The Orion Nebula contains many dozens of highly ionized objects which are generally agreed to be very young stars whose primordial disks have not yet dissipated, called proplyds. (e.g. Laques and Vidal 1979; O'Dell et al. 1993; Churchwell et al. 1987; O'Dell and Wen, 1994; Stauffer et al., 1994). Their radii are typically in the hundreds of AU.  Proplyds represent a unique snapshot of a phase which presumably the Solar System once passed through envelopes are presumed to be similar to the primordial protoplanetary disk of the Sun. Proplyds are thus essential in our understanding of planetary system birth and early evolution.  The work presented here is also an abstract and paper in the 205th Meeting of the American Astronomical Society


The Orion Nebula shown in the HST image on the left is home to dozens of proplyds.  Some of the proplyds are shown above.

Model Input parameters include the Teff, log(g), and mass of the central star.  The central stars are thought from their colors to be late-type (Prosser et al. 1994), but spectra of them have never been obtained and the stars have not been classified.  Further progress in understanding the physical characteristics of proplyds can make good use of spectral types.


This model from Henney & O’Dell 1999 shows the model structure of a proplyd.


Using HST we obtained the first ever spectra of the visible proplyd stars in order to classify them spectrally determine their surface temperatures and luminosities. From these properties, and based on stellar evolutionary models, we can estimate masses and ages.

Observations made using Space Telescope Imaging Spectrograph (Proposal ID 9866; file o8t514010). In spectroscopic mode, the STIS configuration was

  • Slit: 52 arcsec x 0.2 arcsec; P.A.=-67.65OO
  • Grating: G750L
  • Resolving Power: 790 (9.8A); 0.56 arcsec/pixel
  • Central Wavelength 7751A
  • Range: 5236 – 10266A
  • Exposure Time: 120 seconds

We will focus on one proplyd here: N171-340 located at RA (2000): 5h 35m 17.00s and Dec (2000): -5O 23’ 38.87”. The date of Observation was 05 December 2003 UT. Here’s what the spectrum of the central star looks like:


The spectral type appears to be an M3V-M4V and the effective temperature is 3180 K to 3380 K based on comparison with spectra od late type stars published by Kirkpatrick et al. 1993ApJ, 402,643. The luminosity of an M3V is Log(L/Lo): -2.0


Just for fun, the image above shows comparison of the proplyds central star spectrum with a T Tau type star and a very late L4 star.  Neither make very good comparisons.

We want to do a check on the luminosity. A K-band apparent magnitude of 10.05 was measured by McCaughren & Stauffer 1994 and they give the K-band absolute magnitude as +1.53. But…

A V-K for an M4V star is 4.65 and MK = +1.53 which suggests that the absolute MV = +6.18.  This presents a problem because MV = +6.18 is  a K0V star!  So, our spectral type is not consistent with the photometry.  Reasons may include Extinction and Scattering. If we stick with our STIS spectral type for now, then the location of the proplyd central star on the pre-main sequence evolution tracks is shown in the figure below, marked by the red star.


The spectral resolution we used is too low for measurement of the Lithium 6707A absorption line which is needed to pinpoint the star on the isochrones.  If we u se what we have, then the evolutionary tracks suggest an age ~ 10 MYr or younger.

Note that the population of Orion cluster age ~ 0.5 – 1 MYr, so we’d expect our proplyd central star to be in that range.

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November 2004. mwc