arXiv subjects

John Bally

Publications and source records attributed to John Bally.

At least 163 records · Page 9Linked to original sources

arXiv2007

And in the Darkness Bind Them: Equatorial Rings, B[e] Supergiants, and the Waists of Bipolar Nebulae

We report the discovery of two new circumstellar ring nebulae in the western Carina Nebula. The brighter object, SBW1, resembles a lidless staring eye and encircles a B1.5 Iab supergiant. Its size is identical to the inner ring around SN1987A, but SBW1's low N abundance indicates that the star didn't pass through a RSG phase. The fainter object, SBW2, is a more distorted ring, is N-rich, and has a central star that seems to be invisible. We discuss these two new nebulae in context with rings around SN1987A, Sher25, HD168625, RY Scuti, WeBo1, SuWt2, and others. The ring bearers fall into two groups: Five rings surround hot supergiants, and all except for the one known binary are carbon copies of the ring around SN1987A. We propose a link between these rings and B[e] supergiants, where the rings derive from the same material in an earlier B[e] phase. The remaining four rings surround evolved intermediate-mass stars; all members of this ring fellowship are close binaries, hinting that binary interactions govern the forging of such rings. We estimate that there may be several thousand more dark rings in the Galaxy, but we are scarcely aware of their existence due to selection effects. The lower-mass objects might be the equatorial density enhancements often invoked to bind the waists of bipolar PNe.

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arXiv2007

Binary Capture Rates for Massive Protostars

The high multiplicity of massive stars in dense, young clusters is established early in their evolution. The mechanism behind this remains unresolved. Recent results suggest that massive protostars may capture companions through disk interactions with much higher efficiency than their solar mass counterparts. However, this conclusion is based on analytic determinations of capture rates and estimates of the robustness of the resulting binaries. We present the results of coupled n-body and SPH simulations of star-disk encounters to further test the idea that disk-captured binaries contribute to the observed multiplicity of massive stars.

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arXiv2007

Resolving the nature of the Rosette HH1 jet facing strong UV dissipation

The Rosette HH1 jet is a collimated flow immersed in the strong UV radiation field of the Rosette Nebula. We investigate the physical properties of the Rosette HH1 jet using high-quality narrow-band images and high-dispersion spectroscopy. The new images show that the axis of the jet is not precisely aligned with the star near the base of the jet. The high resolution of the spectra allows us to accurately determine the contributions from the \ion{H}{2} region, jet, and star. The appoaching and receding sides of the expanding shell of the Rosette Nebula are at heliocentric velocities of 13 and 40 km s$^{-1}$, while the jet reaches a maximum velocity offset at a heliocentric velocity of $-$30 km s$^{-1}$. The [\ion{S}{2}] doublet ratios indicate an electron density of $\sim$1000 cm$^{-3}$ in the jet and $\le$100 cm$^{-3}$ in the \ion{H}{2} region. With a careful subtraction of the nebular and jet components, we find the stellar H$α$ line is dominated by a broad absorption profile with little or no emission component, indicating a lack of substantial circumstellar material. The circumstellar material has most likely been photo-evaporated by the strong UV radiation field in the Rosette Nebula. The evaporation time scale is 10$^3$ -- 10$^4$ yr. The Rosette HH1 jet source provides evidence for an accelerated evolution from a CTTS to a WTTS due to the strong UV radiation field; therefore, both CTTSs and WTTSs can be spatially mixed in regions with massive star formation.

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arXiv2006

Capture Formed Binaries via Encounters with Massive Protostars

Most massive stars are found in the center of dense clusters, and have a companion fraction much higher than their lower mass siblings; the massive stars of the Trapezium core in Orion have ~ 1.5 companions each. This high multiplicity could be a consequence of formation via a capture scenario, or it could be due to fragmentation of the cores that form the massive stars. During stellar formation circumstellar disks appear to be nearly ubiquitous. Their large radii compared to stellar sizes increase the interaction radius significantly, suggesting that disk interactions with neighboring stars could assist in capturing binary companions. This mechanism has been studied for stars of approximately solar mass and found to be inefficient. In this paper we present simulations of interactions between a 22 Msun star-disk system and less massive impactors, to study the disk-assisted capture formation of binaries in a regime suited to massive stars. The formation of binaries by capture is found to be much more efficient for massive capturers. We discuss the effects of a mass dependent velocity dispersion and mass segregation on the capture rates, and consider the long term survival of the resultant binaries in a dense cluster.

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arXiv2006

Large Area Mapping at 850 Microns. V. Analysis of the Clump Distribution in the Orion A South Molecular Cloud

We present results from a 2300 arcmin^2 survey of the Orion A molecular cloud at 450 and 850 micron using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. The region mapped lies directly south of the OMC1 cloud core and includes OMC4, OMC5, HH1/2, HH34, and L1641N. We identify 71 independent clumps in the 850 micron map and compute size, flux, and degree of central concentration in each. Comparison with isothermal, pressure-confined, self-gravitating Bonnor-Ebert spheres implies that the clumps have internal temperatures T_d ~ 22 +/- K and surface pressures log (k^-1 P cm^-3 K) = 6.0 +/- 0.2. The clump masses span the range 0.3 - 22 Msun assuming a dust temperature T_d ~ 20 K and a dust emissivity kappa_850 = 0.02 cm^2 g^-1. The distribution of clump masses is well characterized by a power-law N(M) propto M^-alpha with alpha = 2.0 +/- 0.5 for M > 3.0 Msun, indicating a clump mass function steeper than the stellar Initial Mass Function. Significant incompleteness makes determination of the slope at lower masses difficult. A comparison of the submillimeter emission map with an H_2 2.122 micron survey of the same region is performed. Several new Class 0 sources are revealed and a correlation is found between both the column density and degree of concentration of the submillimeter sources and the likelihood of coincident H_2 shock emission.

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arXiv2006

Stellar Encounters with Massive Star-Disk Systems

The dense, clustered environment in which massive stars form can lead to interactions with neighboring stars. It has been hypothesized that collisions and mergers may contribute to the growth of the most massive stars. In this paper we extend the study of star-disk interactions to explore encounters between a massive protostar and a less massive cluster sibling using the publicly available SPH code GADGET-2. Collisions do not occur in the parameter space studied, but the end state of many encounters is an eccentric binary with a semi-major axis ~ 100 AU. Disk material is sometimes captured by the impactor. Most encounters result in disruption and destruction of the initial disk, and periodic torquing of the remnant disk. We consider the effect of the changing orientation of the disk on an accretion driven jet, and the evolution of the systems in the presence of on-going accretion from the parent core.

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arXiv2006

Entrainment Mechanisms for Outflows in the L1551 Star-Forming Region

We present high sensitivity 12/13CO(1-0) molecular line maps covering the full extent of the parsec scale L1551 molecular outflow, including the redshifted east-west (EW) flow. We also present 12CO(3-2) data that extends over a good fraction of the area mapped in the 1-0 transition. We compare the molecular data to widefield, narrow-band optical emission in H$α$. While there are multiple outflows in the L1551 cloud, the main outflow is oriented at 50\arcdeg position angle and appears to be driven by embedded source(s) in the central IRS 5 region. The 3-2 data indicate that there may be molecular emission associated with the L1551 NE jet, within the redshifted lobe of main outflow. We have also better defined the previously known EW flow and believe we have identified its blueshifted counterpart. We further speculate that the origin of the EW outflow lies near HH 102. We use velocity dependent opacity correction to estimate the mass and the energy of the outflow. The resulting mass spectral indices from our analysis, are systematically lower (less steep) than the power law indices obtained towards other outflows in several recent studies that use a similar opacity correction method. We show that systematic errors and biases in the analysis procedures for deriving mass spectra could result in errors in the determination of the power-law indices. The mass spectral indices, the morphological appearance of the position-velocity plots and integrated intensity emission maps of the molecular data, compared with the optical, suggest that jet-driven bow-shock entrainment is the best explanation for the driving mechanism of outflows in L1551. The kinetic energy of the outflows is found to be comparable to the binding energy of the cloud and sufficient to maintain the turbulence in the L1551 cloud.

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arXiv2006

Cleaning up Eta Carinae: Detection of Ammonia in the Homunculus

We report the first detection of ammonia in the Homunculus nebula around eta Carinae, which is also the first detection of emission from a polyatomic molecule in this or any other luminous blue variable (LBV) nebula. Observations of the NH3 (J,K)=(3,3) inversion transition made with the Australia Telescope Compact Array reveal emission at locations where infrared H2 emission had been detected previously, near the strongest dust emission in the core of the Homunculus. We also detect ammonia emission from the so-called ``strontium filament'' in the equatorial disk. The presence of NH3 around eta Car hints that molecular shells around some Wolf-Rayet stars could have originated in prior LBV eruptions, rather than in cool red supergiant winds or the ambient interstellar medium. Combined with the lack of any CO detection, NH3 seems to suggest that the Homunculus is nitrogen rich like the ionized ejecta around eta Car. It also indicates that the Homunculus is a unique laboratory in which to study unusual molecule and dust chemistry, as well as their rapid formation in a nitrogen-rich environment around a hot star. We encourage future observations of other transitions like NH3 (1,1) and (2,2), related molecules like N2H+, and renewed attempts to detect CO.

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arXiv2006

Silicate Emission Profiles from Low-Mass Protostellar Disks in the Orion Nebula: Evidence for Growth and Thermal Processing of Grains

We present 8--13 micron low resolution spectra (R~100) of 8 low-mass protostellar objects ("proplyds") in the Orion Nebula using the Long Wavelength Spectrometer (LWS) at the W. M. Keck Observatory. All but one of the sources in our sample show strong circumstellar silicate emission, with profiles that are qualitatively similar to those seen in some T Tauri and Herbig Ae/Be stars. The silicate profile in all cases is significantly flattened compared to the profile for typical interstellar dust, suggesting that the dominant emitting grains are significantly larger than those found in the interstellar medium. The 11.3-to-9.8 micron flux ratio--often used as an indicator of grain growth--is in the 0.8 to 1.0 range for all of our targets, indicating that the typical grain size is around a few microns in the surface layers of the attendant circumstellar disk for each object. Furthermore, the silicate profiles show some evidence of crystalline features, as seen in other young stellar objects. The results of our analysis show that the grains in the photoevaporating protostellar disks of Orion have undergone significant growth and perhaps some annealing, suggesting that grain evolution for these objects is not qualitatively different from other young stellar objects.

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arXiv2006

X-rays from HH210 in the Orion nebula

We report the detection during the Chandra Orion Ultradeep Project (COUP) of two soft, constant, and faint X-ray sources associated with the Herbig-Haro object HH210. HH210 is located at the tip of the NNE finger of the emission line system bursting out of the BN-KL complex, northwest of the Trapezium cluster in the OMC-1 molecular cloud. Using a recent Halpha image obtained with the ACS imager on board HST, and taking into account the known proper motions of HH210 emission knots, we show that the position of the brightest X-ray source, COUP703, coincides with the emission knot 154-040a of HH210, which is the emission knot of HH210 having the highest tangential velocity (425 km/s). The second X-ray source, COUP704, is located on the complicated emission tail of HH210 close to an emission line filament and has no obvious optical/infrared counterpart. Spectral fitting indicates for both sources a plasma temperature of ~0.8 MK and absorption-corrected X-ray luminosities of about 1E30 erg/s (0.5-2.0 keV). These X-ray sources are well explained by a model invoking a fast-moving, radiative bow shock in a neutral medium with a density of ~12000 cm^{-3}. The X-ray detection of COUP704 therefore reveals, in the complicated HH210 region, an energetic shock not yet identified at other wavelengths.

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arXiv2005

Multi-Generational Star Formation in L1551

The L1551 molecular cloud contains two small clusters of Class 0 and I protostars, as well as a halo of more evolved Class II and III YSOs, indicating a current and at least one past burst of star formation. We present here new, sensitive maps of 850 and 450 um dust emission covering most of the L1551 cloud, new CO J=2-1 data of the molecular cloud, and a new, deep, optical image of [SII] emission. No new Class 0/I YSOs were detected. Compact sub-millimetre emitters are concentrated in two sub-clusters: IRS5 and L1551NE, and the HL~Tauri group. Both stellar groups show significant extended emission and outflow/jet activity. A jet, terminating at HH 265 and with a very weak associated molecular outflow, may originate from LkHa 358, or from a binary companion to another member of the HL Tauri group. Several Herbig Haro objects associated with IRS5/NE were clearly detected in the sub-mm, as were faint ridges of emission tracing outflow cavity walls. We confirm a large-scale molecular outflow originating from NE parallel to that from IRS5, and suggest that the "hollow shell" morphology is more likely due to two interacting outflows. We confirm the presence of a prestellar core (L1551-MC) of mass 2-3 Mo north-west of IRS5. The next generation cluster may be forming in this core. The L1551 cloud appears cometary in morphology, and appears to be illuminated and eroded from the direction of Orion, perhaps explaining the multiple episodes of star formation in this cloud. The full paper (including figures) can be downloaded at http://www.jach.hawaii.edu/~gms/l1551/l1551-apj641.pdf, or viewed at http://www.jach.hawaii.edu/~gms/l1551/.

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arXiv2005

The [O III] Veil: Astropause of Eta Carinae's Wind?

We present narrowband images of eta Carinae in the light of [O III] 5007 obtained with HST/WFPC2, as well as a ground-based image in the same emission line with a larger field of view. These images show a thin veil of [O III] emission around eta Car and its ejecta, confirming the existence of an oxygen-bearing ``cocoon'' inferred from spectra. This [O III] veil may be the remnant of the pre-outburst wind of eta Car, and its outer edge probably marks the interface where eta Car's ejecta meet the stellar wind of the nearby O4 V((f)) star HD303308 or other ambient material -- i.e., it marks the ``astropause'' in eta Car's wind. This veil is part of a more extensive [O III] shell that appears to be shaped and ionized by HD303308. A pair of HST images with a 10 yr baseline shows no proper motion, limiting the expansion speed away from eta Car to 12pm13 km/s, or an expansion age of a few times 10^4 yr. Thus, this is probably the decelerated pre-outburst LBV wind of eta Car. The [O III] morphology is very different from that seen in [N II], which traces young knots of CNO-processed material; this represents a dramatic shift in the chemical makeup of material recently ejected by eta Car. This change in the chemical abundances may have resulted from the sudden removal of the star's outer envelope during eta Car's 19th century outburst or an earlier but similar event.

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arXiv2005

X-ray Emission from Orion Nebula Cluster Stars with Circumstellar Disks and Jets

We investigate the X-ray and near-infrared emission properties of a sample of pre-main sequence (PMS) stellar systems in the Orion Nebula Cluster (ONC) that display evidence for circumstellar disks (``proplyds'') and optical jets in Hubble Space Telescope (HST) imaging. Approximately 70% of ~140 proplyds were detected as X-ray sources in the Chandra Orion Ultradeep Program observation of the ONC, including ~25% of proplyds that do not display central stars in HST imaging. Many proplyds display near-infrared excesses, suggesting disk accretion is ongoing onto the central, PMS stars. Among circumstellar disks that are detected in absorption in HST imaging, we find that X-ray absorbing column and apparent disk inclination are well correlated, providing insight into the disk scale heights and metal abundances of UV- and X-ray-irradiated protoplanetary disks. Jet sources display some of the largest near-infrared excesses among the X-ray-detected proplyds, suggesting that the origin of the jets is closely related to ongoing, PMS stellar accretion. Some jet sources display double-peaked X-ray spectral energy distributions, with soft components indicative of strong shocks in the jet collimation region. These results support models in which circumstellar disks collimate and/or launch jets from young stellar objects and, furthermore, demonstrate that star-disk-jet interactions may contribute to PMS X-ray emission.

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arXiv2005

Thermal Dust Emission from Proplyds, Unresolved Disks, and Shocks in the Orion Nebula

We present a new 11.7 micron mosaic image of the Orion nebula obtained with T-ReCS on Gemini South. The map includes the BN/KL region, the Trapezium, and OMC-1 South. Excluding BN/KL, we detect 91 point sources, with 27 known proplyds and over 30 ``naked'' stars showing no extended structure in HST images. Within the region we surveyed, 80 percent of known proplyds show detectable emission, almost 40 percent of naked stars are detected at 11.7 micron, and the fraction of all visible sources with IR excess emission is roughly 50 percent. Thermal dust emission from stars with no extended structure in HST images means that they have dust disks comparable to the size of our solar system. Proplyds and stars with IR excess show a clear anti-correlation in their spatial distribution, with proplyds clustered close to theta1C, and other infrared sources found farther away. We suspect that the clustered proplyds trace the youngest 0.5 Myr age group associated with the Trapezium, while the more uniformly-distributed sources trace the older 1-2 Myr population of the ONC. This suggests that small disks persist for a few Myr in irradiated environments, and hints that hierarchical sub-clustering has been important. Within 30 arcsec of theta1C, all proplyds are detected at 11.7 micron. The star theta1D is associated with the most prominent mid-IR dust arc in the nebula. We propose that this arc is the consequence of theta1D being the closest member of the Trapezium to the background cloud. Finally, we detect dust emission from HH jets in Orion, including HH202, HH529, HH513, and HH514. This is the first detection of mid-IR continuum emission from dust in the body of a collimated HH jet or bow shock.

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arXiv2005

Can photo-evaporation trigger planetesimal formation?

We propose that UV radiation can stimulate the formation of planetesimals in externally-illuminated protoplanetary disks. We present a numerical model of disk evolution including vertical sedimentation and photo-evaporation by an external O or B star. As solid material grows and settles toward the disk midplane, the outer layers of the disk become dust depleted. When such a disk is exposed to UV radiation, heating drives photo-evaporative mass-loss from its surface, generating a dust-depleted outflow. The dust:gas surface density ratio in the disk interior grows until dust in the disk midplane becomes gravitationally unstable. Thus, UV radiation fields may induce the rapid formation of planetesimals in disks where sedimentation has occurred.

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arXiv2005

The Birth of High Mass Stars: Accretion and/or Mergers?

The observational consequences of the merger scenario for massive star formation are explored and contrasted with the gradual accumulation of mass by accretion. Protostellar mergers may produce high luminosity infrared flares lasting years to centuries followed by a luminosity decline on the Kelvin-Helmholtz time-scale of the merger product. Mergers may be surrounded by thick tori of expanding debris, impulsive wide-angle outflows, and shock induced maser and radio continuum emission. Collision products are expected to have fast stellar rotation and a large multiplicity fraction. Close encounters or mergers will produce circumstellar debris disks with an orientation that differs form that of a pre-existing disk. The extremely rare merger of two stars close to the upper-mass end of the IMF may be a possible pathway to hypernova generated gamma-ray bursters. While accretional growth can lead to the formation of massive stars in isolation or in loose clusters, mergers can only occur in high-density cluster environments. It is proposed that the outflow emerging from the OMC1 core in the Orion molecular cloud was produced by a protostellar merger that released between $10^{48}$ to $10^{49}$ ergs less than a thousand years ago.

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arXiv2005

Discovery of a candidate protoplanetary disk around the embedded source IRc9 in Orion

We report the detection of spatially-extended mid-infrared emission around the luminous embedded star IRc9 in OMC-1, as seen in 8.8, 11.7, and 18.3 micron images obtained with T-ReCS on Gemini South. The extended emission is asymmetric, and the morphology is reminiscent of warm dust disks around other young stars. The putative disk has a radius of roughly 1.5 arcsec (700 AU), and a likely dust mass of almost 10 Earth masses. The infrared spectral energy distribution of IRc9 indicates a total luminosity of about 100 Lsun, implying that it shall become an early A-type star when it reaches the main sequence. Thus, the candidate disk around IRc9 may be a young analog of the planetary debris disks around Vega-like stars and the disks of Herbig Ae stars, and may provide a laboratory in which to study the earliest phases of planet formation. A disk around IRc9 may also add weight to the hypothesis that an enhanced T Tauri-like wind from this star has influenced the molecular outflow from the OMC-1 core.

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arXiv2004

An S-shaped outflow from IRAS 03256+3055 in NGC 1333

The IRAS source 03256+3055 in the NGC 1333 star forming region is associated with extended sub-millimeter emission of complex morphology, showing multiple clumps. One of these is found to coincide with the driving source of a bipolar jet of S-shaped morphology seen in the emission lines of H_alpha and [SII] as well as in the H2 emission lines in the K-band. Detailed images of the driving source at the wavelengths of H_alpha and [SII] and in the I, J, H, and K bands as well as a K-band spectrum and polarimetry are discussed. The near-infrared morphology is characterized by a combination of line emission from the jet and scattered light from a source with a steep continuum spectrum. The morphology and proper motion of the jet are discussed in the context of a binary system with a precessing disk. We conclude that the molecular core associated with IRAS 03256+3055 consists of several clumps, only one of which shows evidence of recent star formation at optical and near-infrared wavelengths.We also briefly discuss a second, newly found near-infrared source associated with a compact sub-millimeter continuum source near IRAS 03256+3055, and conclude that this source may be physically unrelated the cluster of molecular clumps.

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