Although "traditionally" observed at short wavelengths, white dwarfs have displayed many surprising features when probed in the infrared. We present an overview of recent results from ground- and space-based near- to mid-infrared observations of white dwarfs. These include the discovery of many new candidate binary stars containing low mass stellar or sub-stellar companions from a sample of objects previously thought to be single white dwarfs, and Spitzer Space Telescope observations that suggest that dust is common in the environs of white dwarfs in cataclysmic variables

Brinkworth, C. S.

Hoard, D. W.

Wachter, S.

English

Caltech Authors - Main

15th European Workshop on White Dwarfs
ASP Conference Series, Vol. 372, 2007
R. Napiwotzki and M. R. Burleigh
Recent Results from Infrared Observations of White
Dwarfs, their Companions, and the Dust that Surrounds
Them
D. W. Hoard, C. S. Brinkworth, and S. Wachter
Spitzer Science Center, California Institute of Technology, MS 220-6,
Pasadena, CA 91125, USA
Abstract. Although “traditionally” observed at short wavelengths, white
dwarfs have displayed many surprising features when probed in the infrared.
We present an overview of recent results from ground- and space-based near- to
mid-infrared observations of white dwarfs. These include the discovery of many
new candidate binary stars containing low mass stellar or sub-stellar companions
from a sample of objects previously thought to be single white dwarfs, and
Spitzer Space Telescope observations that suggest that dust is common in the
environs of white dwarfs in cataclysmic variables.
1. Unusual Variable in Hercules (Var Her 04)
On 2004 June 16, a previously unknown star in Hercules went into outburst
(Nakano et al. 2004). Based on the large outburst amplitude (∆V > 4.5 mag),
presence of superhumps in the light curve, and very short orbital period (Porb =
81.8 min), Var Her 04 was identiﬁed as a Tremendous Outburst Amplitude Dwarf
Nova (TOAD; e.g., Howell et al. 1995). For a given TOAD, superoutbursts are
rare (recurrence times of decades), so we took advantage of this opportunity to
observe the outburst with the Spitzer Space Telescope. During the subsequent
year, we obtained two observations with IRAC (3.0, 4.5, 5.6, 8.0 µm) and four
observations with MIPS (24 µm) to investigate the formation of dust in the
ejecta of TOAD superoutbursts.
Our Spitzer photometry of Var Her 04 is contaminated by a nearby (1′′)
foreground star that is unrelated to the variable. We have identiﬁed this neighbor
as a normal M3.5 V star based on its near-IR colors (e.g., resolved photometry
from Price et al. 2004) and comparison with near- to mid-IR spectral templates.
Regardless, our Spitzer observations show variability over the stable level of the
foreground star that is attributed to Var Her 04 (see Figure 1).
Based on the increase in 24 µm ﬂux density during our MIPS-2 measure-
ment, we estimate that 10−13–10−11M⊙ of dust was produced by Var Her 04, for
dust temperatures of 400–100 K, respectively (see Figure 2). Theory predicts
there are 104–105 TOADs in the Galaxy (e.g., Howell et al. 2001). If each TOAD
outbursts, on average, once every 10 years, then there may be 103–104 outbursts
per year throughout the Galaxy. If each outburst produces the same amount of
dust as Var Her 04, then this corresponds to 10−10–10−8M⊙ injected into the
ISM per year. On average there is one classical nova per year in the Galaxy,
which ejects 10−9–10−6M⊙ of dust into the ISM (Gehrz et al. 1998), perhaps
501
502 Hoard, Brinkworth, and Wachter
Figure 1. Time-resolved 24 µm photometry of Var Her 04 and the unre-
solved foreground star (ﬁlled circles), in comparison to an ensemble of four
ﬁeld stars (unﬁlled squares). The mean of the ensemble has been deﬁned as
the zero level (dashed line). The second MIPS point marks the detection of
excess 24 µm ﬂux density from Var Her 04 due to the formation of dust in
the outburst ejecta.
making the total population of TOADs as important as classical novae in the
recycling and enriching of the ISM (Ciardi et al. 2006).
2. White Dwarf + Red Dwarf Binaries
We are exploring the frequency of binary stars containing a white dwarf (WD)
and low mass (sub-)stellar companion. We have already identiﬁed ∼ 50 new
binary candidates from the WD sample of McCook & Sion (1999) using data
from the 2MASS Second Incremental Data Release (Wachter et al. 2003); we
are now completing this survey using the 2MASS All Sky Data Release. As we
found in Wachter et al. (2003), there is a locus of WDs in the 2MASS color–color
diagram that have colors consistent with low mass main sequence stars – these
are candidate binaries (see Figure 3). The vertical “bridge” at (H −Ks) ≈ 0.3
that joins this locus with the bulk of the WDs in Figure 3 likely contains bi-
naries with very low mass, probably substellar, companions. We predicted the
existence of such a feature in Wachter et al. (2003) using simulated binaries (see
their Figure 2). Many of the candidates from Wachter et al. (2003) have already
been veriﬁed as close binaries from our HST/ACS snapshot imaging survey (see
Figure 4). A large sample of newly identiﬁed binaries will allow us to empiri-
cally test binary star parameters in the post-AGB phase and begin to address
the fundamental astrophysical question of how stellar evolution inﬂuences the
observed distribution of binary star parameters (e.g., orbital separation).
Infrared Observations of White Dwarfs 503
Figure 2. Spectral energy distribution for the unresolved photometry of
Var Her 04 and the foreground M dwarf (points), with an M3.5 V template
(Patten et al. 2006) scaled to the 2MASS J-band point (black line). We
also show four diﬀerent blackbody curves that are forced to pass through the
bright MIPS-2 data point, to represent the dust emission (ordered from top
to bottom on the right side of the plot as listed in the plot legend).
Acknowledgments. This research was carried out, in part, at the Jet
Propulsion Laboratory, California Institute of Technology (CIT), and was spon-
sored by NASA through the Spitzer and Michelson Science Centers. Support
for HST program number 10255 was provided by NASA through a grant from
the Space Telescope Science Institute, which is operated by the Association
of Universities for Research in Astronomy, Inc., under NASA contract NAS5-
26555. Additional support was provided by NASA through a grant from the
Astrophysics Data Program. We used data products from 2MASS, which is a
joint project of the University of Massachusetts and the Infrared Processing and
Analysis Center/CIT, funded by NASA and the National Science Foundation.
References
Ciardi, D. R., Wachter, S., Hoard, D. W., Howell, S. B., & van Belle, G. T. 2006, AJ,
132, 1989
Farihi, J., Hoard, D. W., & Wachter, S. 2006, ApJ, 646, 480
Gehrz, R. D., Truran, J. W., Williams, R. E., & Starrﬁeld, S. 1998, PASP, 110, 3
Howell, S. B., Szkody, P., & Cannizzo, J. K. 1995, ApJ, 439, 337
Howell, S. B., Nelson, L. A., & Rappaport, S. 2001, ApJ, 550, 897
McCook, G. P., & Sion, E. M. 1999, ApJS, 121, 1
Nakano, S., et al. 2004, IAUC, 8363, 1
Patten, B. M., et al. 2006, ApJ, 651, 502
Price, A., et al. 2004, PASP, 116, 1117
Wachter, S., Hoard, D. W., Hansen, K. H., Wilcox, R. E., Taylor, H. M., & Finkelstein,
S. L. 2003, ApJ, 586, 1356
504 Hoard, Brinkworth, and Wachter
Figure 3. In-progress infrared color–color diagram showing nearly all of the
WDs from the McCook & Sion sample that were not contained in the 2MASS
Second Incremental Data Release (ﬁlled circles; unﬁlled circles have one or
more JHKs photometric uncertainty > 0.1 mag). The regions occupied by
the main sequence (green /// hatched area), giant branch (blue \\\ hatched
area), and L dwarfs (red \\\ hatched area) are also shown.
Figure 4. Representative Hubble Space Telescope ACS/HRC (F814W) im-
ages of resolved WD + low mass star binaries selected from the sample of
allegedly single WDs via infrared excess in the 2MASS color–color diagram
(Farihi et al. 2006; Wachter et al. 2003). Note the range of separations of the
binary components (lower left corner of each image panel).


Recent Results from Infrared Observations of White Dwarfs, their Companions, and the Dust that Surrounds Them

15th European Workshop on White Dwarfs
ASP Conference Series, Vol. 372, 2007
R. Napiwotzki and M. R. Burleigh
Recent Results from Infrared Observations of White
Dwarfs, their Companions, and the Dust that Surrounds
Them
D. W. Hoard, C. S. Brinkworth, and S. Wachter
Spitzer Science Center, California Institute of Technology, MS 220-6,
Pasadena, CA 91125, USA
Abstract. Although “traditionally” observed at short wavelengths, white
dwarfs have displayed many surprising features when probed in the infrared.
We present an overview of recent results from ground- and space-based near- to
mid-infrared observations of white dwarfs. These include the discovery of many
new candidate binary stars containing low mass stellar or sub-stellar companions
from a sample of objects previously thought to be single white dwarfs, and
Spitzer Space Telescope observations that suggest that dust is common in the
environs of white dwarfs in cataclysmic variables.
1. Unusual Variable in Hercules (Var Her 04)
On 2004 June 16, a previously unknown star in Hercules went into outburst
(Nakano et al. 2004). Based on the large outburst amplitude (∆V > 4.5 mag),
presence of superhumps in the light curve, and very short orbital period (Porb =
81.8 min), Var Her 04 was identified as a Tremendous Outburst Amplitude Dwarf
Nova (TOAD; e.g., Howell et al. 1995). For a given TOAD, superoutbursts are
rare (recurrence times of decades), so we took advantage of this opportunity to
observe the outburst with the Spitzer Space Telescope. During the subsequent
year, we obtained two observations with IRAC (3.0, 4.5, 5.6, 8.0 µm) and four
observations with MIPS (24 µm) to investigate the formation of dust in the
ejecta of TOAD superoutbursts.
Our Spitzer photometry of Var Her 04 is contaminated by a nearby (1′′)
foreground star that is unrelated to the variable. We have identified this neighbor
as a normal M3.5 V star based on its near-IR colors (e.g., resolved photometry
from Price et al. 2004) and comparison with near- to mid-IR spectral templates.
Regardless, our Spitzer observations show variability over the stable level of the
foreground star that is attributed to Var Her 04 (see Figure 1).
Based on the increase in 24 µm flux density during our MIPS-2 measure-
ment, we estimate that 10−13–10−11M⊙ of dust was produced by Var Her 04, for
dust temperatures of 400–100 K, respectively (see Figure 2). Theory predicts
there are 104–105 TOADs in the Galaxy (e.g., Howell et al. 2001). If each TOAD
outbursts, on average, once every 10 years, then there may be 103–104 outbursts
per year throughout the Galaxy. If each outburst produces the same amount of
dust as Var Her 04, then this corresponds to 10−10–10−8M⊙ injected into the
ISM per year. On average there is one classical nova per year in the Galaxy,
which ejects 10−9–10−6M⊙ of dust into the ISM (Gehrz et al. 1998), perhaps
501
502 Hoard, Brinkworth, and Wachter
Figure 1. Time-resolved 24 µm photometry of Var Her 04 and the unre-
solved foreground star (filled circles), in comparison to an ensemble of four
field stars (unfilled squares). The mean of the ensemble has been defined as
the zero level (dashed line). The second MIPS point marks the detection of
excess 24 µm flux density from Var Her 04 due to the formation of dust in
the outburst ejecta.
making the total population of TOADs as important as classical novae in the
recycling and enriching of the ISM (Ciardi et al. 2006).
2. White Dwarf + Red Dwarf Binaries
We are exploring the frequency of binary stars containing a white dwarf (WD)
and low mass (sub-)stellar companion. We have already identified ∼ 50 new
binary candidates from the WD sample of McCook & Sion (1999) using data
from the 2MASS Second Incremental Data Release (Wachter et al. 2003); we
are now completing this survey using the 2MASS All Sky Data Release. As we
found in Wachter et al. (2003), there is a locus of WDs in the 2MASS color–color
diagram that have colors consistent with low mass main sequence stars – these
are candidate binaries (see Figure 3). The vertical “bridge” at (H −Ks) ≈ 0.3
that joins this locus with the bulk of the WDs in Figure 3 likely contains bi-
naries with very low mass, probably substellar, companions. We predicted the
existence of such a feature in Wachter et al. (2003) using simulated binaries (see
their Figure 2). Many of the candidates from Wachter et al. (2003) have already
been verified as close binaries from our HST/ACS snapshot imaging survey (see
Figure 4). A large sample of newly identified binaries will allow us to empiri-
cally test binary star parameters in the post-AGB phase and begin to address
the fundamental astrophysical question of how stellar evolution influences the
observed distribution of binary star parameters (e.g., orbital separation).
Infrared Observations of White Dwarfs 503
Figure 2. Spectral energy distribution for the unresolved photometry of
Var Her 04 and the foreground M dwarf (points), with an M3.5 V template
(Patten et al. 2006) scaled to the 2MASS J-band point (black line). We
also show four different blackbody curves that are forced to pass through the
bright MIPS-2 data point, to represent the dust emission (ordered from top
to bottom on the right side of the plot as listed in the plot legend).
Acknowledgments. This research was carried out, in part, at the Jet
Propulsion Laboratory, California Institute of Technology (CIT), and was spon-
sored by NASA through the Spitzer and Michelson Science Centers. Support
for HST program number 10255 was provided by NASA through a grant from
the Space Telescope Science Institute, which is operated by the Association
of Universities for Research in Astronomy, Inc., under NASA contract NAS5-
26555. Additional support was provided by NASA through a grant from the
Astrophysics Data Program. We used data products from 2MASS, which is a
joint project of the University of Massachusetts and the Infrared Processing and
Analysis Center/CIT, funded by NASA and the National Science Foundation.
References
Ciardi, D. R., Wachter, S., Hoard, D. W., Howell, S. B., & van Belle, G. T. 2006, AJ,
132, 1989
Farihi, J., Hoard, D. W., & Wachter, S. 2006, ApJ, 646, 480
Gehrz, R. D., Truran, J. W., Williams, R. E., & Starrfield, S. 1998, PASP, 110, 3
Howell, S. B., Szkody, P., & Cannizzo, J. K. 1995, ApJ, 439, 337
Howell, S. B., Nelson, L. A., & Rappaport, S. 2001, ApJ, 550, 897
McCook, G. P., & Sion, E. M. 1999, ApJS, 121, 1
Nakano, S., et al. 2004, IAUC, 8363, 1
Patten, B. M., et al. 2006, ApJ, 651, 502
Price, A., et al. 2004, PASP, 116, 1117
Wachter, S., Hoard, D. W., Hansen, K. H., Wilcox, R. E., Taylor, H. M., & Finkelstein,
S. L. 2003, ApJ, 586, 1356
504 Hoard, Brinkworth, and Wachter
Figure 3. In-progress infrared color–color diagram showing nearly all of the
WDs from the McCook & Sion sample that were not contained in the 2MASS
Second Incremental Data Release (filled circles; unfilled circles have one or
more JHKs photometric uncertainty > 0.1 mag). The regions occupied by
the main sequence (green /// hatched area), giant branch (blue \\\ hatched
area), and L dwarfs (red \\\ hatched area) are also shown.
Figure 4. Representative Hubble Space Telescope ACS/HRC (F814W) im-
ages of resolved WD + low mass star binaries selected from the sample of
allegedly single WDs via infrared excess in the 2MASS color–color diagram
(Farihi et al. 2006; Wachter et al. 2003). Note the range of separations of the
binary components (lower left corner of each image panel).


http://authors.library.caltech.edu/20024/1/Hoard2007p880515Th_European_Workshop_On_White_Dwarfs_Proceedings.pdf

Recent Results from Infrared Observations of White Dwarfs, their Companions, and the Dust that Surrounds Them

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