arXiv subjects

Steven W. Stahler

Publications and source records attributed to Steven W. Stahler.

At least 19 recordsLinked to original sources

arXiv2024

Outward Migration in Nascent Stellar Groups

As a stellar group forms within its parent molecular cloud, new members first appear in the deep interior. These overcrowded stars continually diffuse outward to the cloud boundary, and even beyond. Observations have so far documented only the interior drift. Those stars that actually leave the cloud form an expanding envelope that I call the "stellar mantle." Simple fluid models for the cloud and mantle illustrate their basic structure. The mantle's expansion speed is subsonic with respect to the cloud's dynamical temperature. I describe, in qualitative terms, how the expanding mantle and Galactic tidal radius might together shape the evolution of specific types of stellar groups. The massive stars in OB associations form in clouds that contract before extruding a substantial mantle. In contrast, the more slowly evolving clouds forming open clusters and T associations have extended mantles that encounter a shrinking tidal radius. These clouds are dispersed by internal stellar outflows. If the remaining group of stars is gravitationally bound, it appears as a long-lived open cluster, truncated by the tidal radius. If the group is unbound, it is a late-stage T association that will soon be torn apart by the tidal force. The "distributed" populations of pre-main~sequence stars observed in the outskirts of several star-forming regions are too distant to be stellar mantles. Rather, they could be the remnants of especially low-mass T associations.

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arXiv2021

Neutral vs Ion Linewidths in Barnard 5: Evidence for Penetration by MHD Waves

Dense cores are the final place where turbulence is dissipated. It has been proposed from theoretical arguments that the non-thermal velocity dispersion should be narrower both for molecular ions (compared to neutrals) and for transitions with higher critical densities. To test these hypotheses, we compare the velocity dispersion of N$_2$H$^+$ (1--0) (n$_{\rm crit}$ = $6\times10^4$ cm$^{-3}) and NH$_3$ (n$_{\rm crit}=2\times10^3$ cm$^{-3}), in the dense core Barnard 5. We analyse well resolved and high signal-to-noise observations of NH$_3$ (1,1) and (2,2) obtained with combining GBT and VLA data, and N$_2$H$^+$ (1--0) obtained with GBT Argus, which present a similar morphology. % Surprisingly, the non-thermal velocity dispersion of the ion is systematically higher than that of the neutral by 20\%. The derived sonic Mach number, $\mathcal{M}_s = σ_{\rm NT}/c_s$, has peak values $\mathcal{M}_{s, {\rm N_2H^+}} = 0.59$ and $\mathcal{M}_{s, {\rm NH}_3} = 0.48$ for N$_2$H$^+$ and NH$_3$, respectively. % This observed difference may indicate that the magnetic field even deep within the dense core is still oscillating, as it is in the turbulent region outside the core. The ions should be more strongly dynamically coupled to this oscillating field than the neutrals, thus accounting for their broader linewidth. If corroborated by further observations, this finding would shed additional light on the transition to quiescence in dense cores.

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arXiv2019

The Nature of Class I Sources: Periodic Variables in Orion

We present a quantitative, empirically based argument that at least some Class I sources are low-mass, pre-main-sequence stars surrounded by spatially extended envelopes of dusty gas. The source luminosity arises principally from stellar gravitational contraction, as in optically visible pre-main-sequence stars that lack such envelopes. We base our argument on the fact that some Class I sources in Orion and other star-forming regions have been observed by Spitzer to be periodic variables in the mid-infrared, and with periods consistent with T Tauri rotation rates. Using a radiative transfer code, we construct a variety of dust envelopes surrounding rotating, spotted stars, to see if an envelope that produces a Class I SED at least broadly matches the observed modulations in luminosity. Acceptable envelopes can either be spherical or flattened, and may or may not have polar cavities. The key requirement is that they have a modest equatorial optical depth at the Spitzer waveband of 3.6 $μ$m, typically ${τ_{3.6}}$ ${\approx}$ 0.6. The total envelope mass, based on this limited study, is at most about 0.1 $\text{M}_{\odot}$, less than a typical stellar mass. Future studies should focus on the dynamics of the envelope, to determine whether material is actually falling onto the circumstellar disk.

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arXiv2017

Embedded Binaries and Their Dense Cores

We explore the relationship between young, embedded binaries and their parent cores, using observations within the Perseus Molecular Cloud. We combine recently published VLA observations of young stars with core properties obtained from SCUBA-2 observations at 850 um. Most embedded binary systems are found toward the centres of their parent cores, although several systems have components closer to the core edge. Wide binaries, defined as those systems with physical separations greater than 500 au, show a tendency to be aligned with the long axes of their parent cores, whereas tight binaries show no preferred orientation. We test a number of simple, evolutionary models to account for the observed populations of Class 0 and I sources, both single and binary. In the model that best explains the observations, all stars form initially as wide binaries. These binaries either break up into separate stars or else shrink into tighter orbits. Under the assumption that both stars remain embedded following binary breakup, we find a total star formation rate of 168 Myr^-1. Alternatively, one star may be ejected from the dense core due to binary breakup. This latter assumption results in a star formation rate of 247 Myr^-1. Both production rates are in satisfactory agreement with current estimates from other studies of Perseus. Future observations should be able to distinguish between these two possibilities. If our model continues to provide a good fit to other star-forming regions, then the mass fraction of dense cores that becomes stars is double what is currently believed.

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arXiv2014

Two paths of cluster evolution: global expansion versus core collapse

All gravitationally bound clusters expand, due to both gas loss from their most massive members and binary heating. All are eventually disrupted tidally, either by passing molecular clouds or the gravitational potential of their host galaxies. However, their interior evolution can follow two very different paths. Only clusters of sufficiently large initial population and size undergo the combined interior contraction and exterior expansion that leads eventually to core collapse. In all other systems, core collapse is frustrated by binary heating. These clusters globally expand for their entire lives, up to the point of tidal disruption. Using a suite of direct N-body calculations, we trace the "collapse line" in r_v-N space that separates these two paths. Here, r_v and N are the cluster's initial virial radius and population, respectively. For realistic starting radii, the dividing N-value is from 10^4 to over 10^5. We also show that there exists a minimum population, N_min, for core collapse. Clusters with N < N_min tidally disrupt before core collapse occurs. At the Sun's Galactocentric radius, R_G = 8.5 kpc, we find N_min >~ 300. The minimum population scales with Galactocentric radius as R_G^{-9/8}. The position of an observed cluster relative to the collapse line can be used to predict its future evolution. Using a small sample of open clusters, we find that most lie below the collapse line, and thus will never undergo core collapse. Most globular clusters, on the other hand, lie well above the line. In such a case, the cluster may or may not go through core collapse, depending on its initial size. We show how an accurate age determination can help settle this issue.

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arXiv2013

The Dynamics of Ultracompact HII Regions

Many ultracompact HII regions exhibit a cometary morphology in radio continuum emission. In such regions, a young massive star is probably ablating, through its ultraviolet radiation, the molecular cloud clump that spawned it. On one side of the star, the radiation drives an ionization front that stalls in dense molecular gas. On the other side, ionized gas streams outward into the more rarefied environment. This wind is underpressured with respect to the neutral gas. The difference in pressure draws in more cloud material, feeding the wind until the densest molecular gas is dissipated. Recent, time-dependent simulations of massive stars turning on within molecular gas show the system evolving in a direction similar to that just described. Here, we explore a semi-analytic model in which the wind is axisymmetric and has already achieved a steady state. Adoption of this simplified picture allows us to study the dependence of both the wind and its bounding ionization front on the stellar luminosity, the peak molecular density, and the displacement of the star from the center of the clump. For typical parameter values, the wind accelerates transonically to a speed of about 15 km / s, and transports mass outward at a rate of 10^-4 solar masses per year. Stellar radiation pressure acts to steepen the density gradient of the wind.

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arXiv2013

Dynamical Friction in a Gas: The Supersonic Case

Any gravitating mass traversing a relatively sparse gas experiences a retarding force created by its disturbance of the surrounding medium. In a previous contribution (Lee & Stahler 2011), we determined this dynamical friction force when the object's velocity was subsonic. We now extend our analysis to the supersonic regime. As before, we consider small perturbations created in the gas far from the gravitating object, and thereby obtain the net influx of linear momentum over a large, bounding surface. Various terms in the perturbation series formally diverge, necessitating an approximate treatment of the flow streamlines. Nevertheless, we are able to derive exactly the force itself. As in the subsonic case, we find that F=Mdot*V, where Mdot is the rate of mass accretion onto the object and V its instantaneous velocity with respect to distant background gas. Our force law holds even when the object is porous (e.g., a galaxy) or is actually expelling mass in a wind. Quantitatively, the force in the supersonic regime is less than that derived analytically by previous researchers, and is also less than was found in numerical simulations through the mid 1990s. We urge simulators to revisit the problem using modern numerical techniques. Assuming our result to be correct, it is applicable to many fields of astrophysics, ranging from exoplanet studies to galactic dynamics.

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arXiv2013

Externally Fed Star Formation: A Numerical Study

We investigate, through a series of numerical calculations, the evolution of dense cores that are accreting external gas up to and beyond the point of star formation. Our model clouds are spherical, unmagnetized configurations with fixed outer boundaries, across which gas enters subsonically. When we start with any near-equilibrium state, we find that the cloud's internal velocity also remains subsonic for an extended period, in agreement with observations. However, the velocity becomes supersonic shortly before the star forms. Consequently, the accretion rate building up the protostar is much greater than the benchmark value c_s^3/G, where c_s is the sound speed in the dense core. This accretion spike would generate a higher luminosity than those seen in even the most embedded young stars. Moreover, we find that the region of supersonic infall surrounding the protostar races out to engulf much of the cloud, again in violation of the observations, which show infall to be spatially confined. Similar problematic results have been obtained by all other hydrodynamic simulations to date, regardless of the specific infall geometry or boundary conditions adopted. Low-mass star formation is evidently a quasi-static process, in which cloud gas moves inward subsonically until the birth of the star itself. We speculate that magnetic tension in the cloud's deep interior helps restrain the infall prior to this event.

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arXiv2012

Externally Fed Accretion onto Protostars

The asymmetric molecular emission lines from dense cores reveal slow, inward motion in the clouds' outer regions. This motion is present both before and after the formation of a central star. Motivated by these observations, we revisit the classic problem of steady, spherical accretion of gas onto a gravitating point mass, but now include self-gravity of the gas and impose a finite, subsonic velocity as the outer boundary condition. We find that the accretion rate onto the protostar is lower than values obtained for isolated, collapsing clouds, by a factor that is the Mach number of the outer flow. Moreover, the region of infall surrounding the protostar spreads out more slowly, at a speed close to the subsonic, incoming velocity. Our calculation, while highly idealized, provides insight into two longstanding problems -- the surprisingly low accretion luminosities of even the most deeply embedded stellar sources, and the failure so far to detect spatially extended, supersonic infall within their parent dense cores. Indeed, the observed subsonic contraction in the outer regions of dense cores following star formation appears to rule out a purely hydrodynamic origin for these clouds.

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arXiv2011

Dynamical Friction in a Gas: The Subsonic Case

We study the force of dynamical friction acting on a gravitating point mass that travels through an extended, isothermal gas. This force is well established in the hypersonic limit, but remains less understood in the subsonic regime. Using perturbation theory, we analyze the changes in gas velocity and density far from the mass. We show analytically that the steady-state friction force is Mdot*V, where Mdot is the mass accretion rate onto an object moving at speed V. It follows that the speed of an object experiencing no other forces declines as the inverse square of its mass. Using a modified version of the classic Bondi-Hoyle interpolation formula for Mdot as a function of V, we derive an analytic expression for the friction force. This expression also holds when mass accretion is thwarted, e.g. by a wind, as long as the wind-cloud interaction is sufficiently confined spatially. Our result should find application in a number of astrophysical settings, such as the motion of galaxies through intracluster gas.

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arXiv2010

Star Clusters Under Stress: Why Small Systems Cannot Dynamically Relax

Utilizing a series of N-body simulations, we argue that gravitationally bound stellar clusters of modest population evolve very differently from the picture presented by classical dynamical relaxation theory. The system's most massive stars rapidly sink towards the center and form binary systems. These binaries efficiently heat the cluster, reversing any incipient core contraction and driving a subsequent phase of global expansion. Most previous theoretical studies demonstrating deep and persistent dynamical relaxation have either conflated the process with mass segregation, ignored three-body interactions, or else adopted the artificial assumption that all cluster members are single stars of identical mass. In such a uniform-mass cluster, binary formation is greatly delayed, as we confirm here both numerically and analytically. The relative duration of core contraction and global expansion is effected by stellar evolution, which causes the most massive stars to die out before they form binaries. In clusters of higher N, the epoch of dynamical relaxation lasts for progressively longer periods. By extrapolating our results to much larger populations, we can understand, at least qualitatively, why some globular clusters reach the point of true core collapse.

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arXiv2010

Molecular Line Profiles from Contracting Dense Cores

We recently proposed that molecular cloud dense cores undergo a prolonged period of quasi-static contraction prior to true collapse. This theory could explain the observation that many starless cores exhibit, through their spectral line profiles, signs of inward motion. We now use our model, together with a publicly available radiative transfer code, to determine the emission from three commonly used species - N2H+, CS, and HCN. A representative dense core of 3 Msun that has been contracting for 1 Myr has line profiles that qualitatively match the observed ones. In particular, optically thick lines have about the right degree of blue-red asymmetry, the empirical hallmark of contraction. The J=2-1 rotational transition of CS only attains the correct type of profile if the species is centrally depleted, as has been suggested by previous studies. These results support the idea that a slow, but accelerating, contraction leads to protostellar collapse. In the future, the kind of analysis presented here can be used to assign ages to individual starless cores.

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arXiv2010

The Dynamical Evolution of the Pleiades

We present the results of a numerical simulation of the history and future development of the Pleiades. This study builds on our previous one that established statistically the present-day structure of this system. Our simulation begins just after molecular cloud gas has been expelled by the embedded stars. We then follow, using an N body code, the stellar dynamical evolution of the cluster to the present and beyond. Our initial state is that which evolves, over the 125 Myr age of the cluster, to a configuration most closely matching the current one. We find that the original cluster, newly stripped of gas, already had a virial radius of 4 pc. This configuration was larger than most observed, embedded clusters. Over time, the cluster expanded further and the central surface density fell by about a factor of two. We attribute both effects to the liberation of energy from tightening binaries of short period. Indeed, the original binary fraction was close to unity. The ancient Pleiades also had significant mass segregation, which persists in the cluster today. In the future, the central density of the Pleiades will continue to fall. For the first few hundred Myr, the cluster as a whole will expand because of dynamical heating by binaries. The expansion process is aided by mass loss through stellar evolution, which weakens the system's gravitational binding. At later times, the Galactic tidal field begins to heavily deplete the cluster mass. It is believed that most open clusters are eventually destroyed by close passage of a giant molecular cloud. Barring that eventuality, the density falloff will continue for as long as 1 Gyr, by which time most of the cluster mass will have been tidally stripped away by the Galactic field.

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arXiv2009

The Orbital Decay of Embedded Binary Stars

Young binaries within dense molecular clouds are subject to dynamical friction from ambient gas. Consequently, their orbits decay, with both the separation and period decreasing in time. A simple analytic expression is derived for this braking torque. The derivation utilizes the fact that each binary acts as a quadrupolar source of acoustic waves. The acoustic disturbance has the morphology of a two-armed spiral and carries off angular momentum. From the expression for the braking torque, the binary orbital evolution is also determined analytically. This type of merger may help explain the origin of high-mass stars. If infrared dark clouds, with peak densities up to 10^7 cm^{-3}, contain low-mass binaries, those with separations less than 100 AU merge within about 10^5 yr. During the last few thousand years of the process, the rate of mechanical energy deposition in the gas exceeds the stars' radiative luminosity. Successive mergers may lead to the massive star formation believed to occur in these clouds.

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arXiv2009

The Approach to Collapse of Molecular Clouds

The dense molecular cloud cores that form stars, like other self-gravitating objects, undergo bulk oscillations. Just at the point of gravitational instability, their fundamental oscillation mode has zero frequency. We study, using perturbation theory, the evolution of a spherical cloud that possesses such a frozen mode. We find that the cloud undergoes a prolonged epoch of subsonic, accelerating contraction. This slow contraction occurs whether the cloud is initially inflated or compressed by the oscillation. The subsonic motion described here could underlie the spectral infall signature observed in many starless dense cores.

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arXiv2008

The Distribution of Stellar Mass in the Pleiades

As part of an effort to understand the origin of open clusters, we present a statistical analysis of the currently observed Pleiades. Starting with a photometric catalog of the cluster, we employ a maximum likelihood technique to determine the mass distribution of its members, including single stars and both components of binary systems. We find that the overall binary fraction for unresolved pairs is 68%. Extrapolating to include resolved systems, this fraction climbs to about 76%, significantly higher than the accepted field-star result. Both figures are sensitive to the cluster age, for which we have used the currently favored value of 125 Myr. The primary and secondary masses within binaries are correlated, in the sense that their ratios are closer to unity than under the hypothesis of random pairing. We map out the spatial variation of the cluster's projected and three-dimensional mass and number densities. Finally, we revisit the issue of mass segregation in the Pleiades. We find unambiguous evidence of segregation, and introduce a new method for quantifying it.

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arXiv2003

A Near-Infrared Multiplicity Survey of Class I/Flat-Spectrum Systems in Six Nearby Molecular Clouds

We present new near-IR observations of 76 Class I/flat-spectrum objects in the nearby (d < 320 pc) Perseus, Taurus, Chamaeleon I and II, rho Ophiuchi, and Serpens dark clouds. These observations are part of a larger systematic infrared multiplicity survey of self-embedded objects in the nearest dark clouds. When combined with the results of our previously published near-infrared multiplicity survey, we find a restricted companion star fraction of 14/79 (18% +/- 4%) of the sources surveyed to be binary or higher order multiple systems over a separation range of ~300 - 2000 AU with a magnitude difference $Δ$ K <= 4, and with no correction for background contamination or completeness. This is consistent with the fraction of binary/multiple systems found among older pre-main-sequence T Tauri stars in each of the Taurus, rho Ophiuchi, and Chamaeleon star-forming regions over a similar separation range, as well as the combined companion star fraction for these regions. However, the companion star fraction for solar-type, and lower mass M dwarf, main-sequence stars in the solar neighborhood in this separation range (11% +/- 3%) is approximately one-half that of our sample. Together with multiplicity statistics derived for previously published samples of Class 0 and Class I sources, our study suggests that a significant number of binary/multiple objects may remain to be discovered at smaller separations among our Class I/flat-spectrum sample and/or most of the evolution of binary/multiple systems occurs during the Class 0 phase of early stellar evolution.

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arXiv2003

Trapped Protostellar Winds and their Breakout

Observations show that high-velocity jets stem from deeply embedded young stars, which may still be experiencing infall from their parent cloud cores. Yet theory predicts that, early in this buildup, any outgoing wind is trapped by incoming material of low angular momentum. As collapse continues and brings in more rapidly rotating gas, the wind can eventually break out. Here we model this transition by following the motion of the shocked shell created by impact of the wind and a rotating, collapsing envelope. We first demonstrate, both analytically and numerically, that our previous, quasi-static solutions are dynamically unstable. Our present, fully time-dependent calculations include cases both where the wind is driven back by infall to the stellar surface, and where it erupts as a true outflow. For the latter, we find that the time of breakout is sim 50,000 yr for wind speeds of 200 km/s. The reason for the delay is that the shocked material, including the swept-up infall, must be able to climb out of the star's gravitational potential well. We explore the critical wind speed necessary for breakout as a function of the mass transport rates in the wind and infall, as well as the cloud rotation rate Omega0 and time since the start of infall. Breakout does occur for realistic parameter choices. The actual breakout times would change if we relaxed the assumption of perfect mixing between the wind and infall material. Our expanding shells do not exhibit the collimation of observed jets, but continue to expand laterally. To halt this expansion, the density in the envelope must fall off less steeply than in our model.

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