HD 32297: HR Diagram Position for a Resolved Debris Disk Star
Plot Summary: Shown is a plot of the HR diagram position for the
debris disk host star HD 32297, where the abscissa is log10 of the
effective temperature [Kelvin], and the ordinate is log10 of the
stellar luminosity in units of solar luminosity.
Text Summary: The star HD 32297 is famous for being having its dusty
debris disk resolved with NICMOS on Hubble Space Telescope (see
e.g. Schneider et al. 2005, Astrophysical Journal 629, L117;
http://adsabs.harvard.edu/abs/2005ApJ...629L.117S), however its exact
HR diagram position, stellar parameters, and age have been subject to
considerable uncertainty (e.g. Fitzgerald et al. 2007, Astrophysical
Journal 670, 557; http://adsabs.harvard.edu/abs/2007ApJ...670..557F).
Here we estimate the spectral type of HD 32297 and constrain its HR
diagram position. We surmise that it is young zero-age main sequence
star, almost certainly older than 13 Myr (>95% significance).
Spectral Type: There is considerable uncertainty regarding the
spectral type of the star as published estimates range from A0 (Torres
et al. 2006, Astronomy & Astrophysics 460, 695;
http://adsabs.harvard.edu/abs/2006A%26A...460..695T; Henry Draper
Catalog, classified by A.J. Cannon;
http://vizier.cfa.harvard.edu/viz-bin/Cat?III/135A) to A5 (Dieckvoss &
Heckmann 1975; AGK3 Catalog;
http://cdsads.u-strasbg.fr/abs/1975ascp.book.....H). Based on an
archival optical spectrum of the star taken 2 Feb 2006 with the 300
line grating of the FAST spectrograph on the Tillinghast telescope
(see FAST archive: http://tdc-www.harvard.edu/cgi-bin/arc/fsearch),
and comparison of the spectrum to a dense grid of MK standard stars
using the python tool "sptool" (http://rumtph.org/pecaut/sptool/), we
estimate that the star is approximately "A6 V". A closer look at the
Ca and metal lines shows that the star should probably be classified
"kA3hA6mA6 V" (see the scheme used for classifying chemically peculiar
stars; either at http://ned.ipac.caltech.edu/level5/Gray/Gray14.html
or Chapter 5.4 of Gray & Corbally (2009), see
http://adsabs.harvard.edu/abs/2009ssc..book.....G). The star is very
consistent with a "hydrogen-type" of A6, and while the metal lines
appear normal for A6, the Ca K line is weak and appears consistent
with A3. Morgan classifed such A-type stars with weak Ca K lines but
normal metal lines "proto metallic-line" stars
(http://www.ulo.ucl.ac.uk/catalogues/mkrevised/xhtml/catalogue/19/19.html),
however Gray & Corbally (2009; Sec. 5.4.1) do not make this
distinction, and do not consider stars metallic-line A stars unless
the difference in Ca K and hydrogen types exceeds 5 subtypes. HD
32297 is also clearly not a Lambda Boo-type star. The spectrum of the
star is clearly inconsistent with previously published spectral types
"A0V" (Torres et al. 2006, Astronomy & Astrophysics 460, 695;
http://adsabs.harvard.edu/abs/2006A%26A...460..695T) and the "A0" HD
type (Henry Draper Catalog, classified by A.J. Cannon;
http://vizier.cfa.harvard.edu/viz-bin/Cat?III/135A). The
"hydrogen-type" of the star is A6V, hence we adopt Teff = 8000+-150 K
and and V-band bolometric correction BCv = 0.02+-0.01 mag (assuming
approximately +-1 subtype errors) based on the authors's revised
spectral type-Teff-color-BC calibration
(http://www.pas.rochester.edu/~emamajek/EEM_dwarf_UBVRI_colors_Teff.dat)
Reddening, Extinction, and HR Diagram Position: The HR diagram point
was calculated using the following input data: V magnitude of
8.13+-0.01 (from Hipparcos catalog; ESA 1997;
http://adsabs.harvard.edu/abs/1997ESASP1200.....P), revised Hipparcos
parallax of 8.90+-0.85 mas (van Leeuwen 2007;
http://adsabs.harvard.edu/abs/2007ASSL..350.....V), and BVJHKs
photometry from Hipparcos catalog and 2MASS catalog (Skrutskie et
al. 2006, AJ, 131, 1163;
http://adsabs.harvard.edu/abs/2006AJ....131.1163S). From a fit of the
BVJHKs photometry to SED templates interpolated from the previously
mentioned spectral type-Teff-color-BC calibration, and considering the
photometric errors in the chi^2 fit, I find that the combination of
spectral type and optical-IR photometry is consistent with reddening
E(B-V)=0.03+-0.03 mag, extinction Av = 0.11+-0.11 mag, absolute
magnitude Mv = 2.78+-0.24 mag, apparent bolometric magnitude mbol =
8.04+-0.11 mag, absolute bolometric magnitude Mbol = 2.79+-0.24 mag,
bolometric flux fbol = 0.0156+-0.0016 nW/m^2, and bolometric
luminosity log(L/Lsun) = 0.79+-0.09 dex (all 1sigma
uncertainties). The isochrones in the plot are calculated using the
stellar evolutionary tracks of Dotter et al. (2008, Astrophysical
Journal Supplement Series, 178, 89;
http://adsabs.harvard.edu/abs/2008ApJS..178...89D) for a presumed
protosolar composition of Y = 0.26 and metal fraction Z = 0.017.
Age: From the HR diagram point and isochrones, I infer that the star
is likely to be >13 Myr old at 95% significance, and most likely it is
on or near the zero-age main sequence (ZAMS), with age > 18 Myr. The
star is most likely on the main sequence or zero-age main sequence,
rather than pre-main sequence. At this time we do not provide a
quantitative estimate of the upper limit to the age of the star,
however it is likely to be on the order of ~300 Myr or so.
Comments on Origin: The revised Hipparcos astrometry for HD 32297 (van
Leeuwen 2007; pmRA = +6.67+-1.11 mas/yr, pmDec = -23.32+-0.69 mas/yr)
and radial velocity from Torres et al. (2006) of +23 km/s are both
inconsistent with the star belonging to any of the Taurus subgroups,
Cas-Tau, Pleiades, and Hyades groups (all in the same general
direction as HD 32297). The proper motion does seem consistent with
membership in the ~20 Myr-old 32 Ori group (U, V, W = -11.8, -18.5,
-8.9 +- 0.4, 0.4, 0.3 km/s; calculated by the author), however this
membership would predict a radial velocity of about +18 km/s, and the
predicted kinematic distance would be 137+-8 pc - which is somewhat
larger than the Hipparcos-derived distance of 112+-11 pc, or the
typical distance to 32 Ori members (~90 pc). Interestingly, Fusco et
al. (2013, AAS #221, #144.20;
http://adsabs.harvard.edu/abs/2013AAS...22114420F) find that the mean
radial velocity of the circumstellar material is at RV = +19 km/s, so
it is possible that HD 32297 could very well be an outlying member of
the 32 Ori group. The proper motion is somewhat similar in direction
to that for Taurus members, however the predicted kinematic distances
would place the star at ~60-80 pc, clearly inconsistent with the
observed parallax, HR diagram position, and distances to Taurus
members (~140 pc). Hence, thus far we are unable to constrain a
birthsite for HD 32297, although the ~20 Myr-old 32 Ori group remains
a possibility that should be investigated further.
