APOGEE Caveats

Although ASPCAP currently 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 with the mean fiber number (few hundredths of a dex).

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 concern particularly impacts the bright star sample observed with the NMSU 1-meter.

Users are cautioned against using the tabulated extinctions for very bright stars.

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. .

Because 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. Jönsson et al. (2020) provides a detailed discussion.

The abundance scale of DR16, i.e., which solar abundances are used in the normalization of the supplied abundance ratios, is difficult to detangle, but we are likely to be close to the scale of Grevesse et al. (2007) for many elements. For a complete description of the APOGEE abundance scale, go here and Jönsson et al. (2020) provides a detailed discussion.

For stars in which all of the visit spectra were recorded in regions affected by persistence, the uncertainties for the pixels/wavelengths affected by persistence are significantly inflated. This detector-based problem has the effect of down-weighting pixels/wavelengths relative to those pixels/wavelengths that are not affected by persistence in the ASPCAP fits. Because the S/N that characterizes the combined spectra is the median S/N of all of the pixels, the standard S/N reported for these stars is reduced. To provide a better S/N estimate, we have also calculated an alternate S/N, SNREV, that is determined over a wavelength region in the middle chip that should not have many pixels that can be affected by persistence.

SNREV is the recommended quantity to use for S/N assessment.

A subset of red giant stars with $T_{eff} \lt 4000 K$ exhibit “noding” in the abundances of a few elements. For reasons not yet fully understood ASPCAP prefers discrete abundance values for this subset of stars, resulting in tight sequences in the $[X/Fe]-[Fe/H]$ planes that we believe to be unphysical. The elements most affected by this “noding” are aluminum and chromium, with oxygen and calcium also showing this behavior for stars with supersolar metallicity. Users should be cautious when using the abundances of these elements for red giant stars with $T_{eff} \lt 4000 K$.

Rotation is only included as a fitted parameter for the dwarf grids. For this reason, giant stars that are rotating may have suspect stellar parameters due to the spectral fits trying to compensate for the broadened lines.

In the allVisit and allStar files, the VHELIO entry is in the barycentric frame, not the heliocentric frame as the name suggests. The naming convention has been maintained from earlier releases for historical reasons. The data model has been corrected to more clearly convey this information to the user.

The asVisit and asPlate FITS files in DR16 incorrectly have a header card TELESCOP='apo25m', when they should all have TELESCOP='lco25m'. These files can all easily be identified as being derived from LCO observations by their filename prefixes (as) and by their location in the directory tree.

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.

APOGEE-2S plates have a full field-of-view with a radius of $0.95^{\circ}$. However, most of the APOGEE-2S plates were designed to have a reduced field-of-view of only $0.80^{\circ}$. This choice occurred due to the vignetting along the edges of the du Pont telescope (APOGEE-2S) field-of-view.

Users modeling the selection function for APOGEE-S data are encouraged to check the field-of-view for the plates used in their analysis.

There are two inconsistencies with the Targeting Bits that are worth noting:

• APOGEE2_TARGET1=18 has not been use for APOGEE-2N Stream targeting, but has been used for APOGEE-2S stream targeting. We note that only a limited number of stream observations are available in DR16.
• APOGEE2_TARGET2=23 is set for short cohort stars with the restricted magnitude limit of $10\lt H \lt 12.2$ mag. However, this is not done uniformly over regions of the sky -- e.g., not all disk fields will have this restriction because they were designed before the $H$ magnitude range was restricted. Thus, there could be fields with the restricted limit and with $7\lt H \lt 12.2$ mag in the same part of the sky.
Users should be aware of these inconsistencies.

Targeting for Globular Clusters occurred in a multi-phase strategy (see Star Clusters Targeting Information). While members confirmed via chemical properties, radial velocities, or proper motions were flagged as “members,” those candidates selected purely from photometry were not tagged as “candidates.” Thus, these stars are not readily identified in the catalogs for DR16, albeit the targeting input files will have these stars identified as special targets and particularly savvy users may be able to restore this information. For DR17, we intend to associate these stars with flag apogee2_target4=3.

Users investigating Globular Clusters are encouraged to run member selection on all stars in the field.
Users modeling the selection function in fields containing Globular Clusters are encouraged to consider that some of the targets in the field are not flagged as having been selected as cluster candidates.

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 bitmask that appears in the summary allStar file and in the CAS table.

The EXTRATARG bit should allow users to avoid the duplicate use of the same target in an analysis.

For objects observed with the NMSU 1-meter (telescope = apo1m), alternate object/star names were used for observation and reduction. 2MASS identifications, however, have been adopted in the final summary files and associated database tables. 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 ALT_ID tag.

Users should check the ALT_ID tag before seeking individual spectrum files for NMSU 1-meter observations.

One hundred twenty-eight (128) special targets in the bulge had previously been associated with the wrong object from the targeting files; this has now been corrected in the summary files and associated database tables. The list of affected objects is given in Jönsson et al. (2020). This caveat applies only to APOGEE VACs that have entries for each of the APOGEE targets, as the derived quantities for these 128 objects will be incorrect if it used the previous targeting information (e.g., $H$ magnitude). An effort was taken to update these files, but not all were feasible before the data release.

Users may want to correct these IDs using the attached machine-readable list available here.

For objects observed with the NMSU 1-meter (telescope = apo1m), APOGEE2_TARGET2 bit 22 was not set in DR16. The 1-meter data can still be isolated using the telescope tag.

Users should identify 1-meter data using telescope = 'apo1m' instead of using APOGEE2_TARGET2.