Betsy Mills (Indiana University) Hugo Schwarz (CTIO)

1. 2005 CTIO REU/PIA Program Betsy Mills (Indiana University) Hugo Schwarz (CTIO) Romano Corradi (Instituto Astrofisica de las Canarias)

2. Are We There Yet? An Extinction Method of Determining Model-Independent Distances for a Large Sample of Planetary Nebulae

3. Where We’re Coming From: The majority of planetary nebulae distances are not well known. -Too far for trigonometric parallax -Spectroscopic parallax is also out because we don’t have a sample of stars of the types in the centers of PNe that we can use to calibrate their absolute brightness In general, statistical methods must be used to determine distances. In special cases accurate distances to individual nebulae can also be found.

4. Where We’re Coming From: All large samples of nebulae distances rely on statistical assumptions about nebulae properties, calibrated to some degree by individual well-known Pne distances Constant Ionized Mass Constant Absolute H Flux Constant Radius These methods do not yield distances that are in good agreement

5. Where We’re Coming From: Accurate distances are necessary to better understand planetary nebulae as a stage of stellar evolution. Absolute Luminosity Galactic Distribution Birthrate Central Star Radius Nebular Mass We need a method of determining distance that does not make assumptions about any of these properties

6. Where We’re Coming From: One neglected method of determining planetary nebulae distances is the extinction method. Extinction to each nebula is measured Many stars in PN fields are classified From the spectral type and absolute brightness the extinction and distance for each star are determined A diagram relating distance and extinction for each PN field is then made. Plotting PN extinction on this diagram, you can read off distance. Thus far this method has only been used on inhomogeneous samples of planetary nebulae-- samples taken at different times, with different instruments and under overall different conditions. The samples have also been small because It is a very labor-intensive process. However, unlike other specialized methods to determine model-independent distances, this method can be applied to almost all nebulae

7. Our data: We have long-slit spectra for 50 different planetary nebulae, and for about 4000 foreground and background stars in the same 3’ x 3’ fields. All of the data were taken on La Palma -Long-slit nebular spectra on the Isaac Newton 2.5m with IDS -Stellar spectra on the 4.2m with the multi-slit spectrograph Work has already been done to spectrally classify the stars using neural network software. The nebular spectra have been flux and wavelength calibrated

8. Sky subtraction and collapsing spectra Before sky subtraction: After sky subtraction:

9. Extinction and the Balmer Decrement By comparing the theoretical ratio of Balmer line strengths to those actually observed, you can determine the amount of interstellar extinction along the line of sight between the earth and the nebula. Interstellar dust preferentially scatters blue wavelengths of light As you move beyond H to bluer Balmer lines, they will have fluxes smaller than theory predicts in the absence of extinction.

10. Measurements Measuring the Balmer line fluxes can be difficult. Some lines such as H and H can be blended with nearby lines. Some spectra can include Balmer absorption lines from the central star. As you go further down the Balmer series, signal to noise decreases.

11. Identifying Saturated Lines In order to get accurate Balmer line ratios, any saturated lines must be identified so that they can be avoided. Saturated lines are identified by using imexam to measure the flux in the original 2D images. Lines with counts over 60,000 are considered saturated, and over 50,000 counts the CCD response becomes nonlinear, so those lines are eliminated as well

12. Calculating Extinction I H / I H = ( I H0 / I H0 ) 10-c * A(,) Literature values of extinction can vary due to different models of the variation of interstellar extinction with wavelength.

13. Where we are now: Balmer line fluxes have been measured, and some preliminary extinctions calculated. Next step is to determine model to be used for extinction as a function of wavelength Calculate all extinction values for the nebulae Refine average extinction for each nebula by weighting extinction values according to the S/N of the line fluxes. Once distance and extinction for field stars can be calculated, the distance to the PNe can be determined.

14. Potential Potholes: Internal Extinction Some PNe have internal extinction that will contribute to the observed Balmer decrement If this effect is ignored it will lead to systematically higher distances for those nebulae. This problem can be addressed by measuring variations in extinction in different parts of the 2D spectrum, and selecting the lowest value as the interstellar extinction

15. And Roadblocks : Scale of ISM variations Our data were taken in fields of 3 square arc minutes. ISM variations on this scale are possible By using many field stars (~40-100) we will reduce the extra scatter this will introduce into our plots of distance v. extinction. This may introduce some additional random error into our distances, but it should not be significant.

16. Final destination: Well-determined model-independent distances for a large group of PNe NGC 7026 Soon to be more than just another pretty face.

17. Special Thanks to: My fellow REU students PB+J Quesadillas Computer Support Staff Dr. Alan Whiting