Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userCO in distantly active comets
83
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Activity of most comets near the Sun is dominated by sublimation of frozen water, the most abundant ice in comets. Some comets, however, are active well beyond the water-ice sublimation limit of ~3 AU. Three bodies dominate the observational record and modeling efforts for distantly active comets: the long-period comet C/1995 O1 Hale-Bopp and the short-period comets (with Centaur orbits) 29P/Schwassmann Wachmann 1 and 2060 Chiron. We summarize what is known about these three objects emphasizing their gaseous comae. We calculate their CN/CO and CO2/CO production rate ratios from the literature and discuss implications. Using our own data we derive CO production rates for all three objects, in order to examine a correlation between gas production and different orbital histories and/or size. We find that orbital history does not appear to play a significant role in explaining 29Ps CO production rates. 29P outproduces Hale-Bopp at the same heliocentric distance, even though it has been subjected to much more solar heating. Previous modeling work on such objects predicts that 29P should have been de-volatilized over a fresher comet like Hale-Bopp. This may point to 29P having a different orbital history than current models predict, with its current orbit acquired more recently. On the other hand, Chirons CO measurements are consistent with it being significantly depleted over its original state, perhaps due to increased radiogenic heating made possible by its much larger size or its higher processing due to orbital history. Observed spectral line profiles are consistent with development and sublimation of icy grains at about 5-6 AU for 29P and Hale-Bopp, and this is probably a common feature in distantly active comets, and an important source of volatiles for all comets within 5 AU. In contrast, the narrow CO line profiles indicate a nuclear, and not extended, origin for CO beyond ~4 AU.
rate research
Read More
(60558) 174P/Echeclus is an unusual object that belongs to a class of minor planets called Centaurs, which may be intermediate between Kuiper Belt Objects and Jupiter Family comets. It is sporadically active throughout its orbit at distances too far for water ice to sublimate, the source of activity for most comets. Thus, its coma must be triggered by another mechanism. In 2005, Echeclus had a strong outburst with peculiar behavior that raised questions about the nucleus homogeneity. In order to test nucleus models, we performed the most sensitive search to date for the highly volatile CO molecule via its J=2-1 emission toward Echeclus during 2016 May-June (at 6.1 astronomical units from the Sun) using the Arizona Radio Observatory 10-m Submillimeter Telescope. We obtained a 3.6-sigma detection with a slightly blue-shifted (delta v = -0.55 +- 0.1 km/s) and narrow (FWHM = 0.53 +- 0.23 km/s) line. The data are consistent with emission from a cold gas from the sunward side of the nucleus, as seen in two other comets at 6 AU. We derive a production rate of Q(CO) = (7.7 +- 3.3)x10^26 mol/s, which is capable of driving the estimated dust production rates. Echeclus CO outgassing rate is ~40 times lower than what is typically seen for another Centaur at this distance, 29P/Schwassmann-Wachmann 1. We also used the IRAM 30-m telescope to search for the CO J=2-1 line, and derive an upper limit that is above the SMT detection. Compared to the relatively unprocessed comet C/1995 O1 (Hale-Bopp), Echeclus produces significantly less CO, as do Chiron and four other Centaurs.
Context: Surveys in the visible and near-infrared spectral range have revealed the presence of low-albedo asteroids in cometary like orbits (ACOs). In contrast to Jupiter family comets (JFCs), ACOs are inactive, but possess similar orbital parameters. Aims: In this work, we discuss why ACOs are inactive, whereas JFCs show gas-driven dust activity, although both belong to the same class of primitive solar system bodies. Methods: We hypothesize that ACOs and JFCs have formed under the same physical conditions, namely by the gravitational collapse of ensembles of ice and dust aggregates. We use the memory effect of dust-aggregate layers under gravitational compression to discuss under which conditions the gas-driven dust activity of these bodies is possible. Results: Owing to their smaller sizes, JFCs can sustain gas-driven dust activity much longer than the bigger ACOs, whose sub-surface regions possess an increased tensile strength, due to gravitational compression of the material. The increased tensile strength leads to the passivation against dust activity after a relatively short time of activity. Conclusions: The gravitational-collapse model of the formation of planetesimals, together with the gravitational compression of the sub-surface material simultaneously, explains the inactivity of ACOs and the gas-driven dust activity of JFCs. Their initially larger sizes means that ACOs possess a higher tensile strength of their sub-surface material, which leads to a faster termination of gas-driven dust activity. Most objects with radii larger than $2 , mathrm{km}$ have already lost their activity due to former gravitational compression of their current surface material.
Recent ALMA observations present mounting evidence for the presence of exocometary gas released within Kuiper belt analogues around nearby main sequence stars. This represents a unique opportunity to study their ice reservoir at the younger ages when volatile delivery to planets is most likely to occur. We here present the detection of CO J=2-1 emission co-located with dust emission from the cometary belt in the 440 Myr-old Fomalhaut system. Through spectro-spatial filtering, we achieve a 5.4$sigma$ detection and determine that the rings sky-projected rotation axis matches that of the star. The CO mass derived ($0.65-42 times10^{-7}$ M$_{oplus}$) is the lowest of any circumstellar disk detected to date, and must be of exocometary origin. Using a steady state model, we estimate the CO+CO$_2$ mass fraction of exocomets around Fomalhaut to be between 4.6-76%, consistent with Solar System comets and the two other belts known to host exocometary gas. This is the first indication of a similarity in cometary compositions across planetary systems that may be linked to their formation scenario and is consistent with direct ISM inheritance. In addition, we find tentative evidence that $(49pm 27)$% of the detected flux originates from a region near the eccentric belts pericentre. If confirmed, the latter may be explained through a recent impact event or CO pericentre glow due to exocometary release within a steady state collisional cascade. In the latter scenario, we show how the azimuthal dependence of the CO release rate leads to asymmetries in gas observations of eccentric exocometary belts.
A fundamental question in cometary science is whether the different dynamical classes of comets have different chemical compositions, which would reflect different initial conditions. From the ground or Earth orbit, radio and infrared spectroscopic observations of a now significant sample of comets indeed reveal deep differences in the relative abundances of cometary ices. However, no obvious correlation with dynamical classes is found. Further results come, or are expected, from space exploration. Such investigations, by nature limited to a small number of objects, are unfortunately focussed on short-period comets (mainly Jupiter-family). But these in situ studies provide ground truth for remote sensing. We discuss the chemical differences in comets from our database of spectroscopic radio observations, which has been recently enriched by several Jupiter-family and Halley-type comets.
Comets spend most of their lives at large distances from any star, during which time their interior compositions remain relatively unaltered. Cometary observations can therefore provide direct insight into the chemistry that occurred during their birth at the time of planet formation. To-date, there have been no confirmed observations of parent volatiles (gases released directly from the nucleus) of a comet from any planetary system other than our own. Here we present high-resolution, interferometric observations of 2I/Borisov, the first confirmed interstellar comet, obtained using the Atacama Large Millimeter/submillimeter Array (ALMA) on 15th-16th December 2019. Our observations reveal emission from hydrogen cyanide (HCN), and carbon monoxide (CO), coincident with the expected position of 2I/Borisovs nucleus, with production rates Q(HCN)=$(7.0pm1.1)times10^{23}$ s$^{-1}$ and Q(CO)=$(4.4pm0.7)times10^{26}$ s$^{-1}$. While the HCN abundance relative to water (0.06-0.16%) appears similar to that of typical, previously observed comets in our Solar System, the abundance of CO (35-105%) is among the highest observed in any comet within 2 au of the Sun. This shows that 2I/Borisov must have formed in a relatively CO-rich environment - probably beyond the CO ice-line in the very cold, outer regions of a distant protoplanetary accretion disk, as part of a population of small, icy bodies analogous to our Solar Systems own proto-Kuiper Belt.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
