Harvard Astronomy 201b

ARTICLE: Postrefurbishment mission Hubble Space Telescope images of the core of the Orion Nebula: Proplyds, Herbig-Haro objects, and measurements of a circumstellar disk

Read the paper by C.R. O’Dell and Wen Zheng (1994)

Summary by Jason Dittmann

Abstract

We report on observations of M42 made with the Hubble Space Telescope (HST) immediately after the successful repair and refurbishment mission. Images were made in the strongest optical emission lines of H I, (N II), and (O III) and in a bandpass close to V. In a previous paper, the term proplyd was introduced to describe young stars surrounded by circumstellar material rendered visible by being in an H II region. We confirm the proplyd nature of 17 of 18 objects found earlier with the HST, incorporate 13 previously known sources into the class on the basis of their emission-line appearance, and find 26 additional members not seen previously in other wavelengths. Half of the 110 stars brighter than V = 21 show proplyd structure, which implies that more than half of the stars have circumstellar material since nebular structures are more difficult to detect than stars. The highly variable forms of the proplyds can be explained on the basis of balance of ambient stellar gas pressure and radial pressure arising from the stellar wind and radiation pressure of the dominant stars in the region. Arguments are presented explaining the proplyds as disks or flattened envelopes surrounding young stars, hence they are possible planetary disks. The characteristic mass of ionized material is 2 \times 10^{28} g, which becomes a lower limit to the total mass of the proplyds. A new, coordinate-based, designation scheme for compact sources and stars in the vicinity of M42 is proposed and applied. Evidence is presented that one of the previously known bright Herbig-Haro objects (HH 203) may be the result of a stream of material coming from a proplyd shocking against the neutral lid that covers M42. One object, 183-405, is a proplyd seen only in silhouette against the bright nebular background. It is elliptical, with dimensions 0.9 sec by 1.2 sec and surrounds a pre-main-sequence star of at least 0.2 solar mass. The outer parts of this stellar disk are optically thin and allow column mass densities to be determined. We set a lower limit to this disk to be 0.1-4.4 \times 10^{28} g, dependent on the assumed gas to dust mass ratio.

Summary

This article is a follow-up study of the authors’ previous Hubble Space Telescope study done of the Orion Nebula star forming region. After the refurbishment mission, they obtained higher quality data investigating the proplyd nature of objects they identified in their first paper. The authors conclude:

1. The objects found in their previous study are indeed protoplanetary disks, with sizes of ~200 AU

2. These disks are interacting with bright nearby stars, as the proplyds are not spherical but teardrop shaped, oriented towards the nearby bright star.

3. The ionizing flux from the star is creating a physically thin shell around the protoplanetary disk (where the recombination is occuring)

4. The mass of the protoplanetary disk has a minimum mass of 10 earth masses.

Background Information

The Orion nebula is a nearby star forming region that is one of the most widely studied astronomical objects. The nebula itself is full of gas and dust that is interacting with the many young stars forming within it. A sampling of papers being written on objects in the Orion Nebula at all wavelengths is available here.

Prior to Hubble, ground based radio studies have identified possible protoplanetary disk candidates from their radio signature. Heavily cited in this paper, Churchwell et al. (1987) produce a radio map of orion, identifying “Solar System sized condensates”, which the authors postulate may be protoplanetary disks forming planets around young stars. Their radio map is shown below (17 of 22 sources are associated with stars, and white corresponds to 1 mJy):

The less accepted explanation is that these are aliens from the Orion Empire chattering to each other.

Pre-Refurbishment Observations

Prior to this study, the author’s used a pre-refurbished Hubble to image the Orion Nebula. This study can be found here. They image a small portion of the Orion Nebula:

a

In this image, they find 31 total compact objects that may be protoplanetary disks. Futhermore, all 7 VLA sources from Churchwell et al. (1987) study that were in their field are confirmed to be compact objects.

Proplyd Model

O’Dell et al. (1993), using their pre-refurbishment measurements, describe a qualitative model o the compact objects they see. They believe them to be a large disk of neutral material surrounding a small star at the center. Surrounding this neutral material is a disk of ionized material that is being pushed outward and away from the neutral disk. A diagram of this model from O’Dell et al. (1993) is shown below:

Post Refurbishment Follow-Up

Following the refurbishment of the Hubble Space Telescope to correct for the spherical aberration of its primary mirror, O’Dell et al. (1994) repeated their previous observations of the Orion Nebula. Their new, improved image is shown below:

"All that gas and dust is getting in the way of my quasar survey!" - Sire QuasaringtonIn this image, O’Dell et al. (1994) find a host of proplyd objects:

Shenanigans by the authors: One of these is actually a dust blot

They further confirmed that these disks were interacting with nearby bright stars, whose ionizing flux was creating the surface brightness seen by Hubble. The image below shows some of the proplyds near a bright star in Orion, and that the proplyds are indeed oriented preferentially towards the direction of the bright star.

For the proplyd clouds to actually be neutral, then the incoming ionization flux from the star must be balanced by the recombination rate, which can be directly measured by the surface brightness caused be recombination through the star:

Here, h\nu_{H\alpha} is the energy from recombination, \alpha^{eff}_{H\alpha} and \alpha_B are the recombination rates to n=2 and to any state, respectively, Q is the ionization flux, and r is the distance of the proplyd from the ionization source.

Unfortunately, r^2 is a 3-dimensional distance, and the data that was taken is only a 2-dimensional measurement of the distance. Not to be deterred by a silly thing like the 3-dness of space, the authors ignored this dimension and plotted the surface brightness of the proplyd with the 2-dimensional (projected) distance from the bright ionizing source:

.....it does well to a couple order of magnitude scatter.......Here, the straight line is what would be expected from their model. The authors argue that because this model works as well as it does, despite the assumptions they made and not taking into account the fact that real life is 3-dimensional, then the basic idea must be true and that these are indeed protoplanetary disks interacting with the bright nearby ionizing source. As was discussed in class, they could have done a much better job estimating the 3-dimensional distance, but opted not to for this paper.

Furthermore, O’Dell et al. (1994) find that the layer of ionized material is very thin, as the surface brightness falls off quite sharply (1 pixel ~ 0.1 arcseconds):

Finally, the authors estimate a lower bound for the mass of dust in the protoplanetary disk. Taking the definition of tau and a spherical dust grain:

Making the assumptions:

ie – the average density of a grain is that of water, and the grains are 200nm in radius. They justify using such a large size because they say that most of the mass is going to be coming from the grains of larger size.

Using these assumptions, the authors place a lower bound of 10 Earth masses for the mass of a typical disk, which is in the right ballpark.

References

Churchwell, E., Felli, M., Wood, D.O.S., and Massi, M. (1987) ApJ 321, 516

O’Dell, C.R., Wen, Z., Hu, X. (1993) ApJ 410, 696

O’Dell, C.R., Wen, Z. (1994) ApJ 436, 194

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