Young Vega and Altair Analogs: Rotationally

1
IV. HD 135344 in X-rays
Young Vega and Altair Analogs Rotationally- Enhan
ced Activity in HD 169142 and HD 135344
HD 163296
The ROSAT HRI detected an x-ray source (1RXH
J151548.5 -370917) 1.06 from HD 135344 with a
count rate of 0.009 /-0.0015 c/s. This is
equivalent to the ROSAT HRI count rate for HD
163296, scaled to 140 pc, suggesting a similar
Lx1.5x1029 erg s-1. This luminosity is
consistent either with low-level accretion plus
some coronal activity from HD 135344, or with the
entire Lx originating on a low-mass companion.
The NICMOS coronagraphic data exclude a companion
with spectral type earlier than M2 at r gt 0.85
from the Herbig star.
C. A. Grady1, G. Schneider2, K. Hamaguchi3, M.
Sitko4, W. Carpenter5, K. Collins6, G. Williger6,
B. Woodgate7, R. Petre7, J. Nuth, III7, D.
Hines8, T. Henning9, A. Quirrenbach9, F.
Ménard10, D. Wilner11 1Eureka Scientific and
GSFC, 2U. of Arizona, 3USRA and GSFC, 4Space
Sciences Institute and U. Cincinnati, 5U.
Cincinnati, 6U. Louisville, 7NASA's GSFC, 8Space
Sciences Institute, 9MPIA, Germany, 10Laboratoire
d'Astrophysique de Grenoble, France, 11CfA.
The accreting Herbig Ae star HD 163296 has a
harder X-ray spectrum than seen in HD 169142
(Swartz et al. 2005).
I. The Stars
Debris disks associated with rapidly rotating A
stars, such as Vega, are among the nearest
candidate planetary systems. These stars differ
from Solar analogs in having stronger UV
radiation fields with implications for the
evolution of planetary atmospheres and the
survival of molecular gas in the disk to the
epoch of giant planet formation. Both moderately
rotating A stars (? PsA, Kalas et al. 2005) and
rapidly rotating A stars (Vega, Marsh et al.
2006) have structure in their disks suggesting
that planets have formed. By combining
inclination data derived from either scattered
light imaging of the disk or from millimeter
studies with v sin i data from the literature, we
have identified several systems which appear to
be young Vega analogs in that the stars are
rapidly rotating, and the disks are viewed close
to face-on. Here we concentrate on two of these
stars HD 169142 and HD 135344.
V. UV and FUV Activity
HD 135344
HD 169142
a)
b)
C III
OVI
c)
d)
HD 135344 has a FUV spectrum (black)
characterized by a faint continuum at longer
wavelength (ltfgt 5x10-15 erg cm-2 s-1 Å-1) and
emission in C III?? 977, 1176, and O VI ?? 1032,
1038. The O VI profile is narrow compared to
jet-driving Herbig Ae stars such as MWC 480
(blue), and is similar to other low v sin i older
Herbig stars such as HD 139614 (red). The
absence of the blue-shifted jet component in O VI
suggests that the current accretion rate onto HD
135344 is 3-5x10-9 solar masses yr-1, scaling
down from MWC 480 (10-8 solar masses yr-1). The
stellar photosphere contributes no more than
5x10-16 erg cm-2 s-1 Å-1 (e.g. HD 181327, F5V,
veq 33 km/s). Collectively, these data
indicate that the bulk of the emission is due to
stellar activity, as expected for an F star.
a) HD 169142 appears later in spectral type in
the UV than its optical type of A5Ve would
indicate. b, c) The star also lacks the strong
emission and wind absorption lines seen in
accreting Herbig Ae stars (HD 104237, orange).
However, there is enhanced FUV emission compared
to non-accreting systems (Altair, blue), and with
stronger emission in some chromospheric
transitions such as C II. No HH knots are seen
in ACS F122M imagery, above the background
surface brightness limit of 1.2x10-13 erg cm-2
s-1 arcsec-2 at 1lt r lt 12.
II. The Disks
HD 135344 HST/NICMOS F110W
HD 169142 HST/NICMOS F110W
VI. Conclusions

• By combining inclination data derived from
  circumstellar disk studies with projected
  rotational velocites, we can now separate the
  rapid rotators from more typical A-F stars.
• HD 169142 and HD 135344 exhibit enhanced x-ray
  and emission-line activity over that observed for
  more typically rotating young A-F stars,
  including systems like 51 Eri (Feigelson et al.
  2006). For HD 135344, the majority of this
  emission is from stellar activity at most 20 of
  the FUV line emission is from accretion. For HD
  169142, our Chandra data suggest that stellar
  activity dominates the X-ray data.
• Neither star has the accretion rate inferred from
  model fits to the mid-IR spectral energy
  distribution (DAllessio et al. 2005).
  Emission-line based measures of accretion
  (Garcia-Lopez et al. 2006) may prove more robust.
  
• When combined with FUV data, the X-ray data for
  older late-A to F Herbig stars suggest a gradual
  change over to activity dominated by magnetic
  activity rather than accretion, with the process
  complete by 12 Myr (? Pic Moving Group).
• Herbig Ae stars known to drive jets are shown
  with green stars. Stars with open symbols are
  debris disks. Both HD 135344 and HD 169142 lie at
  the upper end of the observed x-ray activity for
  ? Pic Moving Group (BPMG) F stars, and well above
  BPMG brown dwarfs or A stars.

HD 135344 HST/NICMOS F160W
HD 135344s circumstellar disk has also been
detected in scattered light with NICMOS
PSF-subtracted coronagraphy at both 1.1 mm and,
in a re-analysis of 1.6 ?m archival data, to an
angular distance of 1.05 ( 150 AU) from the
star.
Light scattered from the HD 169142 circumstellar
disk has been imaged with NICMOS 1.1 mm
PSF-subtracted coronagraphy to an angular
distance of 1.4 from the centrally occulted star
( 200 AU in pole-on projection), spatially
coincident with millimeter detection of the disk
to 1.6 (Raman et al. 2006). The radial surface
brightness (SB) of the disk declines as ? r-3,
indicative of a geometrically-flat disk that is
directly illuminated by the star (Whitney
Hartmann 1992). Modeling of the IR spectrum
suggests an inner cavity 24 AU in radius with
material in the inner disk which does not
appreciably shadow the outer disk.
At r gt 0.62, the disk has an very steep radial
SB profile, ? r-9.7, suggesting that the outer
disk is both geometrically flat and shadowed
rather than directly illuminated. As noted by
Dominik et al. (2005), the disk is very
vertically extended at the dust sublimation
radius, suggesting that this star continues to
accrete some material from its disk.
III. HD 169142 in X-rays
HD169142
T Tauri companions
Both HD 169142 and its co-moving binary companion
were detected in a 10ks Chandra ACIS-S image
(left, Grady et al. 2007a is up and East to the
left in the image). The Herbig Ae star can be fit
with a single temperature thin thermal model (the
APEC code) kT0.16 keV, NH3.6x1021 cm-2 model
(histogram, with data-model residuals below,
right), resulting in Lx1029 erg s-1 for d140
pc. The pulse height spectrum more closely
resembles 51 Eri (F0V, t12 Myr, Feigelson et al.
2006) than either young, non-accreting Herbig Ae
stars such as HD 163296 (Swartz et al. 2005) or
accreting Herbig Ae stars.
Multi-wavelength Data Rules! This study included
data obtained with the NASA/ESA Hubble Space
Telescope as part of HST-GO-10177, HST-GO-10764,
data from the Chandra X-ray Observatory under
program P7200493, FUV data obtained with the Far
Ultraviolet Spectrographic Explorer under program
E510, and archival spectra obtained with the
International Ultraviolet Explorer. Data analysis
facilities were provided by the Exoplanets and
Stellar Astrophysics Lab at GSFC, the Steward
Observatory of the University of Arizona, and the
Department of Physics and Astrophysics at the
Johns Hopkins University.