Context.
29P/Schwassmann-Wachmann 1 is a distant Centaur/comet, showing persistent CO-driven activity and frequent outbursts.
Aims.
We aim to better characterize its gas and dust activity from ...multiwavelength observations performed during outbursting and quiescent states.
Methods.
We used the HIFI, PACS and SPIRE instruments of the
Herschel
space observatory on several dates in 2010, 2011, and 2013 to observe the H
2
O 557 GHz and NH
3
573 GHz lines and to image the dust coma in the far-infrared. Observations with the IRAM 30 m telescope were undertaken in 2007, 2010, 2011, and 2021 to monitor the CO production rate through the 230 GHz line, and to search for HCN at 89 GHz. The 70 and 160 µm PACS images were used to measure the thermal flux from the nucleus and the dust coma. Modeling was performed to constrain the size of the sublimating icy grains and to derive the dust production rate.
Results.
HCN is detected for the first time in comet 29P (at 5
σ
in the line area). H
2
O is detected as well, but not NH
3
. H
2
O and HCN line shapes differ strongly from the CO line shape, indicating that these two species are released from icy grains. CO production rates are in the range (2.9–5.6) × 10
28
s
−1
(1400–2600 kg s
−1
). A correlation between the CO production rate and coma brightness is observed, as is a correlation between CO and H
2
O production. The correlation obtained between the excess of CO production and excess of dust brightness with respect to the quiescent state is similar to that established for the continuous activity of comet Hale-Bopp. The measured
Q
(H
2
O)/
Q
(CO) and
Q
(HCN)/
Q
(CO) production rate ratios are 10.0 ± 1.5 % and 0.12 ± 0.03 %, respectively, averaging the April-May 2010 measurements (
Q
(H
2
O) = (4.1 ± 0.6) × 10
27
s
−1
,
Q
(HCN) = (4.8 ± 1.1) × 10
25
s
−1
). We derive three independent and similar values of the effective radius of the nucleus, ~31 ± 3 km, suggesting an approximately spherical shape. The inferred dust mass-loss rates during quiescent phases are in the range 30–120 kg s
−1
, indicating a dust-to-gas mass ratio <0.1 during quiescent activity. We conclude that strong local heterogeneities exist on the surface of 29P, with quenched dust activity from most of the surface, but not in outbursting regions.
Conclusions.
The volatile composition of the atmosphere of 29P strongly differs from that of comets observed within 3 au from the Sun. The observed correlation between CO, H
2
O and dust activity may provide important constraints for the outburst-triggering mechanism.
Aims. We present optical photometry and spectroscopy of the afterglow and host galaxy of gamma-ray burst GRB 040924. This GRB had a rather short duration of T90 ~2.4 s, and a well sampled optical ...afterglow light curve. We aim to use this dataset to find further evidence that this burst is consistent with a massive star core-collapse progenitor. Methods. We combine the afterglow data reported here with those from the literature and compare the host properties with survey data. Results. We find that the global behaviour of the optical afterglow is well fit by a broken power-law, with a break at ~0.03 days. We determine the redshift z = 0.858 ± 0.001 from the detected emission lines in our spectrum. Using the spectrum and photometry we derive global properties of the host, showing it to have similar properties to other long GRB hosts. We detect the Ne III emission line in the spectrum, and compare the fluxes of this line of a sample of 15 long GRB host galaxies with survey data, showing the long GRB hosts to be comparable to local metal-poor emission line galaxies in their Ne III emission. We fit the supernova bump accompanying this burst, and find that it is similar to other long GRB supernova bumps, but fainter. Conclusions. All properties of GRB 040924 (the associated supernova, the spectrum and SED of host and afterglow) are consistent with an origin in the core-collapse of a massive star.
Episodic dust formation by HD 192641 (WR 137) – II Williams, P. M.; Kidger, M. R.; van der Hucht, K. A. ...
Monthly notices of the Royal Astronomical Society,
06/2001, Letnik:
324, Številka:
1
Journal Article
Recenzirano
Odprti dostop
We present new infrared photometry of the WC7-type Wolf–Rayet star HD 192641 (WR 137) from 1985 to 1999. These data track the cooling of the dust cloud formed in the 1982–84 dust-formation episode ...from 1985 to 1991, the increase of the infrared flux from 1994.5 to a new dust-formation maximum in 1997 and its subsequent fading. From these and earlier data we derive a period of 4765±50 d(13.05±0.15 yr) for the dust-formation episodes. Between dust-emission episodes, the infrared spectral energy distribution has the form of a power law, λF∝λ−1.86. The rising branch of the infrared light curve (1994–97) differs in form from that of the episodic dust-maker WR 125. Time-dependent modelling shows that this difference can be attributed to a different time dependence of dust formation in WR 137, which occurred approximately ∝t2 until maximum, whereas that of WR 125 could be described by a step function, akin to a threshold effect. For an adopted distance of 1.6 kpc, the rate of dust formation was found to be 5.0×10−8 M⊙ yr−1 at maximum, accounting for a fraction fC≈1.5×10−3 of the carbon flowing in the stellar wind. The fading branches of the light curves show evidence for secondary ‘mini-eruptions’ in 1987, 1988 and 1990, behaviour very different from that of the prototypical episodic dust-maker HD 193793 (WR 140), and suggesting the presence in the WR 137 stellar wind of large-scale structures that are crossed by the wind–wind collision region.
Context. The goal of the Herschel open time programme “TNOs are Cool!” is to derive the physical and thermal properties for a large sample of Centaurs, and trans-Neptunian objects (TNOs), including ...resonant, classical, detached and scattered disk objects. Aims. Based on observations of two targets we tried (i) to optimise the SPIRE observing technique for faint (close to the background confusion noise), slowly moving targets; (ii) to test different thermal model techniques; (iii) to determine radiometric diameter and albedo values; (iv) to compare with Spitzer results whenever possible. Methods. We obtained SPIRE photometry on two targets and PACS photometry on one of the targets. Results. We present results for the two targets, (90482) Orcus and (136472) Makemake, observed with SPIRE and for one of those targets, Makemake, observed with PACS. We adopt pV = 0.27 and D = 850 km as our best estimate of the albedo and diameter of Orcus using single terrain models. With two-terrain models for Makemake, the bright terrain is fitted by, 0.78 < pV < 0.90, and the dark terrain 0.02 < pV < 0.12, giving 1360 < D < 1480 km. Conclusions. A single terrain model was derived for Orcus through the SPIRE photometry combined with MIPS data. The Makemake data from MIPS, PACS and SPIRE combined are not compatible with a single terrain model, but can be modelled with a two-terrain fit. These science demonstration observations have shown that the scanning technique, which allows us to judge the influence of background structures, has proved to be a good basis for this key programme.
Thermal emission from Kuiper belt object (136108) Haumea was measured with Herschel–PACS at 100 μm and 160 μm for almost a full rotation period. Observations clearly indicate a 100 μm thermal ...lightcurve with an amplitude of a factor of ~2, which is positively correlated with the optical lightcurve. This confirms that both are primarily due to shape effects. A 160 μm lightcurve is marginally detected. Radiometric fits of the mean Herschel- and Spitzer- fluxes indicate an equivalent diameter D ~ 1300 km and a geometric albedo pv ~ 0.70–0.75. These values agree with inferences from the optical lightcurve, supporting the hydrostatic equilibrium hypothesis. The large amplitude of the 100 μm lightcurve suggests that the object has a high projected a/b axis ratio (~1.3) and a low thermal inertia as well as possible variable infrared beaming. This may point to fine regolith on the surface, with a lunar-type photometric behavior. The quality of the thermal data is not sufficient to clearly detect the effects of a surface dark spot.
Cryovolcanic activity near the south pole of Saturn’s moon Enceladus produces plumes of H2O-dominated gases and ice particles, which escape and populate a torus-shaped cloud. Using submillimeter ...spectroscopy with Herschel, we report the direct detection of the Enceladus water vapor torus in four rotational lines of water at 557, 987, 1113, and 1670 GHz, and probe its physical conditions and structure. We determine line-of-sight H2O column densities of ~4 × 1013 cm-2 near the equatorial plane, with a ~50 000 km vertical scale height. The water torus appears to be rotationally cold (e.g. an excitation temperature of 16 K is measured for the 1113 GHz line) but dynamically excited, with non-Keplerian dispersion velocities of ~2 km s-1, and appears to be largely shaped by molecular collisions. From estimates of the influx rates of torus material into Saturn and Titan, we infer that Enceladus’ activity is likely to be the ultimate source of water in the upper atmosphere of Saturn, but not in Titan’s.
We report on an initial analysis of Herschel/HIFI observations of hydrogen chloride (HCl), hydrogen peroxide (H2O2), and molecular oxygen (O2) in the Martian atmosphere performed on 13 and 16 April ...2010 (Ls ~ 77°). We derived a constant volume mixing ratio of 1400 ± 120 ppm for O2 and determined upper limits of 200 ppt for HCl and 2 ppb for H2O2. Radiative transfer model calculations indicate that the vertical profile of O2 may not be constant. Photochemical models determine the lowest values of H2O2 to be around Ls ~ 75° but overestimate the volume mixing ratio compared to our measurements.
Context. Cometary activity at large heliocentric distances is thought to be driven by outgassing of molecular species more volatile than water that are present in the nucleus. The long-period comet ...C/2006 W3 (Christensen) was an exceptional target for a detailed study of its distant gaseous and dust activity. Aims. We aimed to measure the H2O and dust production rates in C/2006 W3 (Christensen) with the Herschel Space Observatory at a heliocentric distance of ~5 AU and compared these data with previous post-perihelion Herschel and ground-based observations at ~3.3 AU from the Sun. Methods. We have searched for emission in the HO and NH3 ground-state rotational transitions, JKaKc (110–101) at 557 GHz and JK (10–00) at 572 GHz, simultaneously, toward comet C/2006 W3 (Christensen) with the Heterodyne Instrument for the Far Infrared (HIFI) onboard Herschel on UT 1.5 September 2010. Photometric observations of the dust coma in the 70 μm and 160 μm channels were acquired with the Photodetector Array Camera and Spectrometer (PACS) instrument on UT 26.5 August 2010. Results. A tentative 4σ H2O line emission feature was found in the spectra obtained with the HIFI wide-band and high-resolution spectrometers, from which we derive a water production rate of (2.0 ± 0.5) × 1027 molec s-1. A 3σ upper limit for the ammonia production rate of <1.5 × 1027 molec s-1 is obtained taking into account the contribution from all hyperfine components. The dust thermal emission was detected in the 70 μm and 160 μm filters, with a more extended emission in the blue channel. We fit the radial dependence of the surface brightness with radially symmetric profiles for the blue and red bands. The dust production rates, obtained for a dust size distribution index that explains the fluxes at the photocenters of the 70 μm and 160 μm PACS images, lie in the range 70 kg s-1 to 110 kg s-1. Scaling the CO production rate measured post-perihelion at 3.20 AU and 3.32 AU, these values correspond to a dust-to-gas production rate ratio in the range 0.3–0.4. Conclusions. The blueshift of the water line detected by HIFI suggests preferential emission from the subsolar point. However, it is also possible that water sublimation occurs in small ice-bearing grains that are emitted from an active region on the nucleus surface at a speed of ~0.2 km s-1. The dust production rates derived in August 2010 are roughly one order of magnitude lower than in September 2009, suggesting that the dust-to-gas production rate ratio remained approximately constant during the period when the activity became increasingly dominated by CO outgassing.
Context. Hydrogen chloride (HCl) and hydrogen fluoride (HF) are expected to be the main reservoirs of fluorine and chlorine over a wide range of conditions, wherever hydrogen is predominantly ...molecular. They are found to be strongly depleted in dense molecular clouds, suggesting freeze-out onto grains in such cold environments. We can then expect that HCl and HF were also the major carriers of Cl and F in the gas and icy phases of the outer solar nebula, and were incorporated into comets. Aims. We aimed to measure the HCl and HF abundances in cometary ices as they can provide insights on the halogen chemistry in the early solar nebula. Methods. We searched for the J(1−0) lines of HCl and HF at 626 and 1232 GHz, respectively, using the Heterodyne Instrument for the Far-Infrared (HIFI) onboard the Herschel Space Observatory. HCl was searched for in comets 103P/Hartley 2 and C/2009 P1 (Garradd), whereas observations of HF were conducted in comet C/2009 P1 (Garradd). In addition, observations of H2O and H218O lines were performed in C/2009 P1 (Garradd) to measure the H2O production rate at the time of the HCl and HF observations. Three lines of CH3OH were serendipitously observed in the HCl receiver setting. Results. HCl is not detected, whereas a marginal (3.6-σ) detection of HF is obtained. The upper limits for the HCl abundance relative to water are 0.011% and 0.022%, for comet 103P/Hartley 2 and C/2009 P1 (Garradd), respectively, showing that HCl is depleted with respect to the solar Cl/O abundance by a factor more than 6\hbox{$^{+6}_{-3}$}+6-3 in 103P/Hartley 2, where the error is related to the uncertainty in the chlorine solar abundance. The marginal HF detection obtained in C/2009 P1 (Garradd) corresponds to an HF abundance relative to water of (1.8 ± 0.5) × 10-4, which is approximately consistent with a solar photospheric F/O abundance. The inferred water production rate in comet C/2009 P1 (Garradd) is (1.1 ± 0.3) × 1029 s-1 and (0.75 ± 0.05) × 1029 s-1 on 17 and 23 February 2012, respectively. CH3OH abundances relative to water are 2.7 ± 0.3% and 3.4 ± 0.6%, for comets 103P/Hartley 2 and C/2009 P1 (Garradd), respectively. Conclusions. The observed depletion of HCl suggests that HCl was not the main reservoir of chlorine in the regions of the solar nebula where these comets formed. HF was possibly the main fluorine compound in the gas phase of the outer solar nebula. However, this needs to be confirmed by future measurements.
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