergone a barium-enrichment phase in its evolution because of a higher mass companion. Despite this, our study reveals that the star has a fairly typical activity level for its rotation period and spectral type. Future studies will enable us to explore the long-term evolution of the field, as well as to measure the stellar rotation period, with greater accuracy.

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arXiv2014

The influence of the magnetic topology on the braking of sun-like stars

Stellar winds are thought to be the main process responsible for the spin down of main-sequence stars. The extraction of angular momentum by a magnetized wind has been studied for decades, leading to several formulations for the resulting torque. However, previous studies generally consider simple dipole or split monopole stellar magnetic topologies. Here we consider in addition to a dipolar stellar magnetic field, both quadrupolar and octupolar configurations, while also varying the rotation rate and the magnetic field strength. 60 simulations made with a 2.5D, cylindrical and axisymmetric set-up and computed with the PLUTO code were used to find torque formulations for each topology. We further succeed to give a unique law that fits the data for every topology by formulating the torque in terms of the amount of open magnetic flux in the wind. We also show that our formulation can be applied to even more realistic magnetic topologies, with examples of the Sun in its minimum and maximum phase as observed at the Wilcox Solar Observatory, and of a young K-star (TYC-0486-4943-1) whose topology has been obtained by Zeeman-Doppler Imaging (ZDI). Ideas about how to compute the open flux from ZDI Maps are discussed.

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arXiv2014

The effect of magnetic topology on thermally-driven winds: towards a general formulation of the braking law

Stellar winds are thought to be the main process responsible for the spin down of main-sequence stars. The extraction of angular momentum by a magnetized wind has been studied for decades, leading to several formulations for the resulting torque. However, previous studies generally consider simple dipole or split monopole stellar magnetic topologies. Here we consider in addition to a dipolar stellar magnetic field, both quadrupolar and octupolar configurations, while also varying the rotation rate and the magnetic field strength. 60 simulations made with a 2.5D, cylindrical and axisymmetric set-up and computed with the PLUTO code were used to find torque formulations for each topology. We further succeed to give a unique law that fits the data for every topology by formulating the torque in terms of the amount of open magnetic flux in the wind. We also show that our formulation can be applied to even more realistic magnetic topologies, with examples of the Sun in its minimum and maximum phase as observed at the Wilcox Solar Observatory, and of a young K-star (TYC-0486- 4943-1) whose topology has been obtained by Zeeman-Doppler Imaging (ZDI).

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arXiv2014

Coronal Mass Ejections and Angular Momentum Loss in Young Stars

In our own solar system, the necessity of understanding space weather is readily evident. Fortunately for Earth, our nearest stellar neighbor is relatively quiet, exhibiting activity levels several orders of magnitude lower than young, solar-type stars. In protoplanetary systems, stellar magnetic phenomena observed are analogous to the solar case, but dramatically enhanced on all physical scales: bigger, more energetic, more frequent. While coronal mass ejections (CMEs) could play a significant role in the evolution of protoplanets, they could also affect the evolution of the central star itself. To assess the consequences of prominence eruption/CMEs, we have invoked the solar-stellar connection to estimate, for young, solar-type stars, how frequently stellar CMEs may occur and their attendant mass and angular momentum loss rates. We will demonstrate the necessary conditions under which CMEs could slow stellar rotation.

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arXiv2012

Mass loss in pre-main sequence stars via coronal mass ejections and implications for angular momentum loss

We develop an empirical model to estimate mass-loss rates via coronal mass ejections (CMEs) for solar-type pre-main-sequence (PMS) stars. Our method estimates the CME mass-loss rate from the observed energies of PMS X-ray flares, using our empirically determined relationship between solar X-ray flare energy and CME mass: log(M_CME [g]) = 0.63 x log(E_flare [erg]) - 2.57. Using masses determined for the largest flaring magnetic structures observed on PMS stars, we suggest that this solar-calibrated relationship may hold over 10 orders of magnitude in flare energy and 7 orders of magnitude in CME mass. The total CME mass-loss rate we calculate for typical solar-type PMS stars is in the range 1e-12 to 1e-9 M_sun/yr. We then use these CME mass-loss rate estimates to infer the attendant angular momentum loss leading up to the main sequence. Assuming the CME outflow rate for a typical ~1 M_sun T Tauri star is < 1e-10 M_sun/yr, the resulting spin-down torque is too small during the first ~1 Myr to counteract the stellar spin-up due to contraction and accretion. However, if the CME mass-loss rate is >1e-10 M_sun/yr, as permitted by our calculations, the CME spin-down torque may influence the stellar spin evolution after an age of a few Myr.

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arXiv2008

Accretion-Powered Stellar Winds III: Spin Equilibrium Solutions

We compare the stellar wind torque calculated in a previous work (Paper II) to the spin-up and spin-down torques expected to arise from the magnetic interaction between a slowly rotating ($\sim 10$% of breakup) pre-main-sequence star and its accretion disk. This analysis demonstrates that stellar winds can carry off orders of magnitude more angular momentum than can be transferred to the disk, provided that the mass outflow rates are greater than the solar wind. Thus, the equilibrium spin state is simply characterized by a balance between the angular momentum deposited by accretion and that extracted by a stellar wind. We derive a semi-analytic formula for predicting the equilibrium spin rate as a function only of the ratio of $\dot M_{\rm w} / \dot M_{\rm a}$ and a dimensionless magnetization parameter, $Ψ\equiv B_*^2 R_*^2 (\dot M_{\rm a} v_{\rm esc})^{-1}$, where $\dot M_{\rm w}$ is the stellar wind mass outflow rate, $\dot M_{\rm a}$ the accretion rate, $B_*$ the stellar surface magnetic field strength, $R_*$ the stellar radius, and $v_{\rm esc}$ the surface escape speed. For parameters typical of accreting pre-main-sequence stars, this explains spin rates of $\sim 10$% of breakup speed for $\dot M_{\rm w} / \dot M_{\rm a} \sim 0.1$. Finally, the assumption that the stellar wind is driven by a fraction of the accretion power leads to an upper limit to the mass flow ratio of $\dot M_{\rm w} / \dot M_{\rm a} \la 0.6$.

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arXiv2008

Accretion-Powered Stellar Winds II: Numerical Solutions for Stellar Wind Torques

[Abridged] In order to explain the slow rotation observed in a large fraction of accreting pre-main-sequence stars (CTTSs), we explore the role of stellar winds in torquing down the stars. For this mechanism to be effective, the stellar winds need to have relatively high outflow rates, and thus would likely be powered by the accretion process itself. Here, we use numerical magnetohydrodynamical simulations to compute detailed 2-dimensional (axisymmetric) stellar wind solutions, in order to determine the spin down torque on the star. We explore a range of parameters relevant for CTTSs, including variations in the stellar mass, radius, spin rate, surface magnetic field strength, the mass loss rate, and wind acceleration rate. We also consider both dipole and quadrupole magnetic field geometries. Our simulations indicate that the stellar wind torque is of sufficient magnitude to be important for spinning down a ``typical'' CTTS, for a mass loss rate of $\sim 10^{-9} M_\odot$ yr$^{-1}$. The winds are wide-angle, self-collimated flows, as expected of magnetic rotator winds with moderately fast rotation. The cases with quadrupolar field produce a much weaker torque than for a dipole with the same surface field strength, demonstrating that magnetic geometry plays a fundamental role in determining the torque. Cases with varying wind acceleration rate show much smaller variations in the torque suggesting that the details of the wind driving are less important. We use our computed results to fit a semi-analytic formula for the effective Alfvén radius in the wind, as well as the torque. This allows for considerable predictive power, and is an improvement over existing approximations.

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arXiv2007

The Nature of Stellar Winds in the Star-Disk Interaction

Stellar winds may be important for angular momentum transport from accreting T Tauri stars, but the nature of these winds is still not well-constrained. We present some simulation results for hypothetical, hot (~1e6 K) coronal winds from T Tauri stars, and we calculate the expected emission properties. For the high mass loss rates required to solve the angular momentum problem, we find that the radiative losses will be much greater than can be powered by the accretion process. We place an upper limit to the mass loss rate from accretion-powered coronal winds of ~1e-11 solar masses per year. We conclude that accretion powered stellar winds are still a promising scenario for solving the stellar angular momentum problem, but the winds must be cool (~1e4 K) and thus are not driven by thermal pressure.

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arXiv2007

Understanding the Spins of Young Stars

We review the theoretical efforts to understand why pre-main-sequence stars spin much more slowly than expected. The first idea put forward was that massive stellar winds may remove substantial angular momentum. Since then, it has become clear that the magnetic interaction between the stars and their accretion disks explains many of the observed emission properties. The disk locking scenario, which assumes the magnetic star-disk interaction also solves the stellar spin problem, has received the most attention in the literature to date. However, recent considerations suggest that the torques in the star-disk interaction are insufficient for disk locking to explain the slow rotators. This prompts us to revisit stellar winds, and we conclude that stellar winds, working in conjunction with magnetospheric accretion, are a promising candidate for solving the angular momentum problem. We suggest future directions for both observations and theory, to help shed light on this issue.

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arXiv2006

Astrophysical Explosions Driven by a Rotating, Magnetized, Gravitating Sphere

We present the results of a numerical magnetohydrodynamic simulation that demonstrates a mechanism by which magnetic fields tap rotational energy of a stellar core and expel the envelope. Our numerical setup, designed to focus on the basic physics of the outflow mechanism, consists of a solid, gravitating sphere, which may represent the compact core of a star, surrounded by an initially hydrostatic envelope of ionized gas. The core is threaded by a dipolar magnetic field that also permeates the envelope. At the start of the simulation, the core begins to rotate at 10% of the escape speed. The magnetic field is sufficiently strong to drive a magneto-rotational explosion, whereby the entire envelope is expelled, confirming the expectation of analytical models. Furthermore, the dipolar nature of the field results in an explosion that is enhanced simultaneously along the rotation axis (a jet) and along the magnetic equator. While the initial condition is simplified, the simulation approximates circumstances that may arise in astrophysical objects such as Type II supernovae, gamma ray bursts, and proto-planetary nebulae.

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arXiv2005

Accretion-powered Stellar Winds as a Solution to the Stellar Angular Momentum Problem

We compare the angular momentum extracted by a wind from a pre-main-sequence star to the torques arising from the interaction between the star and its Keplerian accretion disk. We find that the wind alone can counteract the spin-up torque from mass accretion, solving the mystery of why accreting pre-main-sequence stars are observed to spin at less than 10% of break-up speed, provided that the mass outflow rate in the stellar winds is ~10% of the accretion rate. We suggest that such massive winds will be driven by some fraction $\epsilon$ of the accretion power. For observationally constrained typical parameters of classical T-Tauri stars, $\epsilon$ needs to be between a few and a few tens of percent. In this scenario, efficient braking of the star will terminate simultaneously with accretion, as is usually assumed to explain the rotation velocities of stars in young clusters.

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arXiv2004

The spin of accreting stars: dependence on magnetic coupling to the disc

We formulate a general, steady-state model for the torque on a magnetized star from a surrounding accretion disc. For the first time, we include the opening of dipolar magnetic field lines due to the differential rotation between the star and disc, so the magnetic topology then depends on the strength of the magnetic coupling to the disc. This coupling is determined by the effective slip rate of magnetic field lines that penetrate the diffusive disc. Stronger coupling (i.e., lower slip rate) leads to a more open topology and thus to a weaker magnetic torque on the star from the disc. In the expected strong coupling regime, we find that the spin-down torque on the star is more than an order of magnitude smaller than calculated by previous models. We also use our general approach to examine the equilibrium (`disc-locked') state, in which the net torque on the star is zero. In this state, we show that the stellar spin rate is roughly an order of magnitude faster than predicted by previous models. This challenges the idea that slowly-rotating, accreting protostars are disc locked. Furthermore, when the field is sufficiently open (e.g., for mass accretion rates > 5 x 10^{-9} M_sun / yr, for typical accreting protostars), the star will receive no magnetic spin-down torque from the disc at all. We therefore conclude that protostars must experience a spin-down torque from a source that has not yet been considered in the star-disc torque models--possibly from a stellar wind along the open field lines.

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arXiv2004

Simultaneous Production of Disk and Lobes: A Single-Wind MHD Model for the Eta Carinae Nebula

The luminous blue variable Eta Carinae is surrounded by a complex and highly structured nebula of ejected material. The best-studied and axisymmetric components of this outflow consist of bipolar lobes (the ``homunculus'') and an equatorial ``skirt.'' Recent proper motion measurements suggest that the skirt was ejected at the same time as the lobes, contrary to the assumptions of all current theoretical models for the formation of the nebula (which use the skirt to collimate stellar winds into lobes). We present a magnetohydrodynamic (MHD) stellar wind model that produces an outflowing disk and bipolar lobes in a single, steady-state wind. The basic model consists of a wind from a rotating star with a rotation-axis-aligned dipole magnetic field. The azimuthal component of the magnetic field, generated by stellar rotation, compresses the wind toward the equator and also toward the rotation axis, simultaneously producing an outflowing disk and jet. We use numerical MHD simulations to study the wind for various amounts of stellar rotation and to show a range of wind morphologies. In order to produce wide angle lobes similar to the homunculus (which have roughly a 30 degree opening angle), a high-speed polar wind (with enhanced energy density) from the star is also required. In that case, the structure of the wind bears a remarkable resemblance to the skirt plus homunculus morphology of the Eta Car nebulae, and a significant fraction of the stellar angular momentum is carried away by the wind. Although the model assumes a steady-state wind (rather than an eruption) and thermal wind driving (rather than radiation pressure), the structure of the wind is encouraging.

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arXiv2004

Does Disk Locking Solve the Stellar Angular Momentum Problem?

We critically examine the theory of disk locking, which assumes that the angular momentum deposited on an accreting protostar is exactly removed by torques carried along magnetic field lines connecting the star to the disk. In this letter, we consider that the differential rotation between the star and disk naturally leads to an opening (i.e., disconnecting) of the magnetic field between the two. We find that this significantly reduces the spin-down torque on the star by the disk. Thus, disk-locking cannot account for the slow rotation ($\sim$10% of breakup speed) observed in several systems and for which the model was originally developed.

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arXiv2004

The Spin History of Protostars: Disk Locking, Revisited

In this talk, we take a new look at the theory of disk locking, which assumes that an accreting protostar rids itself of accreted angular momentum through a magnetic coupling with the accretion disk. We consider that differential rotation between the star and disk twists the field lines. For large enough twist, the magnetic field lines connecting the star and disk open and disconnect. This significantly reduces the spin-down torque on the star by the disk, and so we find that disk-locking theory predicts spin periods that are much too short to account for typical observed systems.

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arXiv2003

The Last Hurrah: PPN Formation by a Magnetic Explosion

We discuss a mechanism by which a giant star can expel its envelope in an outburst, leaving its core exposed. The outburst is powered by rotational kinetic energy of the core, transferred to the envelope via the twisting of magnetic fields. We show that, if the core is magnetized, and if it has sufficient angular momentum, this mechanism may be triggered at the end of the asymptotic giant branch phase, and drive a proto-planetary nebula (pPN) outflow. This explosion of magnetic energy self-consistently explains some of the asymmetries and dynamics of pPNe.

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arXiv2003

Collimation of a Central Wind by a Disc-Associated Magnetic Field

Studies of jets from young stellar objects (YSO's) suggest that material is launched from a small central region at wide opening angles and collimated by an interaction with the surrounding environment. Using time-dependent, numerical magnetohydrodynamic simulations, we follow the detailed launching of a central wind via the coupling of a stellar dipole field to the inner edge of an accretion disc. Our method employs a series of nested computational grids, which allows the simulations to follow the central wind out to scales of tens of AU, where it may interact with its surroundings. The coupling between the stellar magnetosphere and disc inner edge has been known to produce an outflow containing both a highly collimated jet plus a wide-angle flow. The jet and wide-angle wind flow at roughly the same speed (100--200 km/s), and most of the energy and mass is carried off at relatively wide angles. We show that the addition of a weak disc-associated field (<< 0.1 Gauss) has little effect on the wind launching, but it collimates the entire flow (jet + wide wind) at a distance of several AU. The collimation is inevitable, regardless of the relative polarity of the disc field and stellar dipole, and the result is a more powerful and physically broader collimated flow than from the star-disc interaction alone. Within the collimation region, the morphology of the large-scale flow resembles a pitchfork, in projection. We compare these results with observations of outflows from YSO's and discuss the possible origin of the disc-associated field.

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