Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Observations of [O I] emission in Comets C/2014 Q2 (Lovejoy) and C/2007 N3 (Lulin): Possible Influence of Solar Activity on Oxygen Line Ratios

Ella J. Mayfield, Adam J. McKay, Michael S. P. Kelley, Anita L. Cochran

TL;DR

This study tests the validity of using the forbidden O I line ratio $R$ as a ground-based proxy for CO$_2$/H$_2$O in comets by observing Lovejoy near solar maximum and Lulin near solar minimum. It contrasts empirical and theoretical [O I] release rates, derives CO$_2$/H$_2$O from $R$, and benchmarks against space-based CO$_2$ measurements from Spitzer and AKARI. The findings show that empirical release rates accurately reproduce Lovejoy's CO$_2$ abundance but not consistently for Lulin, while theoretical rates underperform in both cases; solar activity likely modulates the photochemistry governing $O(^1S)$/$O(^1D)$ production, necessitating solar-cycle-aware calibrations. Overall, the work highlights both the promise and the limitations of the oxygen-line ratio method for inferring cometary CO$_2$ and underscores the need for a larger sample across solar activity levels to quantify this dependency.

Abstract

Observing [O I] emission to calculate an "oxygen line ratio" has been proposed as a potential proxy for direct CO$_2$ measurement in comets. However, the photochemistry governing [O I] release into the coma is not well understood, and using theoretical release rates often yields different results than using empirical release rates determined in conjunction with direct space-based measurements of CO$_2$. We hypothesize that the accuracy of the release rates could depend on the level of solar activity at the time the comet is observed, which will be influenced by the solar cycle. We present observations and analysis of [O I] emission in one comet observed near solar maximum, C/2014 Q2 (Lovejoy), and one near solar minimum, C/2007 N3 (Lulin). Our [O I] measurements were obtained using two high spectral resolution optical spectrographs: the Tull Coudè spectrometer at McDonald Observatory and the ARCES spectrometer at Apache Point Observatory. We use empirical and theoretical models for [O I] emission from the literature to derive multiple sets of inferred CO$_2$ abundances for these comets and compare to contemporaneous space-based measurements of CO$_2$. We find that the empirical model, which was developed based on comet observations obtained near solar maximum, reproduces the directly measured CO$_2$ abundances better for Lovejoy. Neither model accurately reproduces the direct measurement for Lulin. We discuss the implications of our findings for the accuracy and dependencies of the oxygen line ratio method for inferring CO$_2$ abundances in cometary comae.

Observations of [O I] emission in Comets C/2014 Q2 (Lovejoy) and C/2007 N3 (Lulin): Possible Influence of Solar Activity on Oxygen Line Ratios

TL;DR

This study tests the validity of using the forbidden O I line ratio as a ground-based proxy for CO/HO in comets by observing Lovejoy near solar maximum and Lulin near solar minimum. It contrasts empirical and theoretical [O I] release rates, derives CO/HO from , and benchmarks against space-based CO measurements from Spitzer and AKARI. The findings show that empirical release rates accurately reproduce Lovejoy's CO abundance but not consistently for Lulin, while theoretical rates underperform in both cases; solar activity likely modulates the photochemistry governing / production, necessitating solar-cycle-aware calibrations. Overall, the work highlights both the promise and the limitations of the oxygen-line ratio method for inferring cometary CO and underscores the need for a larger sample across solar activity levels to quantify this dependency.

Abstract

Observing [O I] emission to calculate an "oxygen line ratio" has been proposed as a potential proxy for direct CO measurement in comets. However, the photochemistry governing [O I] release into the coma is not well understood, and using theoretical release rates often yields different results than using empirical release rates determined in conjunction with direct space-based measurements of CO. We hypothesize that the accuracy of the release rates could depend on the level of solar activity at the time the comet is observed, which will be influenced by the solar cycle. We present observations and analysis of [O I] emission in one comet observed near solar maximum, C/2014 Q2 (Lovejoy), and one near solar minimum, C/2007 N3 (Lulin). Our [O I] measurements were obtained using two high spectral resolution optical spectrographs: the Tull Coudè spectrometer at McDonald Observatory and the ARCES spectrometer at Apache Point Observatory. We use empirical and theoretical models for [O I] emission from the literature to derive multiple sets of inferred CO abundances for these comets and compare to contemporaneous space-based measurements of CO. We find that the empirical model, which was developed based on comet observations obtained near solar maximum, reproduces the directly measured CO abundances better for Lovejoy. Neither model accurately reproduces the direct measurement for Lulin. We discuss the implications of our findings for the accuracy and dependencies of the oxygen line ratio method for inferring CO abundances in cometary comae.

Paper Structure

This paper contains 16 sections, 7 equations, 7 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Observations and data analysis
  3. Optical observations
  4. Data reduction
  5. Deblending of telluric and cometary lines
  6. Data analysis
  7. Infrared observations
  8. Results
  9. Discussion
  10. Lovejoy: Changes in [O1] ratio with heliocentric and cometocentric distance
  11. Water production rates
  12. Lovejoy
  13. Lulin
  14. Inferred CO$_2$ abundances
  15. Solar activity effects
  16. ...and 1 more sections

Figures (7)

  • Figure 1: Energy level diagram for atomic oxygen [O1]. In analysis of the oxygen line ratio, the 2972 and 2958Å lines can be ignored as they only comprise 10% of transitions from the $^1S$ level to the $^1D$ level. Note that all atoms that decay through the 5577Å line will then also decay through either the 6300 or 6364Å line. Figure adapted from bhardwaj2012coupled.
  • Figure 2: Region of the spectrum obtained of Lovejoy on UT May 11, 2015 that contains the forbidden oxygen line at 6300Å. Note that the cometary line appears redward of 6300Å due to Doppler shifting caused by the comet's geocentric velocity. The data are represented by a histogram (black) with $1\sigma$ errorbars. The data are fitted with two separate Gaussians corresponding to the telluric (red) and cometary (blue). The sum of the two Gaussian components is plotted in green.
  • Figure 3: Spitzer IRAC images of comet C/2014 Q2 (Lovejoy). Top row: 3.6 images. Center row: 4.5 images. Bottom row: derived CO$_2$ gas maps. All images are displayed with an asinh color scale intended to show both the low-surface brightness instrumental background and the bright cometary core. Arrows indicate the projected Celestial north (N), Sun ($\sun$) and velocity ($v$) vectors. Each sub-panel is 81$\times$81.
  • Figure 4: Regions of the spectra obtained of Lovejoy on UT 2015 February 3 that contain the forbidden oxygen lines at 5577Å (top row), 6300Å (middle row), and 6364Å (bottom row). Note that the cometary line appears redward of the telluric in all plots due to Doppler shifting caused by the comet's geocentric velocity. The data are represented by a histogram (black) with $1\sigma$ errorbars. The right column shows the cometary lines fitted with Gaussian curves (blue). The left column displays a zoomed-out view such that both the cometary and telluric lines are visible as well as the surrounding signal-to-noise. In the top left panel, the weaker features surrounding the two oxygen lines are due to emission from C$_2$.
  • Figure 5: Regions of the spectra obtained of Lulin on UT 2009 February 14 that contain the forbidden oxygen lines at 5577Å (top row), 6300Å (middle row), and 6364Å (bottom row). Note that the cometary line appears blueward of the telluric in all plots due to Doppler shifting caused by the comet's geocentric velocity. The data are represented in the same ways described in Figure \ref{['res-fig:prettyfit-doublepanel-lovejoy']}. In the top left panel, the weaker features surrounding the two oxygen lines are due to emission from C$_2$.
  • ...and 2 more figures