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SSPP Caveats

It is important that the SSPP be able to identify situations where the quoted atmospheric parameters may be in doubt, or simply to make the user away where possible anomalies might be found in the spectra of a given star. We have designed a number of flags which serve this purpose.

Brief Descriptions of SSPP Flags

Flag Comment
n All appears normal
D Likely white dwarf
d Likely sdO or sdB
H Hot star with Teff > 10,000 K
h Helium line detected, possibly very hot star
l Likely solar abundance, late-type star
E Emission lines in spectrum
S Sky spectrum
V No radial velocity information available
N Noisy spectrum at extrema
C The photometric g-r color may be incorrect
B Unexpected Hα line strength predicted from Hδ
G Strong G-band feature
g Mild G-band feature

There are two primary categories of flags -- critical flags and cautionary flags. Critical flags are discussed below. When a critical flag is raised, the SSPP is set to either ignore the determinations of atmospheric parameters for a given source, or it is forced (in the case of the color flag described below) to take steps that differ from normal processing in an attempt to rescue this information. Obviously, even when information is salvaged, the presence of a critical flag means the user must be aware that special steps have been taken, and the reported estimated parameters must be viewed with this knowledge in mind.

The second category of flags are the cautionary flags, which are provided for user consideration, but are not necessarily cause for undue concern. Indeed, sometimes these flags are raised when all is in fact OK, but the flag has been raised due to a peculiarity in the spectrum that is relatively harmless, and which will not unduly influence the determinations of atmospheric parameters. The user should nevertheless be aware of the existence of these flags.

The flags are combined into a single set of four letters, the meanings of which are summarized in the table above, and described below in more detail. Four placeholders are used in order to accommodate cases where more than one sort of flag is raised.

The nominal condition for the four letter flag combination is `nnnn', which indicates that the SSPP is satisfied that a given stellar spectrum (and its reported g-r colors) has passed all of the tests that have been performed, and the stellar parameters should be considered as well determined.

The first letter in this combination is set to one of 10 different values: n, D, d, H, h, l, E, S, V, and N. Their explanations follow:

  • n: The letter n indicates nominal.
  • D: The letter D indicates that a comparison of the breadth of the Hδ line at 20% below its continuum, D0.2, and the line depth below the continuum, Rc, relative to their expected relationship for "normal stars", provided below, does not apply. The expected relationship is given by:
     
    Rc = -0.009503 + 0.027740D0.2 - 0.000590D0.22 + 0.000006D0.23
     
    If D0.2 is greater than 35.0 Å and the predicted Rc from the above relationship is less than the measured value, then the star is most likely a white dwarf. This is a critical flag.
  • d: This flag is raised if D0.2 is less than 35.0 Å and the predicted Rc from above is less than the measured value. In this case, the star is most likely a sdO or sdB star. This is a critical flag.
  • H: This flag is raised when the estimated Teff from the SSPP is greater than 10,000K, and is meant to indicate a hot star. This is a critical flag.
  • h: This flag is raised if the estimated Teff from the SSPP is greater than 8,000K, and either of the line indices of He I (at 4026.2 Å) or He I (at 4471.7 Å) is greater than 1.0 Å. This indicates that the star is likely to be a hot star. This is a critical flag.
  • l: This flag is raised if the SSPP judges the star to have a high likelihood of being a late type star (generally late K, M, or later spectral type), beyond the ability of the present pipeline to determine acceptable atmospheric parameter estimates. The condition used for raising the l flag is that the Na line (5892.9 Å) index, as measured over a 24 Å band centered on this feature, is larger than 10 Å, and the g-r color is greater than 0.80. This is a critical flag.
  • E: This flag is raised if significant emission lines are detected in a spectrum. This is a critical flag.
  • S: This flag is raised if the spectrum (according to the header information) is a night sky spectrum. This is a critical flag.
  • V: This flag is raised when an adequate radial velocity could not be found for a given spectrum. This is a critical flag.
  • N: This flag is raised if the spectrum is considered noisy at the extremes of the wavelength range (e.g., around Ca II K and the Ca II triplet). This is a cautionary flag.

The flags that are used to fill out the remaining three positions of the four letter flag combination are C, B, G, or g, as described below:

  • C: This flag is raised if the SSPP is concerned that the reported g-r color is incorrect. As mentioned in Lee et al 2007, we calculate three estimates of predicted g-r colors. For each of these three predicted colors, we find the one which is closest to the reported g-r color based on the photometry. If the difference between the reported color and the closest predicted color is larger than 0.2 magnitudes, the color flag is raised. The SSPP is set up to proceed with its calculations of atmospheric parameters using the predicted g-r color. This flag is always found in the second position of the combination flag parameters. This is a critical flag.
  • B: This flag is raised if the SSPP is concerned that there exists a strong mismatch between the strength of the predicted Hα line index HA24, based on the measured Hδ line index, HD24. For the great majority of normal stars, the predicted value of the H\alpha line index is found to be HA24 = 2.737 + 0.775HD24. For stars with significant HA24 and HD24 measurements (which we take to mean that the values of these indices exceed zero by more than 2σ, where σ is the error in the measured line index), if the difference between the predicted HA24 line index and the measured HA24 index is larger than 2.5 Å, then the B flag is raised. This flag is always found in the third position of the combination flag parameters. This is a cautionary flag.
  • G or g: This flag is raised if the SSPP suggests that the star may exhibit a strong (G) or mild (g) CH G-band (around 4,300 Å), relative to expectation for normal stars. This flag is always found in the fourth position of the combination flag parameters. This is a cautionary flag.


 


 
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