APOGEE Caveats

For stars fit with the the dwarf grids, the C and N abundances are not valid. The dwarf grids do not have separate dimensions for C and N, so the [M/H] dimension is used to determine C and N abundances. However, since most of the information for C and N come from molecules, varying C and N with [M/H] is incorrect, since whenever the C or N abundance is varied, the abundances of all other elements are also varied! Since many of the windows for C and N are the same, this also has the effect of making the C and N abundance almost the same. This was only recognized after the data release files were frozen -- in fact, this was a problem in DR13 that was never recognized. Users should not use C and N abundances for stars fit with the dwarf grids (ASPCAP_CLASS = Fd, GKd, or Md)!

Since ASPCAP works by varying element families together, inaccuracies can occur if aberrant/non-standard element abundance ratios are present in stars. For example, the ASPCAP parameter derivation is somewhat flawed in second generation globular cluster stars, where non-standard oxygen abundances lead to systematic offsets in effective temperature and gravity, which in turn result in offsets in other quantities.

While the improved treatment of persistence seems to result in significantly better performance in the derivation of stellar parameters and abundances for stars affected by persistence, there are still some stars for which persistence likely leads to issues. Users of the APOGEE spectra should pay careful attention to the persistence flags and should consider the use of the inflated uncertainties for persistence-affected pixels.

Although DR14 ASPCAP attempts to account for LSF variations by splitting the analysis into groups by mean fiber number, there are indications that the mean and scatter in abundances of stars varies at a low level (few hundredths of a dex) with the mean fiber number.

For certain fields which overlap one another, a few stars were inadvertently targeted in both fields. Since spectra are combined only within a field, these stars appear more than once in the combined spectra and summary files/tables. When multiple combined spectra of the same object exist, the lower S/N observations all have a bit set in the EXTRATARG flag that appears in the summary allStar file and in the CAS table. The EXTRATARG bit should allow users to avoid duplicate use of the same target in analysis.

Some targets may have been selected independently for different programs within different visits to the same field. When we combine spectra, the target flag that we adopt for the combined spectrum is a bitwise OR of all of the target flags of the individual visit spectra. As a result, there are stellar spectra constructed from multiple visits for which a target flag bit may be set in the combined spectra, but not in all of the visit spectra that were used to construct it.

APOGEE2_TARGET2 bit is not set for bit flags 22 (APOGEE2_1M_TARGET), 23 (APOGEE2_MOD_BRIGHT_LIMIT) and 30 (APOGEE2_OBJECT). This will be corrected in future releases.

APOGEE_ANCILLARY bit is not set for all APOGEE_KEPLER_SEISMO and APOGEE_KEPLER_HOST targets

For a few objects, alternate object/star names were used during the reduction. 2MASS identifications have been adopted in the final summary files and associated database tables, but if users want to find the individual star spectrum files (e.g., the apStar or aspcapStar files), they will need to know the star name used during the reduction, which is stored in the REDUCTION_ID tag. This is particularly true for the 1m+APOGEE observations.

For very bright objects, the WISE photometry employed to generate some extinction estimates can have significant problems, which leads to estimated extinctions that may be appreciably in error. This is particularly true for the bright star sample observed with the 1m+APOGEE. Users are cautioned against using the tabulated extinctions for these stars.