Searcharxiv⌕ Search

arXiv subjects

Mark Morris

Publications and source records attributed to Mark Morris.

At least 37 records · Page 2Linked to original sources
arXiv2007

A Spitzer Study of the Mass Loss Histories of Three Bipolar Pre-Planetary Nebulae

We present the results of far-infrared imaging of extended regions around three bipolar pre-planetary nebulae, AFGL 2688, OH 231.8+4.2, and IRAS 16342$-$3814, at 70 and 160 $μ$m with the MIPS instrument on the Spitzer Space Telescope. After a careful subtraction of the point spread function of the central star from these images, we place constraints on the existence of extended shells and thus on the mass outflow rates as a function of radial distance from these stars. We find no apparent extended emission in AFGL 2688 and OH 231.8+4.2 beyond 100 arcseconds from the central source. In the case of AFGL 2688, this result is inconsistent with a previous report of two extended dust shells made on the basis of ISO observations. We derive an upper limit of $2.1\times10^{-7}$ M$_\odot$ yr$^{-1}$ and $1.0\times10^{-7}$ M$_\odot$ yr$^{-1}$ for the dust mass loss rate of AFGL 2688 and OH 231.8, respectively, at 200 arcseconds from each source. In contrast to these two sources, IRAS 16342$-$3814 does show extended emission at both wavelengths, which can be interpreted as a very large dust shell with a radius of $\sim$ 400 arcseconds and a thickness of $\sim$ 100 arcseconds, corresponding to 4 pc and 1 pc, respectively, at a distance of 2 kpc. However, this enhanced emission may also be galactic cirrus; better azimuthal coverage is necessary for confirmation of a shell. If the extended emission is a shell, it can be modeled as enhanced mass outflow at a dust mass outflow rate of $1.5\times10^{-6}$ M$_\odot$ yr$^{-1}$ superimposed on a steady outflow with a dust mass outflow rate of $1.5\times10^{-7}$ M$_\odot$ yr$^{-1}$. It is likely that this shell has swept up a substantial mass of interstellar gas during its expansion, so these estimates are upper limits to the stellar mass loss rate.

astro-ph↗
arXiv2007

Discovery of hot supergiant stars near the Galactic center

We report new results of a campaign to find Wolf-Rayet and O (WR/O) stars and high-mass X-ray binaries (HMXBs) in the Galactic center. We searched for candidates by cross-correlating the 2MASS catalog with a deep Chandra catalog of X-ray point sources in the Radio Arches region. Following up with K-band spectroscopy, we found two massive stellar counterparts to CXOGC J174555.3-285126 and CXOGC J174617.0-285131, which we classify as a broad-lined WR star of sub-type WN6b and an O Ia supergiant, respectively. Their X-ray properties are most consistent with those of known colliding-wind binaries in the Galaxy and the Large Magellanic Cloud, although a scenario involving low-rate accretion onto a compact object is also possible. The O Ia star lies 4.4 pc in projection from the Quintuplet cluster, and has a radial velocity consistent with that of the Quintuplet, suggesting that this star might have escaped from the cluster. We also present the discovery of a B2 Ia supergiant, which we identified as a candidate massive star using 8 micron Spitzer maps of the Galactic center in a region near the known massive X-ray-emitting star CXOGC J174516.1-290315. We discuss the origin of these stars in the context of evolving stellar clusters in the Galactic center.

astro-ph↗
arXiv2007

The Galactic Center Magnetosphere

The magnetic field within a few hundred parsecs of the center of the Galaxy is an essential component of any description of that region. The field has several pronounced observational manifestations: 1) morphological structures such as nonthermal radio filaments (NTFs) -- magnetic flux tubes illuminated by synchrotron emission from relativistic electrons -- and a remarkable, large-scale, helically wound structure, 2) relatively strong polarization of thermal dust emission from molecular clouds, presumably resulting from magnetic alignment of the rotating dust grains, and 3) synchrotron emission from cosmic rays. Because most of the NTFs are roughly perpendicular to the Galactic plane, the implied large-scale geometry of the magnetic field is dipolar. Estimates of the mean field strength vary from tens of microgauss to ~ a milligauss. The merits and weaknesses of the various estimations are discussed here. If the field strength is comparable to a milligauss, then the magnetic field is able to exert a strong influence on the dynamics of molecular clouds, on the collimation of a Galactic wind, and on the lifetimes and bulk motions of relativistic particles. Related to the question of field strength is the question of whether the field is pervasive throughout the central zone of the Galaxy, or whether its manifestations are predominantly localized phenomena. Current evidence favors the pervasive model.

astro-ph↗
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.

astro-ph↗
arXiv2005

High Spectral Resolution Observations of the Massive Stars in the Galactic Center

We present high-resolution near-infrared spectra, obtained with the NIRSPEC spectrograph on the W. M. Keck II Telescope, of a collection of hot, massive stars within the central 25 arcseconds of the Galactic center. We have identified a total of twenty-one emission-line stars, seven of which are new radial velocity detections with five of those being classified as He I emission-line stars for the first time. These stars fall into two categories based on their spectral properties: 1) those with narrow 2.112, 2.113 micron He I doublet absorption lines, and 2) those with broad 2.058 micron He I emission lines. These data have the highest spectral resolution ever obtained for these sources and, as a result, both components of the absorption doublet are separately resolved for the first time. We use these spectral features to measure radial velocities. The majority of the measured radial velocities have relative errors of 20 kms, smaller than those previously obtained with proper-motion or radial velocity measurements for similar stellar samples in the Galactic center. The radial velocities estimated from the He I absorption doublet are more robust than those previously estimated from the 2.058 micron emission line, since they do not suffer from confusion due to emission from the surrounding ISM. Using this velocity information, we agree that the stars are orbiting in a somewhat coherent manner but are not as defined into a disk or disks as previously thought. Finally, multi-epoch radial velocity measurements for IRS 16NE show a change in its velocity presumably due to an unseen stellar companion.

astro-ph↗
arXiv2005

The Double Helix Nebula: a magnetic torsional wave propagating out of the Galactic centre

Radioastronomical studies have indicated that the magnetic field in the central few hundred parsecs of our Milky Way Galaxy has a dipolar geometry and a strength substantially larger than elsewhere in the Galaxy, with estimates ranging up to a milligauss. A strong, large-scale magnetic field can affect the Galactic orbits of molecular clouds by exerting a drag on them, it can inhibit star formation, and it can guide a wind of cosmic rays away from the central region, so a characterization of the magnetic field at the Galactic center is important for understanding much of the activity there. Here, we report Spitzer Space Telescope observations of an unprecedented infrared nebula having the morphology of an intertwined double helix. This feature is located about 100 pc from the Galaxy's dynamical centre toward positive Galactic latitude, and its axis is oriented perpendicular to the Galactic plane. The observed segment is about 25 pc in length, and contains about 1.25 full turns of each of the two continuous, helically wound strands. We interpret this feature as a torsional Alfven wave propagating vertically away from the Galactic disk, driven by rotation of the magnetized circumnuclear gas disk. As such, it offers a new morphological probe of the Galactic center magnetic field. The direct connection between the circumnuclear disk and the double helix is ambiguous, but the MSX images show a possible meandering channel that warrants further investigation.

astro-ph↗
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.

astro-ph↗
arXiv2005

A Candidate Neutron Star Associated with Galactic Center Supernova Remnant Sagittarius A East

We present imaging and spectral studies of the supernova remnant (SNR) Sagittarius (Sgr) A East from deep observations with the {\it Chandra X-Ray Observatory}. The spatially-resolved spectral analysis of Sgr A East reveals the presence of a two-temperature thermal plasma ($kT$ $\sim$ 1 keV and 5 keV) near the center of the SNR. The central region is dominated by emission from highly-ionized Fe-rich ejecta. We estimate a conservative upper limit on the total Fe ejecta mass of the SNR, M$_{Fe}$ $<$ 0.27 M$_{\odot}$. Comparisons with standard SN nucleosynthesis models suggest that this Fe mass limit is consistent with a Type II SN explosion for the origin of Sgr A East. On the other hand, the soft X-ray emission extending toward the north of the SNR can be described by a single-temperature ($kT$ $\sim$ 1.3 keV) thermal plasma with normal chemical composition. This portion of the SNR is thus X-ray emission from the heated interstellar medium rather than the metal-rich stellar ejecta. We point out that a hard pointlike source CXOGC J174545.5$-$285829 (the so-called ``cannonball'') at the northern edge of the SNR shows unusual X-ray characteristics among other Galactic center sources. The morphological, spectral, and temporal characteristics of this source suggest an identification as a high-velocity neutron star. Based on the suggested Type II origin for the SNR Sgr A East and the proximity between the two, we propose that CXOGC J174545.5$-$285829 is a high-velocity neutron star candidate, born from the core-collapse SN which also created the SNR Sgr A East.

astro-ph↗
arXiv2005

Intra-day Variability of Sagittarius A* at 3 Millimeters

We report observations and analysis of flux monitoring of Sagittarius A* at 3-mm wavelength using the OVRO millimeter interferometer over a period of eight days (2002 May 23-30). Frequent phase and flux referencing (every 5 minutes) with the nearby calibrator source J1744-312 was employed to control for instrumental and atmospheric effects. Time variations are sought by computing and subtracting, from each visibility in the database, an average visibility obtained from all the data acquired in our monitoring program having similar uv spacings. This removes the confusing effects of baseline-dependent, correlated flux interference caused by the static, thermal emission from the extended source Sgr A West. Few-day variations up to ~20% and intra-day variability of \~20% and in some cases up to ~40% on few-hour time scales emerge from the differenced data on SgrA*. Power spectra of the residuals indicate the presence of hourly variations on all but two of the eight days. Monte Carlo simulation of red-noise light curves indicates that the hourly variations are well described by a red-noise power spectrum with P(f) ~ f^(-1). Of particular interest is a ~2.5 hour variation seen prominently on two consecutive days. An average power spectrum from all eight days of data reveals noteworthy power on this time scale. There is some indication that few-hour variations are more pronounced on days when the average daily flux is highest. We briefly discuss the possibility that these few-hour variations are due to the dynamical modulation of accreting gas around the central supermassive black hole, as well as the implications for the structure of the SgrA* photosphere at 3 mm. Finally, these data have enabled us to produce a high sensitivity 3-mm map of the extended thermal emission surrounding SgrA*.

astro-ph↗
arXiv2004

350 micron Galactic Center Dust Observations with SHARC II

We present first and preliminary submillimeter continuum images of the Galactic Center region obtained with the new Caltech Submillimeter Observatory facility camera SHARC II. The instrument allows 350 micron observations with unprecedented sensitivity and instantaneous spatial coverage. The stability of the SHARC II detectors combined with the large number of available pixels allows a non-differential scanning mode that does not require the observation of a reference off-source position. Here we present large-scale 350 micron dust continuum images from the Sgr A and Sgr C regions, as well as the detection of sub-mm dust continuum emission from the IRAS 25 micron source AFGL5376. This source is produced in a large-scale shock that extends well above the Galactic plane.

astro-ph↗
arXiv2004

Kinematic and structural analysis of the Minispiral in the Galactic Center from BEAR spectro-imagery

Integral field spectroscopy of the inner region of the Galactic Center, over a field of roughly 40"x40" was obtained at 2.06 microns (He I) and 2.16 microns (Brackett-gamma) using BEAR, an imaging Fourier Transform Spectrometer, at spectral resolutions respectively of 52.9 km/s and 21.3 km/s, and a spatial resolution of ~0.5". The analysis of the data was focused on the kinematics of the gas flows, traditionally called the "Minispiral", concentrated in the neighborhood of the central black hole, Sgr A*. From the decomposition into several velocity components of the line profile extracted at each point of the field, velocity features were identified. Nine distinguishable structures are described: the standard Northern Arm, Eastern Arm, Bar, Western Arc, and five additional, coherently-moving patches of gas. From this analysis, the Northern Arm appears not limited, as usually thought, to the bright, narrow North-South lane seen on intensity images, but it instead consists of a weak, continuous, triangular-shaped surface, drawn out into a narrow stream in the vicinity of Sgr A* where it shows a strong velocity gradient, and a bright western rim. The Eastern Arm is split into three components. We also report extinction of some interstellar structures by others, providing information on their relative position along the line of sight. A system of Keplerian orbits can be fitted to most of the Northern Arm, and the bright rim of this feature can be interpreted in terms of line-of-sight orbit crowding caused by the warping of the flowing surface at the western edge facing Sgr A*. The question of the origin of the ionized gas is addressed and a discussion of the lifetime of these features is presented.

astro-ph↗
arXiv2004

Thermal-IR Detection of Optical Outflow Sources in OMC1 South

We present the first thermal-infrared imaging photometry for several embedded sources in the OMC1 South cloud core in the Orion nebula, and propose that some of these drive optical Herbig-Haro jets emerging from the region. Thermal-infrared images at 8.8 and 11.7 micron obtained at Gemini South show a handful of sources in OMC1-S with no visual-wavelength counterparts, although a few can be seen in recent near-infrared data. For the three brightest mid-infrared sources, we also present 18.75 micron photometry obtained with the Keck telescope. The most prominent blueshifted outflows in the Orion nebula at visual wavelengths such as HH202, HH203/204, HH529, and HH269 all originate from OMC1-S. The brightest infrared source in OMC1-S at 11.7 micron is located at the base of the prominent jet that powers HH202 and is likely to be the sought-after driver of this outflow. The second brightest infrared source is located at the base of the HH529 jet. We consider the possibility that HH203/204 and HH269 trace parts of a single bent outflow from the third-brightest infrared source. While there may be some lingering ambiguity about which infrared stars drive specific jets, there is now a sufficient number of embedded sources to plausibly account for the multiple outflows from OMC1-S.

astro-ph↗
arXiv2004

Dynamical Friction of Galactic Center Star Clusters with an Intermediate-Mass Black Hole

Numerical simulations of the dynamical friction suffered by a Galactic center star cluster harboring an intermediate-mass black hole (IMBH) have been performed. Gerhard has suggested that dynamical friction, which causes a cluster to lose orbital energy and spiral in toward the Galactic center, may explain the presence of a cluster of very young stars in the central parsec, where star formation might be prohibitively difficult because of strong tidal forces. However, numerical simulations by Kim & Morris showed that this is only possible if the cluster initially has an extremely dense core. Hansen & Milosavljevic recently suggested that the presence of an IMBH in the cluster core might stabilize the core against tidal disruption during the inspiral through dynamical friction, and thus might easily deliver young stars down to the central parsec. We find that the presence of an IMBH does lower the minimum initial core density required to transport young stars down to the central parsec, but this is possible only when the mass of the IMBH is at least ~10% of the total cluster mass. This fraction is significantly higher than that estimated by Portegies Zwart & McMillan with numerical simulations of IMBH formation by successive merging of stars in the cluster core, so it does not appear that a realistic IMBH can help transport young stars into the central parsec.

astro-ph↗
arXiv2004

A new mid-infrared map of the BN/KL region using the Keck telescope

We present a new mid-infrared (12.5micron) map of the BN/KL high-mass star-forming complex in Orion using the LWS instrument at Keck I. Despite poor weather we achieved nearly diffraction-limited images (FWHM = 0.38'') over a roughly 25'' X 25'' region centered on IRc2 down to a flux limit of ~250 mJy. Many of the known infrared (IR) sources in the region break up into smaller sub-components. We have also detected 6 new mid-IR sources. Nearly all of the sources are resolved in our mosaic. The near-IR source ''n'' is slightly elongated in the mid-IR along a NW--SE axis and perfectly bisects the double-peaked radio source ''L''. Source n has been identified as a candidate for powering the large IR luminosity of the BN/KL region (L = 10^5 L_sun). We postulate that the 12 micron emission arises in a circumstellar disk surrounding source n. The morphology of the mid-IR emission and the Orion ''hot core'' (as seen in NH_3 emission), along with the location of water and OH masers, is very suggestive of a bipolar cavity centered on source n and aligned with the rotation axis of the hypothetical circumstellar disk. IRc2, once thought to be the dominant energy source for the BN/KL region, clearly breaks into 4 sub-sources in our mosaic, as seen previously at 3.8 -- 5.0 micron. The anti-correlation of mid-IR emission and NH_3 emission from the nearby hot core indicates that the IRc2 sources are roughly coincident (or behind) the dense hot core. The nature of IRc2 is not clear: neither self-luminous sources (embedded protostars) nor external heating by source I can be definitively ruled out. We also report the discovery of a new arc-like feature SW of the BN object, and some curious morphology surrounding near-IR source ''t".

astro-ph↗
arXiv2003

X-Ray Emission from the Pre-Planetary Nebula Henize 3-1475

We report the first detection of X-ray emission in a pre-planetary nebula, Hen 3-1475. Pre-planetary nebulae are rare objects in the short transition stage between the Asymptotic Giant Branch and planetary nebula evolutionary phases, and Hen 3-1475, characterised by a remarkable S-shaped chain of optical knots, is one of the most noteworthy members of this class. Observations with the Advanced CCD Imaging Spectrometer (ACIS) onboard the Chandra X-Ray observatory show the presence of compact emission coincident with the brightest optical knot in this bipolar object, which is displaced from the central star by 2.7 arcsec along the polar axis. Model fits to the X-ray spectrum indicate an X-ray temperature and luminosity, respectively, of (4.3-5.7) 10^6 K and (4+/-1.4) 10^{31} (D/5 kpc)^2 erg s^{-1}, respectively. Our 3-sigma upper limit on the luminosity of compact X-ray emission from the central star in Hen 3-1475 is ~5 10^{31} (D/5 kpc)^2 erg s^{-1}. The detection of X-rays in Hen 3-1475 is consistent with models in which fast collimated post-AGB outflows are crucial to the shaping of planetary nebulae; we discuss such models in the context of our observations.

astro-ph↗
arXiv2003

Characteristics of Diffuse X-Ray Line Emission within 20 pc of the Galactic Center

Over the last 3 yrs, the Galactic center (GC) region has been monitored with the Chandra X-Ray Observatory. With 11 Chandra observations through 2002 June, the total effective exposure reaches ~590 ks, providing significant photon statistics on the faint, filamentary, diffuse X-ray emission. The true-color X-ray image and the equivalent width (EW) images for the detected elemental species demonstrate that the diffuse X-ray features have a broad range of spatio-spectral properties. Enhancements of the low-ionization-state, or ``neutral'' Fe line emission (E~6.4 keV) to the northeast of Sgr A* can be interpreted as fluorescence within the dense ISM resulting from irradiation by hard, external X-ray sources. They may also be explained by emission induced by the bombardments by high energy particles on the ISM, such as unresolved supernova (SN) ejecta intruding into dense ISM. The detection of molecular cloud counterparts to the 6.4 keV Fe line features indicates that these Fe line features are associated with dense GC clouds and/or active star-forming regions, which supports the X-ray reflection and/or SN ejecta origins for the Fe line emission. We detect highly ionized S and Si lines which are generally coincident with the neutral Fe line emission and the dense molecular clouds in the northeast of Sgr A*. These hot plasmas are likely produced by massive star-forming activities and/or SNRs. In contrast, we find that highly ionized He-like Fe line emission (E~6.7 keV) is primarily distributed along the plane instead of being concentrated in the northeast of Sgr A*. The implied high temperature and the alignment along the plane are consistent with the magnetic confinement model.

astro-ph↗
arXiv2003

An Extended Star Formation History for the Galactic Center from Hubble Space Telescope/NICMOS Observations

We present Hubble Space Telescope (HST) Near-Infrared Camera and Multiobject Spectrometer (NICMOS) observations as evidence that continuous star formation has created much of the central stellar cusp of the Galaxy. The data are the deepest ever obtained for a Galactic Center (GC) population, being $>$50% complete for \mnk$<19.3$, or initial stellar masses $\gtrsim$2 \Msun. We use Geneva and Padova stellar evolution models to produce synthetic luminosity functions for burst and continuous star formation scenarios, finding that the observations are fit best by continuous star formation at a rate that is consistent with the recent star formation activity that produced the three massive young clusters in the central 50 \pc. Further, it is not possible to fit the observations with ancient burst models, such as would be appropriate for an old population like that in Baade's Window or NGC6528.

astro-ph↗