Showing papers by "Nathan Smith published in 2003"

Mass and Kinetic Energy of the Homunculus Nebula around η Carinae

Abstract: We examine thermal infrared images of η Car at wavelengths from 4.8 to 25 μm obtained with the MIRAC3 camera system at the Magellan Observatory, and we conclude that η Car's circumstellar Homunculus Nebula contains much more mass (12 M⊙) and kinetic energy (1049.6–1050 ergs) than had previously been recognized. Excess far-infrared emission from a large mass of cold dust had been discovered recently by Morris et al., which they attributed to 110 K dust in a compact equatorial torus. Our images suggest, however, that roughly 10 M⊙ or more resides instead in the rapidly expanding polar lobes of the Homunculus, known to be ejected during η Car's "Great Eruption" in the 19th century. Combining this large mass with reliable expansion speeds for the polar lobes allows us to estimate the large kinetic energy and efficient momentum transfer associated with the eruption. Most mass resides in a cool outer layer of the lobes, with dust at ~140 K. This material is only revealed at 18–25 μm with high spatial resolution images that can separate adjacent dust components. Warmer dust at ~200 K that has been recognized for some time (responsible for previous mass estimates of 2–3 M⊙) blankets the inner surfaces of the mostly hollow polar lobes as they are irradiated more directly by the central engine.

Latitude-dependent Effects in the Stellar Wind of η Carinae

Abstract: The Homunculus reflection nebula around eta Carinae provides the rare opportunity to observe the spectrum of a star from more than one direction In the case of eta Car, the nebula's geometry is known well enough to infer how wind profiles vary with latitude We present STIS spectra of several positions in the Homunculus, showing directly that eta Car has an aspherical and axisymmetric stellar wind P Cygni absorption in Balmer lines depends on latitude, with relatively high velocities and strong absorption near the polar axis Stronger absorption at high latitudes is surprising, and it suggests higher mass flux toward the poles, perhaps resulting from equatorial gravity darkening on a rotating star Reflected profiles of He I lines are more puzzling, and offer clues to eta Car's wind geometry and ionization structure During eta Car's high-excitation state in March 2000, the wind had a fast, dense polar wind, with higher ionization at low latitudes Older STIS data obtained since 1998 reveal that this global stellar-wind geometry changes during eta Car's 55 year cycle, and may suggest that this star s spectroscopic events are shell ejections Whether or not a companion star triggers these outbursts remains ambiguous The most dramatic changes in the wind occur at low latitudes, while the dense polar wind remains relatively undisturbed during an event The apparent stability of the polar wind also supports the inferred bipolar geometry The wind geometry and its variability have critical implications for understanding the 55 year cycle and long-term variability, but do not provide a clear alternative to the binary hypothesis for generating eta Car s X-rays

Latitude-dependent effects in the stellar wind of Eta Carinae

Abstract: The Homunculus reflection nebula around eta Carinae provides a rare opportunity to observe the spectrum of a star from multiple latitudes. We present STIS spectra of several positions in the Homunculus, showing directly that eta Car has an aspherical stellar wind. P Cygni absorption in Balmer lines depends on latitude, with high velocities and strong absorption near the poles. Stronger absorption at high latitudes is surprising, and it suggests higher mass flux toward the poles, perhaps resulting from radiative driving with equatorial gravity darkening on a rotating star. Reflected profiles of He I lines are more puzzling, offering clues to the wind geometry and ionization structure. During eta Car's high-excitation state in March 2000, the wind was fast and dense at the poles, with higher ionization at low latitudes. Older STIS data obtained since 1998 reveal that this global stellar-wind geometry changes during eta Car's 5.5 year cycle, and may suggest that this star's spectroscopic events are shell ejections. Whether or not a companion star triggers these outbursts remains ambiguous. The most dramatic changes in the wind occur at low latitudes, while the dense polar wind remains relatively undisturbed during an event. The apparent stability of the polar wind also supports the inferred bipolar geometry. The wind geometry and its variability have critical implications for understanding the 5.5 year cycle and long-term variability, but do not provide a clear alternative to the binary hypothesis for generating eta Car's X-rays.

Discovery of a Little Homunculus within the Homunculus Nebula of η Carinae

Abstract: We report long-slit spectroscopic mapping of the η Carinae nebula obtained using the Space Telescope Imaging Spectrograph. The observations reveal the presence of a previously unknown bipolar emission nebula (roughly ±2'' along its major axis) embedded within the well-known and larger Homunculus Nebula. A preliminary analysis suggests that this embedded nebula may have originated from a minor eruption event circa 1890, 50 years after the formation of the larger Homunculus.

Numerous proplyd candidates in the harsh environment of the Carina Nebula

Abstract: We report the discovery of dozens of compact objects in the Carina Nebula (NGC 3372) that closely resemble proplyds (photoablating protoplanetary disks and dark silhouette disks) seen previously in the Orion Nebula. This is the first detection of a large number of such objects outside Orion. They imply that low- and intermediate-mass star formation is proceeding actively in Carina, despite threatening conditions imposed by very hot massive stars. The proplyd candidates that we have detected are larger than those in Orion but more compact than irregular molecular globules within the H II region. Smaller proplyds may still be lurking in Carina, waiting to be discovered. Orion apparently lacks objects of comparable size, and we consider several explanations. Larger proplyds may come from larger and more massive circumstellar disks surrounding young Herbig Ae/Be stars that should be numerous in Carina. Alternatively, far-UV radiation from the massive star η Car may have, in the recent past, enhanced photoablation and expanded proplyd ionization fronts. Some unusual proplyd candidates may also imply that time-dependent effects associated with η Car's current evolutionary state may be critical.

Spatial distribution of near‐infrared and optical emission properties in the bipolar nebula Menzel 3

Abstract: Ground-based optical spectra combined with near-infrared spectra and images of the young bipolar planetary nebula Menzel 3 (Mz 3) reveal positional variations in extinction, excitation, density and other characteristics. Interstellar extinction is probably less than A V = 2.65, while extinction toward the nucleus is approximately 4.6 mag. The lobes show stratified ionization, with high-excitation emission localized at high latitudes. Quantitative analysis using the CLOUDY spectral synthesis code suggests that the polar lobes 'see' an excitation source similar to a blackbody of roughly 36000 K and 10000 L O ., and chemical abundances reveal that the ejecta have He enhanced by a factor of ∼2, and N enhanced more strongly, with N/O∼1. The lobes are probably radiatively excited, but shocks may heat the 'blisters' at the polar axis. The bright, unresolved nucleus has a crowded emission spectrum distinct from the diffuse bipolar lobes, and it indicates high electron densities of 10 6 -10 7 cm - 3 . An equatorial disc-like geometry for dense gas in the nucleus is likely. The nucleus also shows a nearly power-law continuum; when corrected for reddening it may require two stars, one hot star and one cool giant, along with hot ∼900-K dust. A distance up to ∼2.5 kpc is possible. In general, the spectral characteristics of Mz 3 are similar to those of the well-studied nebula M 2-9, and the two objects may share a similar evolutionary history. However, an important difference between them is that Mz 3 appears to be mostly devoid of molecular hydrogen, while infrared H 2 lines are conspicuous in M 2-9.

The integrated optical spectrum of the Crab nebula and evidence for its fading synchrotron continuum

Abstract: A flux-calibrated optical spectrum integrated over the entire Crab nebula was obtained by making drift scans with a long-slit spectrograph. Compared to observations obtained over the past 40 years, these new data confirm an earlier controversial result that the [O III] λλ4959, 5007 equivalent width is increasing with time, although the rate of ∼0.9 per cent yr - 1 is somewhat slower than that measured previously. Additionally, the Hβ equivalent width is increasing at a comparable rate, but the measured fluxes of both Hβ and [O III] have changed less than their respective equivalent widths. The different rates of change in the measured fluxes and equivalent widths of these lines suggest that the optical synchrotron continuum from the Crab nebula is indeed fading rapidly. The apparent decline is consistent with a rate around -0.5 (′0.2) per cent yr - 1 at wavelengths near 5000 A inferred independently from measurements of the optical continuum flux during the same time period.

A gallery of cool hypergiants --- imaging their circumstellar environments

Abstract: High resolution imaging with HST-WFPc2 of seven of the most luminous evolved cool stars shows that the three OH/IR stars NML Cyg, VX Sgr and S Per have circumstellar nebulae while no ejecta was detected for the M-type supergiant J.tCep and the yellow hypergiants pCas, HR8752 and HR5171a. Cool hypergiants are highly unstable, evolved massive stars that lie just below the upper luminosity boundary in the H-R diagram with apparent temperatures ~ 10000 K (spectral types A to M). Observations of two of these stars, IRC+I0420 (Humphreys et al. 1997, 2002) and VYCMa (Smith et ol. 2001) have revealed very complex circumstellar environments with multiple arcs, rings, knots and jet-like features imbedded in more uniform, diffuse nebulae. We have obtained high-resolution, multi-wavelength images of seven additional very luminous cool stars, to better understand their mass loss histories and to sample them at a variety of stages in their brief evolution as cool hypergiants. Their positions on the H-R diagram is shown in Figure 1. The OH/IR M-type supergiants NML Cyg, VX Sgr, and S Per, the normal M-type supergiant j-tCep, and the 'yellow' hypergiants peas, HR8752 and HR5171a were imaged with HST-WFPc2 . The images were all processed with the standard routines in lRAF/STSDAS with the most recent reference files. The pSF-patched long exposures were drizzled to maximize the spatial resolution. The powerful OH/IR source NML eyg has a very obvious circumstellar nebula rv 500x 1000AU with a peculiar asymmetric bean shape, which may be due to interaction with the UV radiation and winds from the nearby Cyg OB2 association. The shape is similar to the theoretical contours calculated by Morris & Jura (1983) for the associated H II region, but on a much smaller scale. The equally well-studied OH/IR source VX Sgr is non-stellar with an image equal to HSTs diffraction limit at 0.5 usu, implying a size of ('oJ 100AU for its extended envelope. SPer has an elongated circumstellar envelope rv 800 AU across which unlike NML Cyg and VX Sgr does not obscure the star. However, we find no evidence for extended structure for pCes, HR8752, HR5171a and j.tCep within fi!l of the stars and down to the wFPc2 read noise rv 2"away. Cornelis de Jager (1998) has suggested that very massive stars evolving blueward encounter a region of increasing dynamical instability, that he calls the 'yellow void'. He specifically suggests that pCas, HR8752 and IRC+10420 are post-red supergiants and that all of the yellow hypergiants may be in a post-

The Mysterious Ring in the Open Cluster NGC 3572: Planetary Nebula or Photoevaporating Globule?

Abstract: We discuss optical and infrared emission from the putative planetary nebula in the young open cluster NGC 3572. Velocity images of [N ii] λ6583 obtained with the Rutgers/CTIO Fabry‐Perot interferometer reveal that most gas in the nebula is expanding at velocities ≲5 km s \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} ewcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} ormalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $^{-1}$ \end{document} , with marginal evidence for bipolar expansion. A few outer condensations are seen at faster redshifted velocities, but their origin is uncertain. Optical spectra reveal a spatial exc...