Tangential Motions and Spectroscopy within NGC 6720, the Ring Nebula

We have combined recent Hubble Space Telescope WFPC2 images in the [O III] 5007 and [N II] 6583 lines with similar images made 9.557 years earlier to determine the motion of the Ring Nebula within the plane of the sky. Scaled ratio images argue for homologous expansion, that is, larger velocities scale with increasing distance from the central star. The rather noisy pattern of motion of individual features argues for the same conclusion and that the silhouetted knots move at the same rate as the surrounding gas. These tangential velocities are combined with information from a recent high resolution radial velocity study to determine a dynamic distance, which is in basic agreement with the distance determined from the parallax of the central star. We have also obtained very high signal to noise ratio moderate resolution spectra (9.4 Angstrom) along the major and minor axes of the nebula and from this determined the electron temperatures and density in the multiple ionization zones present. These results confirm the status of the Ring Nebula as one of the older planetary nebulae, with a central star transitioning to the white dwarf cooling curve. (Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA Contract No. NAS 5-26555 and the San Pedro Martir Observatory operated by the Universidad Nacional Autonoma de Mexico.)Comment: Astronomical Journal, in pres

Which Stars are Ionizing the Orion Nebula ?

The common assumption that Theta-1-Ori C is the dominant ionizing source for the Orion Nebula is critically examined. This assumption underlies much of the existing analysis of the nebula. In this paper we establish through comparison of the relative strengths of emission lines with expectations from Cloudy models and through the direction of the bright edges of proplyds that Theta-2-Ori-A, which lies beyond the Bright Bar, also plays an important role. Theta-1-Ori-C does dominate ionization in the inner part of the Orion Nebula, but outside of the Bright Bar as far as the southeast boundary of the Extended Orion Nebula, Theta-2-Ori-A is the dominant source. In addition to identifying the ionizing star in sample regions, we were able to locate those portions of the nebula in 3-D. This analysis illustrates the power of MUSE spectral imaging observations in identifying sources of ionization in extended regions.Comment: 7 pages with 5 figure

Properties of the ionized gas in HH202. I: Results from integral field spectroscopy with PMAS

We present results from integral field spectroscopy with the Potsdam multi-Aperture Spectrograph of the head of the Herbig-Haro object HH 202 with a spatial sampling of 1"x1". We have obtained maps of different emission lines, physical conditions --such as electron temperature and density-- and ionic abundances from recombination and collisionally excited lines. We present the first map of the Balmer temperature and of the temperature fluctuation parameter, t^2. We have calculated the t^2 in the plane of the sky, which is substantially smaller than that determined along the line of sight. We have mapped the abundance discrepancy factor of O^{2+}, ADF(O^{2+}), finding its maximum value at the HH 202-S position. We have explored the relations between the ADF(O^{2+}) and the electron density, the Balmer and [O III] temperatures, the ionization degree as well as the t^2 parameter. We do not find clear correlations between these properties and the results seem to support that the ADF and t^2 are independent phenomena. We have found a weak negative correlation between the O^{2+} abundance determined from recombination lines and the temperature, which is the expected behaviour in an ionized nebula, hence it seems that there is not evidence for the presence of super-metal rich droplets in H II regions.Comment: 12 pages, 11 figures. Accepted for publication in MNRA

Measuring x-ray polarization in the presence of systematic effects: Known background

The prospects for accomplishing x-ray polarization measurements of astronomical sources have grown in recent years, after a hiatus of more than 37 years. Unfortunately, accompanying this long hiatus has been some confusion over the statistical uncertainties associated with x-ray polarization measurements of these sources. We have initiated a program to perform the detailed calculations that will offer insights into the uncertainties associated with x-ray polarization measurements. Here we describe a mathematical formalism for determining the 1- and 2-parameter errors in the magnitude and position angle of x-ray (linear) polarization in the presence of a (polarized or unpolarized) background. We further review relevant statistics-including clearly distinguishing between the Minimum Detectable Polarization (MDP) and the accuracy of a polarization measurement.Comment: 12 pages, 4 figures, for SPIE conference proceeding

On understanding the figures of merit for detection and measurement of x-ray polarization

The prospects for accomplishing X-ray polarization measurements appear to have grown in recent years after a more than 35-year hiatus. Unfortunately, this long hiatus has brought with it some confusion over the statistical uncertainties associated with polarization measurements of astronomical sources. The heart of this confusion stems from a misunderstanding (or potential misunderstanding) of a standard figure of merit-the minimum detectable polarization (MDP)-that one of us introduced many years ago. We review the relevant statistics, and quantify the differences between the MDP and the uncertainty of an actual polarization measurement. We discuss the implications for future missions.Comment: 5 pages, 2 figures, to be presented at SPIE conference 7732 (paper 13), corrected typo

Using ACIS on the Chandra X-ray Observatory as a particle radiation monitor

The Advanced CCD Imaging Spectrometer (ACIS) is one of two focal-plane instruments on the Chandra X-ray Observatory. During initial radiation-belt passes, the exposed ACIS suffered significant radiation damage from trapped soft protons scattering off the x-ray telescope's mirrors. The primary effect of this damage was to increase the charge-transfer inefficiency (CTI) of the ACIS 8 front-illuminated CCDs. Subsequently, the Chandra team implemented procedures to remove the ACIS from the telescope's focus during high-radiation events: planned protection during radiation-belt transits; autonomous protection triggered by an on-board radiation monitor; and manual intervention based upon assessment of space-weather conditions. However, as Chandra's multilayer insulation ages, elevated temperatures have reduced the effectiveness of the on-board radiation monitor for autonomous protection. Here we investigate using the ACIS CCDs themselves as a radiation monitor. We explore the 10-year database to evaluate the CCDs' response to particle radiation and to compare this response with other radiation data and environment models.Comment: 10 pages, 5 figures. To appear in Proc. SPIE vol. 773

Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--II:Physical Conditions

We have performed a detailed analysis of the electron temperature and density in the the Ring Nebula using the calibrated HST WFC3 images described in the preceding paper. The electron temperature (Te) determined from [N II] and [O III] rises slightly and monotonically towards the central star. The observed equivalent width (EW) in the central region indicates that Te rises as high as 13000 K. In contrast, the low EW's in the outer regions are largely due to scattered diffuse Galactic radiation by dust. The images allowed determination of unprecedented small scale variations in Te. These variations indicate that the mean square area temperature fluctuations are significantly higher than expected from simple photoionization. The power producing these fluctuations occurs at scales of less than 3.5E15 cm. This scale length provides a strong restriction on the mechanism causing the large t^2 values observed.Comment: Accepted for publication in the Astronomical Journa

Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--III:Tangential Motions using AstroDrizzle Images

We have been able to compare with astrometric precision AstroDrizzle processed images of NGC 6720 (the Ring Nebula) made using two cameras on the Hubble Space Telescope. The time difference of the observations was 12.925 yrs. This large time-base allowed determination of tangential velocities of features within this classic planetary nebula. Individual features were measured in [N II] images as were the dark knots seen in silhouette against background nebular [O III] emission. An image magnification and matching technique was also used to test the accuracy of the usual assumption of homologous expansion. We found that homologous expansion does apply, but the rate of expansion is greater along the major axis of the nebula, which is intrinsically larger than the minor axis. We find that the dark knots expand more slowly that the nebular gas, that the distance to the nebula is 720 pc +/-30%, and the dynamic age of the Ring Nebula is about 4000 yrs. The dynamic age is in agreement with the position of the central star on theoretical curves for stars collapsing from the peak of the Asymptotic Giant Branch to being white dwarfs