A giant planet undergoing extreme-ultraviolet irradiation by its hot massive-star host

TLDR: Observations of the bright star HD 195689 are reported, which reveal a close-in (orbital period of about 1.48 days) transiting giant planet, KELT-9b, which is as hot as stars of stellar type K4 and receives 700 times more extreme-ultraviolet radiation than WASP-33b.

Abstract: The amount of ultraviolet irradiation and ablation experienced by a planet depends strongly on the temperature of its host star. Of the thousands of extrasolar planets now known, only six have been found that transit hot, A-type stars (with temperatures of 7,300–10,000 kelvin), and no planets are known to transit the even hotter B-type stars. For example, WASP-33 is an A-type star with a temperature of about 7,430 kelvin, which hosts the hottest known transiting planet, WASP-33b (ref. 1); the planet is itself as hot as a red dwarf star of type M (ref. 2). WASP-33b displays a large heat differential between its dayside and nightside, and is highly inflated–traits that have been linked to high insolation. However, even at the temperature of its dayside, its atmosphere probably resembles the molecule-dominated atmospheres of other planets and, given the level of ultraviolet irradiation it experiences, its atmosphere is unlikely to be substantially ablated over the lifetime of its star. Here we report observations of the bright star HD 195689 (also known as KELT-9), which reveal a close-in (orbital period of about 1.48 days) transiting giant planet, KELT-9b. At approximately 10,170 kelvin, the host star is at the dividing line between stars of type A and B, and we measure the dayside temperature of KELT-9b to be about 4,600 kelvin. This is as hot as stars of stellar type K4 (ref. 5). The molecules in K stars are entirely dissociated, and so the primary sources of opacity in the dayside atmosphere of KELT-9b are probably atomic metals. Furthermore, KELT-9b receives 700 times more extreme-ultraviolet radiation (that is, with wavelengths shorter than 91.2 nanometres) than WASP-33b, leading to a predicted range of mass-loss rates that could leave the planet largely stripped of its envelope during the main-sequence lifetime of the host star.

Figures

Figure 1: KELT-9 discovery and confirmation data. Shown are the discovery light curve, follow-up primary transit and secondary eclipse light curves, and reflex Doppler signal due to the transiting companion. (a) Field 11 of the northern KELT survey was observed 6001 times from UT 2007 May 30 to UT 2013 June 14, following the same procedures as described for the first KELT planet29 The black points show the unbinned data folded at the period of the planet (P = 1.4810932 days), while the red points show these points binned in phase using a bin size of 0.05 in phase. (Methods). (b) Combined, binned follow-up light curves of the primary transit, where TC is the time of the center of the transit. The transit shape is consistent with an opaque, circular planet occulting a star with the limb darkening expected for KELT-9’s spectral type of B9.5–A0 The black points show the average of all follow-up light curves, combined in 5-minute bins. The combined best-fit models binned the same way are shown as a solid red line. (c) Combined, binned follow-up light curves of the secondary eclipse. The magnitude of the signal gives a temperature for the day-side of the planet of∼4600 K. This is consistent with the range of expected equilibrium temperatures of the planet and suggests poor heat redistribution to the night side of the planet. The data points and red curve are binned in the same manner as in b. TS is the time of the center of the secondary eclipse. (d) Doppler reflex curve of the primary star. Only the data that were not used in the Doppler Tomography analysis (see Fig. 2) are shown and were used to measure the Doppler reflex curve. The best-fit model of the Doppler reflex curve is shown as the red line. Note the weak Rossiter-McLaughlin signal near phase zero, which is a prediction based on the Doppler Tomographic signal exhibited in Figure 2. A periodogram of this data alone yields a significant peak with an emphemeris that matches that of the photometric transit data to within . 10−4 days, thereby confirming the reflex signal is due to the transiting planet and verifying the planet mass estimate.

Figure 2: Combined Doppler tomographic measurements from three separate transits. All three transits (see Methods) clearly show a “Doppler shadow” at the time of transit, with the velocity width as expected given the spectroscopically-measured v sin I∗ and the photometricallymeasured transit depth and impact parameter, thus confirming the reality of the Doppler tomographic signal and that the planet orbits KELT-9. The color coding shows the fractional variation in the spectroscopic signal from the null hypothesis of no shadow due to a transiting planet. Darker regions indicate the Doppler shadow as the planet crosses the face of the host star. The horizontal line indicate the expected beginning (bottom) and end (top) of the transit, and the vertical lines indicate the negative and positive width of the spectral lines due to broadening caused by the project rotation speed of the star (see Methods). The projected path of the planet across the face of the star is nearly coincident with the stellar rotation axis, with an impact parameter in units of the stellar radius of ∼ 0.2 and a projected spin-orbit misalignment of ∼ −85 degrees, indicating that the planet is on a nearly polar orbit. The host star’s rapid rotation and resulting oblateness implies that the orbit of the planet is likely to exhibit precession, which should be detectable by the year 202230.

Figure 1: KELT-9 discovery and confirmation data. Shown are the discovery light curve, follow-up primary transit and secondary eclipse light curves, and reflex Doppler signal due to the transiting companion. (a) Field 11 of the northern KELT survey was observed 6001 times from UT 2007 May 30 to UT 2013 June 14, following the same procedures as described for the first KELT planet29 The black points show the unbinned data folded at the period of the planet (P = 1.4810932 days), while the red points show these points binned in phase using a bin size of 0.05 in phase. (Methods). (b) Combined, binned follow-up light curves of the primary transit, where TC is the time of the center of the transit. The transit shape is consistent with an opaque, circular planet occulting a star with the limb darkening expected for KELT-9’s spectral type of B9.5–A0 The black points show the average of all follow-up light curves, combined in 5-minute bins. The combined best-fit models binned the same way are shown as a solid red line. (c) Combined, binned follow-up light curves of the secondary eclipse. The magnitude of the signal gives a temperature for the day-side of the planet of∼4600 K. This is consistent with the range of expected equilibrium temperatures of the planet and suggests poor heat redistribution to the night side of the planet. The data points and red curve are binned in the same manner as in b. TS is the time of the center of the secondary eclipse. (d) Doppler reflex curve of the primary star. Only the data that were not used in the Doppler Tomography analysis (see Fig. 2) are shown and were used to measure the Doppler reflex curve. The best-fit model of the Doppler reflex curve is shown as the red line. Note the weak Rossiter-McLaughlin signal near phase zero, which is a prediction based on the Doppler Tomographic signal exhibited in Figure 2. A periodogram of this data alone yields a significant peak with an emphemeris that matches that of the photometric transit data to within . 10−4 days, thereby confirming the reflex signal is due to the transiting planet and verifying the planet mass estimate.

Table 1: Median values and 68% confidence intervals for the physical properties of the KELT-9 system from a global fit to the light curves, radial velocities, and Doppler tomography data, using a custom version of the EXOFAST transit fitting code23 (Methods). We use the Yonsei-Yale (YY) isochrones24 as well as empirically-calibrated stellar relations25 as constraints. In addition, KELT-9 has a measured parallax from Hipparcos26 and Gaia27, thereby allowing us to estimate the stellar radius empirically28 from its distance, effective temperature, bolometric (wavelength-integrated) flux, and interstellar extinction (Methods). We adopt the values derived using constraints from the YY isochrones and Hipparcos-derived radius as our fiducial system parameters.

Figure 3: Doppler tomographic line profile plots. The data, models, and residuals from all three nights where KELT-9 was observed during transit are shown in three columns. In each case, the top plot shows the spectroscopic date, the middle panels show the derived model, and the bottom panels show the residuals. In each panel, the vertical blue lines denote the width of the convolution kernel (i.e., v sin I∗), and the horizontal blue lines show the duration of the transit. Time increases vertically for each panel. The apparent extension of the Doppler shadow before ingress on UT 20014-10-05 is an artifact of uneven and widely-spaced sampling in time. The greyscale shows the fractional variation in the spectroscopic signal from the null hypothesis of no shadow due to a transiting planet. Darker regions indicate the Doppler shadow as the planet crosses the face of the host star. Note the transit is nearly coincident with the projected stellar spin axis, implying a nearly polar orbit for the planet.

Figure 2: Combined Doppler tomographic measurements from three separate transits. All three transits (see Methods) clearly show a “Doppler shadow” at the time of transit, with the velocity width as expected given the spectroscopically-measured v sin I∗ and the photometricallymeasured transit depth and impact parameter, thus confirming the reality of the Doppler tomographic signal and that the planet orbits KELT-9. The color coding shows the fractional variation in the spectroscopic signal from the null hypothesis of no shadow due to a transiting planet. Darker regions indicate the Doppler shadow as the planet crosses the face of the host star. The horizontal line indicate the expected beginning (bottom) and end (top) of the transit, and the vertical lines indicate the negative and positive width of the spectral lines due to broadening caused by the project rotation speed of the star (see Methods). The projected path of the planet across the face of the star is nearly coincident with the stellar rotation axis, with an impact parameter in units of the stellar radius of ∼ 0.2 and a projected spin-orbit misalignment of ∼ −85 degrees, indicating that the planet is on a nearly polar orbit. The host star’s rapid rotation and resulting oblateness implies that the orbit of the planet is likely to exhibit precession, which should be detectable by the year 202230.

Full text

A giant planet undergoing extreme ultraviolet irradiation
by its hot massive-star host
B. Scott Gaudi
1
, Keivan G. Stassun
2,3
, Karen A. Collins
2
, Thomas G. Beatty
4,5
, George Zhou
6
,
David W. Latham
6
, Allyson Bieryla
6
, Jason D. Eastman
6
, Robert J. Siverd
7
, Justin R. Crepp
8
, Er-
ica J. Gonzales
8
, Daniel J. Stevens
1
, Lars A. Buchhave
9,10
, Joshua Pepper
11
, Marshall C. Johnson
1
,
Knicole D. Colon
12,13
, Eric L. N. Jensen
14
, Joseph E. Rodriguez
6
, Valerio Bozza
15,16
, Sebas-
tiano Calchi Novati
15,17
, Giuseppe D‘Ago
18,19
, Mary T. Dumont
20,21
, Tyler Ellis
22,23
, Clement
Gaillard
20
, Hannah Jang-Condell
22
, David H. Kasper
22
, Akihiko Fukui
24
, Joao Gregorio
25
, Ayaka
Ito
26,27
, John F. Kielkopf
28
, Mark Manner
29
, Kyle Matt
20
, Norio Narita
26,30,31
, Thomas E. Oberst
32
,
Phillip A. Reed
33
, Gaetano Scarpetta
15,17
, Denice C. Stephens
20
, Rex R. Yeigh
22
, Roberto Zambelli
34
,
B.J. Fulton
35,36
, Andrew W. Howard
35
, David J. James
37
, Matthew Penny
1,38
, Daniel Bayliss
39
,
Ivan A. Curtis
40
, D.L. DePoy
41
, Gilbert A. Esquerdo
6
, Andrew Gould
1,42
, Michael D. Joner
20
,
Rudolf B. Kuhn
43
, Jonathan Labadie-Bartz
11
, Michael B. Lund
2
, Jennifer L. Marshall
41
, Kim
K. McLeod
44
, Richard W. Pogge
1
, Howard Relles
6
, Chistopher Stockdale
45
, T.G. Tan
46
, Mark
Trueblood
47
, Patricia Trueblood
47
1
Department of Astronomy, The Ohio State University, Columbus, OH, 43210, USA
2
Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Center, Nashville,
TN 37235, USA
3
Department of Physics, Fisk University, 1000 17th Avenue North, Nashville, TN 37208, USA
4
Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, 525 Davey Lab,
University Park, PA 16802, USA
5
Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Lab,
University Park, PA 16802, USA
6
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
7
Las Cumbres Observatory Global Telescope Network, 6740 Cortona Dr., Suite 102, Santa Bar-
1
arXiv:1706.06723v1 [astro-ph.EP] 21 Jun 2017

bara, CA 93117, USA
8
Department of Physics, University of Notre Dame, 225 Nieuwland Science Hall, Notre Dame, IN
46556, USA
9
Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 21S00 Copenhagen, Den-
mark
10
Centre for Star and Planet Formation, Geological Museum, ster Voldgade 5, 1350 Copenhagen,
Denmark
11
Department of Physics, Lehigh University, 16 Memorial Drive East, Bethlehem, PA 18015, USA
12
NASA Ames Research Center, M/S 244-30, Moffett Field, CA 94035, USA
13
Bay Area Environmental Research Institute, 625 2nd St. Ste 209 Petaluma, CA 94952, USA
14
Department of Physics and Astronomy, Swarthmore College, Swarthmore, PA 19081, USA
15
Dipartimento di Fisica “E. R. Caianiello”, Universit
`
a di Salerno, Via Giovanni Paolo II 132,
84084 Fisciano (SA), Italy
16
Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, 80126 Napoli, Italy
17
IPAC, Mail Code 100-22, Caltech, 1200 E. California Blvd., Pasadena, CA 91125
18
Istituto Internazionale per gli Alti Studi Scientifici (IIASS), Via G. Pellegrino 19, 84019 Vietri
sul Mare (SA), Italy
19
INAF-Observatory of Capodimonte, Salita Moiariello, 16, 80131, Naples, Italy
20
Department of Physics and Astronomy, Brigham Young University, Provo, UT 84602, USA
21
Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz,
CA, 95064, USA
22
Department of Physics and Astronomy, University of Wyoming, 1000 E. University, Laramie,
WY 82071, USA
23
Department of Physics & Astronomy, Louisiana state University, 202 Nicholson Hall, Baton
Rouge, LA 70803, USA
24
Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, NINS,
Asakuchi, Okayama 719-0232, Japan
25
Atalaia Group & Crow-Observatory, Portalegre, Portugal
26
National Astronomical Observatory of Japan, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588,
Kanto, Japan
27
Graduate School of Science and Engineering, Hosei University, 3-7-2 Kajino-cho, Koganeishi,
Tokyo 184-8584, Japan
28
Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292, USA
29
Spot Observatory, Nashville, TN 37206 USA
30
Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033,
Japan
31
Astrobiology Center, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
32
Department of Physics, Westminster College, New Wilmington, PA, 16172, USA
33
Department of Physical Sciences, Kutztown University, Kutztown, PA 19530, USA
34
Societ
`
a Astronomica Lunae, Castelnuovo Magra 19030, Italy
35
Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822-1839,
USA
2

36
NSF Graduate Research Fellow
37
Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195, USA
38
Sagan Fellow
39
Observatoire Astronomique de l’Universit
´
e de Gen
`
eve, 51 Chemin des Maillettes, 1290 Versoix,
Switzerland
40
ICO, Adelaide, Australia
41
George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, and
Department of Physics and Astronomy, Texas A & M University, College Station, TX 77843-4242,
USA
42
Max Planck Institute for Astronomy, K
¨
onigstuhl 17, D-69117 Heidelberg, Germany
43
South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa
44
Wellesley College, 106 Central St, Wellesley, MA 02481, USA
45
Hazelwood Observatory, Victoria, Australia
46
Perth Exoplanet Survey Telescope, Perth, Australia
47
Winer Observatory, Sonoita, AZ 85637, USA
The amount of ultraviolet irradiation and ablation experienced by a planet depends strongly
on the temperature of its host star. Of the thousands of extra-solar planets now known,
only four giant planets have been found that transit hot, A-type stars (temperatures of 7300–
10,000 K), and none are known to transit even hotter B-type stars. WASP-33 is an A-type star
with a temperature of ∼7430 K, which hosts the hottest known transiting planet
1
; the planet
is itself as hot as a red dwarf star of type M
2
. The planet displays a large heat differential
between its day-side and night-side
2
, and is highly inflated, traits that have been linked to
high insolation
3, 4
. However, even at the temperature of WASP-33b’s day-side, its atmosphere
likely resembles the molecule-dominated atmospheres of other planets, and at the level of
ultraviolet irradiation it experiences, its atmosphere is unlikely to be significantly ablated
over the lifetime of its star. Here we report observations of the bright star HD 195689, which
reveal a close-in (orbital period ∼1.48 days) transiting giant planet, KELT-9b. At ∼10,170 K,
the host star is at the dividing line between stars of type A and B, and we measure the KELT-
9b’s day-side temperature to be ∼4600 K. This is as hot as stars of stellar type K4
5
. The
molecules in K stars are entirely dissociated, and thus the primary sources of opacity in the
day-side atmosphere of KELT-9b are likely atomic metals. Furthermore, KELT-9b receives
∼700 times more extreme ultraviolet radiation (wavelengths shorter than 91.2 nanometers)
than WASP-33b, leading to a predicted range of mass-loss rates that could leave the planet
largely stripped of its envelope during the main-sequence lifetime of the host star
6
.
The first transiting planets were discovered around cool, solar-type stars
7, 8
, primarily because
hot stars have few spectral lines and rotate rapidly, making Doppler confirmation of planets more
difficult. Only in the past few years have transiting planets been confirmed around hot stars of
3

types early-F and A
9, 10
, inspired by the discovery of WASP-33b
1
. That discovery demonstrated
that it is possible to confirm transiting planets around rapidly rotating hot stars via a combination
of relatively low-precision radial-velocity measurements and Doppler tomography. However, even
the hottest of these few A-type transiting-planet host stars only reach temperatures of ∼7500 K.
Thus, while transit surveys, in particular Kepler
11
, have extended the census of planets around
low-mass stars, our understanding of planets around massive, hot stars remains poor.
Massive stars cool and spin down as they evolve, enabling precise Doppler measurements.
Thus the primary strategy to search for planets around high-mass stars has been surveys of “retired
A-stars”
12
, high-mass stars that have already evolved into subgiant and giant stars. These stars
have revealed a paucity of short-period giant planets relative to sun-like main-sequence stars
12
.
One interpretation is that the initial planet population of high-mass stars is similar to that seen in
unevolved sun-like stars, but that the short-period planets are subsequently engulfed during the
evolution of their parent stars or ablated by the intense irradiation of their host stars while they
are still hot
6
. Another interpretation is that these stars actually have masses similar to the Sun
13
,
implying that the paucity of short-period planets among the retired A-stars is indeed a signature of
planet engulfment
14
, as sun-like stars do not emit strong ultraviolet radiation with which to ablate
their planets
6
.
It is therefore critical to assay the population of short-period planets around bona fide high-
mass stars while they are still on the main sequence, and then to map the evolution of these planets
through to their later evolutionary phases. Although there have been radial-velocity surveys tar-
4

geting unevolved high-mass stars
15, 16
, there are still no known transiting planets around unevolved
stars more massive than 2 M

that produce high levels of extreme ultraviolet irradiation.
The Kilodegree Extremely Little Telescope (KELT) is an all-sky survey for planets transiting
bright (visual magnitude 8–11) stars
17, 18
. HD 195689 (hereafter KELT-9), exhibited repeating
transit-like events of ∼0.6% depth with a period of P ∼1.48 d (Figure 1), and was selected as
a candidate transiting planet (see the Methods). KELT-9’s basic properties (Table 1) include a
high effective temperature and rapid rotation. Following the approach that led to the discovery of
WASP-33b, we obtained follow-up observations (see Figure 1, Figure 2, and the Methods) that
ultimately confirmed KELT-9b as a transiting planet.
KELT-9 is a hot (T
eff
' 10, 170 K), massive ( M
∗
' 2.5 M

) star of spectral type B9.5–A0,
with a relatively young age of 300 million years (Methods), comfortably in its 500-million-year
main-sequence phase of evolution; it has evidently not yet begun its evolution toward becoming
a “retired A star”. Indeed, from comparison with other known planet-hosting stars (Fig. 3), it is
clear that KELT-9 is a likely progenitor of at least a subset of the putative “retired A star” hosts
of planets detected by radial-velocity surveys. KELT-9, and other A-star transiting planet hosts,
thereby provides an important missing link between these samples of planets, and planets detected
in more traditional radial-velocity surveys of sun-like stars.
KELT-9 is only the fifth A-type star known to host a transiting giant companion, and is by
a significant margin the hottest, most massive, and most luminous known transiting giant planet
host. The host star also has the brightest V -band magnitude of any transiting hot Jupiter host, being
5

Frequently asked questions

Q1. What are the contributions in "A giant planet undergoing extreme ultraviolet irradiation by its hot massive-star host" ?

In this paper, Gaudi et al. present a survey of the authors ' work in the field of computer science. 

Q2. How many planets are known to transit hot?

Of the thousands of extra-solar planets now known, only four giant planets have been found that transit hot, A-type stars (temperatures of 7300– 10,000 K), and none are known to transit even hotter B-type stars. 

Q3. What are the three main instruments used to measure KELT-9b?

Observations usingground-based facilities, Spitzer, the Hubble Space Telescope (HST), and ultimately the James WebbSpace Telescope, will allow for the measurement of the phase-resolved spectrum of its thermal emission from the far-optical through the infrared (∼30 µm). 

Q4. How many observations of KELT-9b were obtained?

To constrain the mass and enable eventual Doppler tomographic (DT) detection of KELT-9b, weobtained a total of 115 spectroscopic observations of the host star with the Tillinghast ReflectorEchelle Spectrograph (TRES) on the 1.5 m telescope at the Fred Lawrence Whipple Observatory,Arizona, USA. 

Q5. What is the final age estimate using the global fit parameters?

From the values obtained with the initial SED fit, the authors infer a system age of ≈0.4 Gyr; the final age estimate using the final global fit parameters is ≈0.3 Gyr. 

Q6. Why does the variability affect the field 12 lightcurve?

The variability from the neighbor does not affect the field 12 lightcurve because the point spread function of KELT-9 in field 12 is smaller and is elongated in adifferent direction than in field 11. 

Q7. How many out-of-transit RVs did the authors use?

The authors measured the relativeradial velocity from 104 of the observations (see Extended Data Table ) and used a total of 43 out-of-transit RVs (40 plus one out-of-transit RV from each of the spectroscopic transit observations)to constrain the planet’s orbit and mass. 

Q8. How many observations were made to constrain the mass of the planet?

This includes 40 observationscovering the entire orbital phase to constrain the mass of the planet, and 75 observations made in-transit over three epochs to perform the tomographic line profile analysis. 

Q9. How long does KELT-9 take to reach the base of the red giant?

At the upper end of these rates, the planetmay be completely stripped of its outer envelope in < 600 Myr, roughly the time scale for thehost to evolve from the main-sequence to the base of the red giant branch (see Methods). 

Q10. What is the temperature of the secondary eclipse?

This is as hot as a late K-type star5, and thus the authors expected a large thermal emission signal, which the authors easily confirmed with their z′-band detection of the secondary eclipse with a depth of ∼ 0.1% (Figure 1).