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Classifying the SEDs of Herbig Ae/Be stars

Last modification: F. Jiménez Nov 24 2014

The overall goal of this tutorial is to become familiar with VOSpec and SPLAT. For that, we are going to build the Spectral Energy Distribution (SED) of a couple of Herbig Ae/Be stars.

Background:

A Herbig Ae/Be star is a 2-8 Solar mass, pre-main-sequence star (that is, young objects with typical ages of <10Myr) of spectral types A or B. These stars are still embedded in gas-dust envelopes and are sometimes accompanied by circumstellar disks. They are in the gravitational contraction stage and approaching the main sequence (i.e. they are not burning hydrogen in their centre).

Herbig  Ae/Be stars show Balmer emission lines in their stellar spectrum and infrared excess due to circumstellar dust. Sometimes Herbig Ae/Be stars show significant brightness variability associated to clumps (protoplanets and planetesimals) in the circumstellar disk.

The SEDs of Herbig Ae/Be stars fall roughly into two groups: Group I sources have a relatively strong far-IR flux, which is energetically comparable with the flux in the near-IR showing an almost flat spectral energy distribution. Group II sources show a similar near-IR excess as group I sources but their flux falls off strongly towards the far-IR. These were first classified by Meeus et al (2001, A&A, 365, 476), see below figure.

 

 

Dullemond and Dominik (2004, A&A, 417, 159) provided a physical explanation for this difference: Group II sources have an outer disk which is protected against direct stellar radiation by a puffed-up inner disc. If the outer disc emerges from the inner disc's shadow, i.e. has a large flaring angle, then its SED resembles that of a Group I source.

Uses VOSpec

Tutorial:

  • Open VOSPEC running the .jar file.
  • Type 'HD 100546' in the "Target" box, '0.001' in the "Size" box and click 'Query'.
  • The Server Selector window opens. The Spectra, Photometry and Theoretical Services available in the VO are listed on the left hand side of the window. Click on each branch to open up the lists. 
  • Select the following spectral services
    • Far Ultraviolet Spectroscopic Explorer (http://archive.stsci.edu/ssap/search2.php?id=FUSE)
    • The Hubble Space Telescope Spectra (http://archive.stsci.edu/ssap/search.php?id=HST)
    • INES: The IUE Newly Extracted Spectra
    • The ISO Data Archive Interoperability System
  • Select the following photometric service
    • CDS Multicatalogue Photometry Service
  • Click 'Query'. The spectra and photometry are then loaded into the 'Spectra List' region of the main VOSpec window. The list of spectra can also be viewed as a table by clicking the 'Tree/Table view' icon on the top right of the main VOSpec window.
  • Select the following spectra
    • FUSE: p219040... Tip: Use the Coverage.Bounds/Start/Stop columns to get an idea of the covered spectral range. Other important information like the time coverage, the S/N ratio or the resolution can be obtained in a similar way.
    • HST: O5C901080, O6GY10010.Tip: Look for the ssa: Query.LName column.
    • INES: LWP16052RL,SWP39712RL. Tip: Click on the column label "name" to sort by name.
    • ISO: SWS01, LWS01
  • Select the following photometric services: 2MASS, AKARI, HIP, IRAS, SDSS, WISE. Tip: Reorganize tree by catalogues. Click "Retrieve"
  • The selected spectra and photometric points are loaded into the main VOSpec window. Change the Y axis from Jy to erg/cm2/s by clicking on the 'Flux Unit' field to the left of the main VOSpec window.
  • Which group does HD100546 belong to?

Additional:

  • Fitting lines:
    • Put the spectral and flux axis in linear scale (top left in the VOSpec main window)
    • Zoom into a region with a strong spectroscopic line (e.g. 6.26 microns in the SWS ISO spectrum).
    • Select Operations -> Fitting Utilities. The Fitting Window then opens. Select the Gaussian tab and click 'Generate'. The fitted Gaussian is plotted over the spectrum and the parameters used in the calculation are shown in the Fitting Window (where A=amplitude, x0 = central peak, sigma = standard deviation).

 

  • Filtering:
    • In the LWP INES spectrum, select the 2400-2800A region.
    • Select Operations -> De-noising -> Averaging Filters -> Median. Select 10 as "number of points". Click 'Filter'. A filtered spectrum is created.

 

  • Identifying lines with SLAP:
    • Zoom into the region with the strong spectroscopic line (2800A).
    • Select the Simple Line Access (Operations -> SLAP).
    • Highlight the region across the emission line. The selected region appears yellow and the SLAP Viewer window opens. Within this window open the SLAP Services tree and select the NIST atomic spectra service. Then click 'select'.
    • A table appears for each service within the Slap Services Output field (in our case, only NIST).
    • Go back to the main VOSpec window. By moving the cursor across the yellow region each atomic line that lies closest to the cursor is displayed. One can then identify the most likely atomic or molecular line. Our two lines are the MgII 2795A and MgII 2805A.
    • Click again on SLAP to deactivate this functionality.
  • Model comparison:
    • Select HD100546 (a B9 star)
    • Select Operations -> Fitting Utilities. The Fitting Window then opens. Select the Blackbody, use a Teff:12000K and click 'Generate'. The blackbody will be overplotted to the observational SED.

 

 

SPLAT

  • Open SPLAT: run the <home>/bin/splat/splat file.
  • File --> SSAP. The 'query VO for Spectra' window opens.
  • Type Nova Cygni 1992 into the 'Object' field, and click 'Lookup'. The coordinates of the object appear in the 'RA' and 'Dec' fields. Select 0.1 arcminutes in the 'Radius' field. Select 'Query format' all and Wave Band "UV". Click 'Send Query'. New window opens.
  • Click on the IUE tag. Select ten lwp spectra (Tip: look for the "title" column label). Click "Display selected". The selected spectra will be loaded into the 'Global list of spectra' on the main SPLAT window and they will be overplotted in the same graphic window. The differences in the MgII 2800 emission line are clearly visible.
  • Click on "Animate selected spectra by ....". Select several spectra and click "Start". New windows open where the selected spectra are displayed one at time, as a movie. Change the X axis range by clicking on "Plot configuration window" and setting the Lower Y to zero. Click "draw". The differences in the MgII 2800 emission line are clearly visible.

 

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