arXiv subjects

Robert A. Gruendl

Publications and source records attributed to Robert A. Gruendl.

At least 55 records · Page 3Linked to original sources

arXiv2007

Were All Massive Stars Born in OB Associations and Clusters?

It has been commonly conjectured that all massive >10 Msun stars are born in OB associations or clusters. Many O and B stars in the Galaxy or the Magellanic Clouds appear to exist in isolation, however. While some of these field OB stars have been ejected from their birthplaces, some are too far away from massive star forming regions to be runaways. Can massive stars form in isolation? The Spitzer survey of the Large Magellanic Cloud (aka SAGE) provides a unique opportunity for us to investigate and characterize the formation sites of massive stars for an entire galaxy. We have identified all massive young stellar objects (YSOs) in the Large Magellanic Cloud. We find that ~85% of the massive YSOs are in giant molecular clouds and ~65% are in OB associations. Only ~7% of the massive YSOs are neither in OB associations nor in giant molecular clouds. This fraction of isolated massive stars in the Large Magellanic Cloud is comparable to the 5-10% found in the Galaxy.

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arXiv2007

Structure of Supergiant Shells in the Large Magellanic Cloud

Nine supergiant shells (SGSs) have been identified in the Large Magellanic Cloud (LMC) based on H-alpha images, and twenty-three SGSs have been reported based on HI 21-cm line observations, but these sets do not always identify the same structures. We have examined the physical structure of the optically identified SGSs using HI channel maps and P-V diagrams to analyze the gas kinematics. There is good evidence for seven of the nine optically identified SGSs to be true shells. Of these seven H-alpha SGSs, four are the ionized inner walls of HI SGSs, while three are an ionized portion of a larger and more complex HI structure. All of the H-alpha SGSs are identified as such because they have OB associations along the periphery or in the center, with younger OB associations more often found along the periphery. After roughly 12 Myrs, if no new OB associations have been formed a SGS will cease to be identifiable at visible wavelengths. Thus, the presence and location of ionizing sources is the main distinction between shells seen only in HI and those also seen in H-alpha. Based on our analysis, H-alpha observations alone cannot unambiguously identify SGSs, especially in distant galaxies.

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arXiv2007

Large-Scale Gravitational Instability and Star Formation in the Large Magellanic Cloud

Large-scale star formation in disk galaxies is hypothesized to be driven by global gravitational instability. The observed gas surface density is commonly used to compute the strength of gravitational instability, but according to this criterion star formation often appears to occur in gravitationally stable regions. One possible reason is that the stellar contribution to the instability has been neglected. We have examined the gravitational instability of the Large Magellanic Cloud (LMC) considering the gas alone, and considering the combination of collisional gas and collisionless stars. We compare the gravitationally unstable regions with the on-going star formation revealed by Spitzer observations of young stellar objects. Although only 62% of the massive young stellar object candidates are in regions where the gas alone is unstable, some 85% lie in regions unstable due to the combination of gas and stars. The combined stability analysis better describes where star formation occurs. In agreement with other observations and numerical models, a small fraction of the star formation occurs in regions with gravitational stability parameter Q > 1. We further measure the dependence of the star formation timescale on the strength of gravitational instability, and quantitatively compare it to the exponential dependence expected from numerical simulations.

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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

X-ray Observations of the Bipolar PNe NGC 2346 and NGC 7026

We have obtained X-ray observations of the bipolar planetary nebulae (PNe) NGC 2346 and NGC 7026 with XMM-Newton. These observations detected diffuse X-ray emission from NGC 7026 but not from NGC 2346. The X-ray emission from NGC 7026 appears to be confined within the bipolar lobes of the PN and has spectral properties suggesting a thermal plasma emitting at a temperature of 1.1 +0.5/-0.2 times 10^6 K. The X-ray spectrum of NGC 7026 is modeled using nebular and stellar abundances to assess whether a significant amount of nebular material has been mixed into the shocked-wind, but the results of this comparison are not conclusive owing to the small number of counts detected. Observations of bipolar PNe indicate that diffuse X-ray emission is much less likely detected in open-lobed nebulae than closed-lobed nebulae, possibly because open-lobed nebulae do not have strong fast winds or are unable to retain hot gas.

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arXiv2005

An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources

We have studied the stellar and interstellar environments of two luminous X-ray sources and five ultraluminous X-ray sources (ULXs) in order to gain insight into their nature. Archival Hubble Space Telescope images were used to identify the optical counterparts of the ULXs Ho IX X-1 and NGC 1313 X-2, and to make photometric measurements of the local stellar populations of these and the luminous source IC 10 X-1. We obtained high-dispersion spectroscopic observations of the nebulae around these seven sources to search for He II lambda-4686 emission and to estimate the expansion velocities and kinetic energies of these nebulae. Our observations did not detect nebular He II emission from any source, with the exception of LMC X-1; this is either because we missed the He III regions or because the nebulae are too diffuse to produce He II surface brightnesses that lie within our detection limit. We compare the observed ionization and kinematics of the supershells around the ULXs Ho IX X-1 and NGC 1313 X-2 with the energy feedback expected from the underlying stellar population to assess whether additional energy contributions from the ULXs are needed. In both cases, we find insufficient UV fluxes or mechanical energies from the stellar population; thus these ULXs may be partially responsible for the ionization and energetics of their supershells. All seven sources we studied are in young stellar environments and six of them have optical counterparts with masses >~7 M_sun; thus, these sources are most likely high-mass X-ray binaries.

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arXiv2004

The Nebular Environment and Enigmatic Hard X-ray Emission of the Hot DO White Dwarf KPD 0005+5106

We have detected an ionized nebula around the hot DO white dwarf KPD 0005+5106, and used the [OIII]/H-alpha ratios and nebular velocities to separate this nebula from the background HII region of AO Cas. The angular size of the [OIII] nebula of KPD 0005+5106 is ~3 deg. The velocity of this nebula is similar to those of the local interstellar HI gas and the interstellar/circumstellar absorption lines in UV spectra of KPD 0005+5106, but has a large offset from those of the stellar photospheric lines. The mass of the ionized nebula, ~70 Msun, indicates that it consists of interstellar material and that the velocity offset between the star and the ambient medium should not be interpreted as a wind outflow. We have also analyzed the ROSAT PSPC observation of KPD 0005+5106 to determine its hard X-ray luminosity. Using the L_X/L_bol relation for late-type stars and the lack of obvious near-IR excess of KPD 0005+5106, we exclude the possible existence of a binary companion with coronal activity. Without a wind outflow, the presence of stellar OVIII emission requires that X-rays at energies greater than 0.871 keV are present in the vicinity of KPD 0005+5106. This hard X-ray emission is most puzzling as neither photospheric emission at such high energies nor a high-temperature corona is expected from current stellar atmospheric models of KPD 0005+5106. X-ray observations with high angular resolution and sensitivity are needed to confirm the positional coincidence and to enable X-ray spectral analyses for constraining the physical origin of the hard X-ray emission from KPD 0005+5106.

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arXiv2004

Physical Structure of Planetary Nebulae. III. The Large and Evolved NGC 1360

NGC 1360 is a large planetary nebula (PN) without an obvious shell morphology. We have analyzed H-alpha images and high-dispersion echelle spectra of NGC 1360 in order to construct spatio-kinematic models and to determine its density distribution. The best-fit model indicates that NGC 1360 is a prolate ellipsoidal shell whose major axis is twice as long as its minor axis and is tilted by 60 arcdeg with respect to the line of sight. The large kinematic age of the shell, ~10,000 yr, and the low density of the nebula, <130 H-atom cm^-3, imply that NGC 1360 is an evolved PN and has begun to merge with the interstellar medium. The observed morphology and surface brightness profiles of NGC 1360 can be described well as a thick shell with a Gaussian radial density profile without a sharp inner edge, indicating a lack of on-going compression by a fast stellar wind. The FLIERs observed in NGC 1360 near the end of its major axis expand faster and are younger than the nebular shell.

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arXiv2004

The Nature of SN 1961V

The nature of SN 1961V has been uncertain. Its peculiar optical light curve and slow expansion velocity are similar to those of super-outbursts of luminous blue variables (LBVs), but its nonthermal radio spectral index and declining radio luminosity are consistent with decades-old supernovae (SNe). We have obtained Hubble Space Telescope STIS images and spectra of the stars in the vicinity of SN 1961V, and find Object 7 identified by Filippenko et al. to be the closest to the optical and radio positions of SN 1961V. Object 7 is the only point source detected in our STIS spectra and only its H-alpha emission is detected; it cannot be the SN or its remnant because of the absence of forbidden lines. While the H-alpha line profile of Object 7 is remarkably similar to that of eta Car, the blue color (similar to an A2Ib supergiant) and lack of appreciable variability are unlike known post-outburst LBVs. We have also obtained Very Long Baseline Array (VLBA) observations of SN 1961V at 18 cm. The non-detection of SN 1961V places a lower limit on the size of the radio-emitting region, 7.6 mas or 0.34 pc, which implies an average expansion velocity in excess of 4,400 km/s, much higher than the optical expansion velocity measured in 1961. We conclude the following: (1) A SN occurred in the vicinity of SN 1961V a few decades ago. (2) If the SN 1961V light maximum originates from a giant eruption of a massive star, Object 7 is the most probable candidate for the survivor, but its blue color and lack of significant variability are different from a post-outburst eta Car. (3) The radio SN and Object 7 could be physically associated with each other through a binary system. (4) Object 7 needs to be monitored to determine its nature and relationship to SN 1961V.

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arXiv2002

An Inside Out View of Bubbles

Fast stellar winds can sweep up ambient media and form bubbles. The evolution of a bubble is largely controlled by the content and physical conditions of the shocked fast wind in its interior. This hot gas was not clearly observed until the recent advent of Chandra and XMM-Newton X-ray observatories. To date, diffuse X-ray emission has been unambiguously detected from two circumstellar bubbles blown by WR stars, four planetary nebulae, and two superbubbles blown by young clusters. Model fits to the X-ray spectra show that the circumstellar bubbles are dominated by hot gas with low temperatures (<= 3x10^6 K), while the interstellar bubbles contain significant fractions of hotter gas (>= 5x10^6 K). In all cases, large discrepancies in the X-ray luminosity are found between observations and conventional models of bubbles. Future theoretical models of bubbles need to re-examine the validity of heat conduction and take into account realistic microscopic processes such as mass loading from dense clumps/knots and turbulent mixing. Chandra ACIS-S observation of NGC 6888 will shed light on these astrophysical processes.

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arXiv2002

Structure and Dynamics of Candidate O Star Bubbles in N44

Dynamical studies of superbubbles and Wolf-Rayet ring nebulae show discrepancies from the standard, adiabatic model for wind-blown bubbles. We therefore study the physical properties and kinematics of three candidate bubbles blown by single O stars, to evaluate whether these discrepancies are also found in these simpler objects. Our sample candidates are N44F, N44J, and N44M, in the outskirts of the HII complex N44 in the Large Magellanic Cloud. We have obtained ground-based and HST emission-line images and high dispersion echelle spectra for these objects. From the Halpha luminosities and the [OIII]/Halpha ratios of these nebulae, we estimate the spectral types of the ionizing stars to be O7V, O9.5V and O9.5V for N44F, N44J, and N44M, respectively. We find that the observed expansion velocity of 12 km/s for N44F is consistent with the stellar wind luminosity expected from the central ionizing star, as predicted by the standard bubble model. The observed upper limits for the expansion velocities of N44J and N44M are also compatible with the expected values, within the uncertainties. We also report the discovery in N44F of strongly-defined dust columns, similar to those seen in the Eagle Nebula. The photoevaporation of these dense dust features may be kinematically important and may actually govern the evolution of the shell. The inclusion of photoevaporation processes may thus undermine the apparent agreement between the observed bubble dynamics and the simple adiabatic models.

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arXiv2002

Diffuse X-ray emission from the planetary nebula NGC 7009

XMM-Newton EPIC observations of the planetary nebula (PN) NGC 7009, the Saturn Nebula, have detected extended X-ray emission in its central cavity. The diffuse X-ray emission must originate in the shocked fast stellar wind. The spectra show that the temperature of the hot gas is 1.8*10^6 K. The rms density derived from the volume emission measure is a few tens H-atom/cm^3. The hot gas does not appear over-pressurized with respect to the nebular shell. NGC 7009 represents an evolutionary stage at which the influence of the hot gas in the PN interior starts to decline due to the diminishing strength of the fast stellar wind and the expansion of the central cavity.

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arXiv2002

X-ray and UV Views of Hot Gas in Planetary Nebulae

The interior of a planetary nebula (PN) is expected to be filled with shocked fast wind from the central star. This hot gas plays the most important role in the dynamical evolution of the PN; however, its physical conditions are not well-known because useful X-ray and far-UV observations were not available until the advent of Chandra, XMM-Newton, and FUSE. This paper reviews X-ray observations of the hot gas in PN interiors and far-UV observations of the interfaces between the hot gas and the dense nebular shells.

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arXiv2002

Narrow Lines in Type II Supernovae -- Probing the Circumstellar Nebulae of the Progenitors

We have carried out a high-dispersion (R~30,000) echelle spectroscopic survey of 16 Type II supernovae (SNe) to search for narrow emission lines from circumstellar nebulae ejected by their massive progenitors. Circumstellar nebulae, if detected, provide invaluable opportunities to probe SN progenitors. Of the 16 SNe observed, SN ejecta are clearly detected in 4 SNe and possibly in another 2 SNe, interstellar gas is detected in 12 SNe, and circumstellar material is detected only in SN 1978K and SN 1998S. In the case of SN 1978K we are able to place an upper limit of ~2.2 pc for the size of the circumstellar ejecta nebula and note that this is more consistent with the typical sizes observed for ejecta nebulae around luminous blue variables rather than Wolf-Rayet stars. In the case of SN 1998S, our observations of the narrow lines \~1 year after the SN explosion show variations compared to early epochs. The nebular lines we observe from SN 1998S originate from either the low--density, outer region of a circumstellar nebula or have become dominated by an interstellar component.

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arXiv2001

Distorted HI Gas in the Widely Separated LIRG Arp 256

We present new interferometric HI and CO (1-0) observations of the luminous infrared source, Arp 256. Arp 256 consists of two spiral galaxies in an early stage of merging, with a projected nuclear separation of 29 kpc (54") and an infrared luminosity of 2.0E11 L_sun. Despite the large separation of the galaxies' nuclei and mildly disrupted stellar components, the HI disks are found to be strongly disrupted, and the southern galaxy in Arp 256 shows an elevated star formation efficiency, which is consistent with a nuclear starburst. Both of these results run contrary to expectations, posing interesting questions on the physical mechanisms involved in stimulating star formation during an interaction.

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arXiv2001

The Enigmatic X-ray Point Sources at the Central Stars of NGC 6543 and NGC 7293

Recent Chandra ACIS-S observations have detected a point source at the central star of NGC 6543 and confirmed the point source nature of the hard X-ray emission from NGC 7293. The X-ray spectra of both sources peak between 0.5 keV and 1.0 keV and show line features indicating a thin plasma at temperatures of a few times 10^6 K. Their X-ray luminosities are 10^30 erg/s and 3x10^29 erg/s, respectively. We have considered four different mechanisms to explain the nature of these sources. The X-ray emission from the central star of NGC 6543 may originate from the coronal activity of an undetected companion star or from shocks in its fast stellar wind, while the hard X-ray emission from NGC 7293 might be ascribed to an undetected dMe companion. Follow-up observations are needed to determine the existence and natures of these stellar companions.

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arXiv2001

Variable H-alpha Line Emission from the Central Star of the Helix Nebula

The central star of the Helix Nebula is a hot white dwarf with unexpected hard X-ray emission. With an effective temperature of ~100,000 K, the star is a powerful source of H-ionizing radiation; the atmosphere of a stellar or planetary companion, if present, will be ionized and emit recombination lines. To probe the origin of hard X-ray emission from the Helix Nebula's central star, we have obtained multi-epoch high-dispersion spectra of the star, and found temporal variation in the H-alpha line profile over a time span of one week. The observed width and strength of the variable H-alpha emission component are consistent with the hypothesized dMe companion proposed to explain the hard X-ray emission. A dMe companion, however, cannot explain the possible detection of variable HeII and [NII] emission. Follow-up spectroscopic monitoring of the Helix Nebula central star is needed to better establish the identification of the spectral lines and their temporal behavior in order to determine the origin of the optical variability and hard X-ray emission.

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arXiv2001

Gas Distribution and Starburst Activity in the Widely Separated Interacting Galaxies NGC 6670

We present high resolution H I 21 cm line, 20 cm radio continuum, and CO(1-0) line observations of the luminous infrared galaxy NGC 6670. NGC 6670 consists of two edge-on disk galaxies (NGC 6670E and NGC 6670W) with a projected nuclear separation of ~16 kpc. While there are no optically identified tidal features and the molecular disks are not distorted much, we have detected a 90 kpc long H I tail which suggests that the galaxies are interacting and have already experienced at least one close encounter. Our observations suggest that the previous encounter between the galaxies had a large impact parameter and that the system is still in an early stage of interaction. Even though NGC 6670 is in an early stage of interaction, we find evidence of starburst. The CO(1-0) observations show large molecular gas reservoirs in the central regions and the 20 cm radio continuum observations reveal enhanced star formation activity in the nuclear regions of both galaxies. We estimate the ratio L_IR/M_H2, which is often used as an indicator of star formation efficiency, in the nuclear regions of NGC 6670E and NGC 6670W to be 18 and 11 L_sun/M_sun, respectively. The nuclear star formation efficiency of these two galaxies has been elevated to the level observed in other nearby starburst galaxies (L_IR/M_H2 > 10 L_sun/M_sun). Other indicators of starburst activity such as CO brightness temperature and infrared surface brightness are also observed.

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