arXiv subjects

Lars Bildsten

Publications and source records attributed to Lars Bildsten.

At least 37 records · Page 2Linked to original sources

arXiv2022

Limiting the accretion disk light in two mass transferring hot subdwarf binaries

We report the results from follow-up observations of two Roche-lobe filling hot subdwarf binaries with white dwarf companions predicted to have accretion disks. ZTF J213056.71+442046.5 (ZTF J2130) with a 39-minute period and ZTF J205515.98+465106.5 (ZTF J2055) with a 56-minute period were both discovered as subdwarf binaries with light curves that could only be explained well by including an accretion disk in their models. We performed a detailed high-resolution spectral analysis using Keck/ESI to search for possible accretion features for both objects. We also employed polarimetric analysis using the Nordic Optical Telescope (NOT) for ZTF J2130. We did not find any signatures of an accretion disk in either object, and placed upper limits on the flux contribution and variation in degree of polarisation due to the disk. Owing to the short 39-minute period and availability of photometric data over six years for ZTF J2130, we conducted an extensive $O - C$ timing analysis in an attempt to look for orbital decay due to gravitational wave radiation. No such decay was detected conclusively, and a few more years of data paired with precise and consistent timing measurements were deemed necessary to constrain $\dot P$ observationally.

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arXiv2022

Synthesizing Spectra from 3D Radiation Hydrodynamic Models of Massive Stars Using Monte Carlo Radiation Transport

Observations indicate that turbulent motions are present on most massive star surfaces. Starting from the observed phenomena of spectral lines with widths much larger than thermal broadening (e.g. micro- and macroturbulence) to the detection of stochastic low-frequency variability (SLFV) in the Transiting Exoplanet Survey Satellite photometry, these stars clearly have large scale turbulent motions on their surfaces. The cause of this turbulence is debated, with near-surface convection zones, core internal gravity waves, and wind variability being proposed. Our 3D grey radiation hydrodynamic (RHD) models characterized the surfaces' convective dynamics driven by near-surface convection zones and provided a reasonable match to the observed SLFV in the most luminous massive stars. We now explore the complex emitting surfaces of these 3D RHD models, which strongly violate the 1D assumption of a plane parallel atmosphere. By post-processing the grey RHD models with the Monte Carlo radiation transport code SEDONA, we synthesize stellar spectra and extract information from the broadening of individual photospheric lines. The use of SEDONA enables the calculation of the viewing angle and temporal dependence of spectral absorption line profiles. Combining uncorrelated temporal snapshots together, we compare the broadening from the 3D RHD models' velocity fields to the thermal broadening of the extended emitting region, showing that our synthesized spectral lines closely resemble the observed macroturbulent broadening from similarly luminous stars. More generally, the new techniques we have developed will allow for systematic studies of the origin of turbulent velocity broadening from any future 3D simulations.

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arXiv2022

3D Hydrodynamics of Pre-supernova Outbursts in Convective Red Supergiant Envelopes

Eruptive mass loss likely produces the energetic outbursts observed from some massive stars before they undergo core-collapse supernovae (CCSNe). The resulting dense circumstellar medium (CSM) may also cause the subsequent SNe to be observed as Type IIn events. The leading hypothesis of the cause of these outbursts is the response of the envelope of the red supergiant (RSG) progenitor to energy deposition in the months to years prior to collapse. Early theoretical studies of this phenomena were limited to 1D, leaving the 3D convective RSG structure unaddressed. Using FLASH's hydrodynamic capabilities, we explore the 3D outcomes by constructing convective RSG envelope models and depositing energies less than the envelope binding energies on timescales shorter than the envelope dynamical time deep within them. We confirm the 1D prediction of an outward moving acoustic pulse steepening into a shock, unbinding the outermost parts of the envelope. However, we find that the initial 2-4 km/s convective motions seed the intrinsic convective instability associated with the high entropy material deep in the envelope, enabling gas from deep within the envelope to escape, increasing the amount of ejected mass compared to an initially "quiescent" envelope. The 3D models reveal a rich density structure, with column densities varying by 10x along different lines of sight. Our work highlights that the 3D convective nature of RSG envelopes impacts our ability to reliably predict the outburst dynamics, the amount, and the spatial distribution of the ejected mass associated with deep energy deposition.

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arXiv2022

Shock Breakout in 3-Dimensional Red Supergiant Envelopes

Using Athena++, we perform 3D Radiation-Hydrodynamic calculations of the radiative breakout of the shock wave in the outer envelope of a red supergiant (RSG) which has suffered core collapse and will become a Type IIP supernova. The intrinsically 3D structure of the fully convective RSG envelope yields key differences in the brightness and duration of the shock breakout (SBO) from that predicted in a 1D stellar model. First, the lower-density `halo' of material outside of the traditional photosphere in 3D models leads to a shock breakout at lower densities than 1D models. This would prolong the duration of the shock breakout flash at any given location on the surface to $\approx$1-2 hours. However, we find that the even larger impact is the intrinsically 3D effect associated with large-scale fluctuations in density that cause the shock to break out at different radii at different times. This substantially prolongs the SBO duration to $\approx$3-6 hours and implies a diversity of radiative temperatures, as different patches across the stellar surface are at different stages of their radiative breakout and cooling at any given time. These predicted durations are in better agreement with existing observations of SBO. The longer durations lower the predicted luminosities by a factor of 3-10 ($L_\mathrm{bol}\sim10^{44}\mathrm{erg\ s^{-1}}$), and we derive the new scalings of brightness and duration with explosion energies and stellar properties. These intrinsically 3D properties eliminate the possibility of using observed rise times to measure the stellar radius via light-travel time effects.

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arXiv2022

Numerical Simulations of Convective 3-Dimensional Red Supergiant Envelopes

We explore the three-dimensional properties of convective, luminous ($L\approx10^{4.5}-10^{5}L_\odot$), Hydrogen-rich envelopes of Red Supergiants (RSGs) based on radiation hydrodynamic simulations in spherical geometry using $\texttt{Athena++}$. These computations comprise $\approx30\%$ of the stellar volume, include gas and radiation pressure, and self-consistently track the gravitational potential for the outer $\approx 3M_\odot$ of the simulated $M\approx15M_\odot$ stars. This work reveals a radius, $R_\mathrm{corr}$, around which the nature of the convection changes. For $r>R_\mathrm{corr}$, though still optically thick, diffusion of photons dominates the energy transport. Such a regime is well-studied in less luminous stars, but in RSGs, the near- (or above-) Eddington luminosity (due to opacity enhancements at ionization transitions) leads to the unusual outcome of denser regions moving outwards rather than inward. This region of the star also has a large amount of turbulent pressure, yielding a density structure much more extended than 1D stellar evolution predicts. This "halo" of material will impact predictions for both shock breakout and early lightcurves of Type II-P supernovae. Inside of $R_\mathrm{corr}$, we find a nearly flat entropy profile as expected in the efficient regime of mixing-length-theory (MLT). Radiation pressure provides $\approx1/3$ of the support against gravity in this region. Our comparisons to MLT suggest a mixing length of $α=3-4$, consistent with the sizes of convective plumes seen in the simulations. The temporal variability of these 3D models is mostly on the timescale of the convective plume lifetimes ($\approx300$ days), with amplitudes consistent with those observed photometrically.

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arXiv2022

Discovery of a double detonation thermonuclear supernova progenitor

We present the discovery of a new double detonation progenitor system consisting of a hot subdwarf B (sdB) binary with a white dwarf companion with an P=76.34179(2) min orbital period. Spectroscopic observations are consistent with an sdB star during helium core burning residing on the extreme horizontal branch. Chimera light curves are dominated by ellipsoidal deformation of the sdB star and a weak eclipse of the companion white dwarf. Combining spectroscopic and light curve fits we find a low mass sdB star, $M_{\rm sdB}=0.383\pm0.028$ M$_\odot$ with a massive white dwarf companion, $M_{\rm WD}=0.725\pm0.026$ M$_\odot$. From the eclipses we find a blackbody temperature for the white dwarf of 26,800 K resulting in a cooling age of $\approx$25 Myrs whereas our MESA model predicts an sdB age of $\approx$170 Myrs. We conclude that the sdB formed first through stable mass transfer followed by a common envelope which led to the formation of the white dwarf companion $\approx$25 Myrs ago. Using the MESA stellar evolutionary code we find that the sdB star will start mass transfer in $\approx$6 Myrs and in $\approx$60 Myrs the white dwarf will reach a total mass of $0.92$ M$_\odot$ with a thick helium layer of $0.17$ M$_\odot$. This will lead to a detonation that will likely destroy the white dwarf in a peculiar thermonuclear supernova. PTF1 2238+7430 is only the second confirmed candidate for a double detonation thermonuclear supernova. Using both systems we estimate that at least $\approx$1% of white dwarf thermonuclear supernovae originate from sdB+WD binaries with thick helium layers, consistent with the small number of observed peculiar thermonuclear explosions.

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arXiv2021

Still Brighter than Pre-Explosion, SN 2012Z Did Not Disappear: Comparing Hubble Space Telescope Observations a Decade Apart

Type Iax supernovae represent the largest class of peculiar white-dwarf supernovae. The type Iax SN~2012Z in NGC 1309 is the only white dwarf supernova with a detected progenitor system in pre-explosion observations. Deep \textit{Hubble Space Telescope} images taken before SN~2012Z show a luminous, blue source that we have interpreted as a helium-star companion (donor) to the exploding white dwarf. We present here late-time \textit{HST} observations taken $\sim$1400 days after the explosion to test this model. We find the SN light curve can empirically be fit by an exponential decay model in magnitude units. The fitted asymptotic brightness is within $10\%$ of our latest measurements and approximately twice the brightness of the pre-explosion source. The decline of the light curve is too slow to be powered by $^{56}$Co or $^{57}$Co decay: if radioactive decay is the dominate power source, it must be from longer half-life species like $^{55}$Fe. Interaction with circumstellar material may contribute to the light curve, as may shock heating of the companion star. Companion-star models underpredict the observed flux in the optical, producing most of their flux in the UV at these epochs. A radioactively-heated bound remnant, left after only a partial disruption of the white dwarf, is also capable of producing the observed excess late-time flux. Our analysis suggests that the total ejecta + remnant mass is consistent with the Chandrasekhar mass for a range of type Iax supernovae.

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arXiv2021

Stochastic Low Frequency Variability in 3-Dimensional Radiation Hydrodynamical Models of Massive Star Envelopes

Increasing main sequence stellar luminosity with stellar mass leads to the eventual dominance of radiation pressure in stellar envelope hydrostatic balance. As the luminosity approaches the Eddington limit, additional instabilities (beyond conventional convection) can occur. These instabilities readily manifest in the outer envelopes of OB stars, where the opacity increase associated with iron yields density and gas pressure inversions in 1D models. Additionally, recent photometric surveys (e.g. TESS) have detected excess broadband low frequency variability in power spectra of OB star lightcurves, called stochastic low frequency variability (SLFV). This motivates our novel 3D Athena++ radiation hydrodynamical (RHD) simulations of two 35$\,$M$_\odot$ star envelopes (the outer $\approx$15$\%$ of the stellar radial extent), one on the zero-age main sequence and the other in the middle of the main sequence. Both models exhibit turbulent motion far above and below the conventional iron opacity peak convection zone (FeCZ), obliterating any ``quiet" part of the near-surface region and leading to velocities at the photosphere of 10-100$\,$km$\,$s$^{-1}$, directly agreeing with spectroscopic data. Surface turbulence also produces SLFV in model lightcurves with amplitudes and power-law slopes that are strikingly similar to those of observed stars. The characteristic frequencies associated with SLFV in our models are comparable to the thermal time in the FeCZ ($\approx$3-7$\,$days$^{-1}$). These simulations, which have no free parameters, are directly validated by observations and, though more models are needed, we remain optimistic that 3D RHD models of main sequence O star envelopes exhibit SLFV originating from the FeCZ.

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arXiv2021

Mass Transfer and Stellar Evolution of the White Dwarfs in AM CVn Binaries

We calculate the stellar evolution of both white dwarfs (WDs) in AM CVn binaries with orbital periods of $P_{\mathrm{orb}} \approx 5-70$ minutes. We focus on the cases where the donor starts as a $M_{\mathrm{He}} < 0.2 \, M_{\odot}$ Helium WD and the accretor is a $M_{\mathrm{WD}} > 0.6 \, M_{\odot}$ WD. Using Modules for Experiments in Stellar Astrophysics (MESA), we simultaneously evolve both WDs assuming conservative mass transfer and angular momentum loss from gravitational radiation. This self-consistent evolution yields the important feedback of the properties of the donor on the mass transfer rate, $\dot{M}$, as well as the thermal evolution of the accreting WD. Consistent with earlier work, we find that the high $\dot{M}$'s at early times forces an adiabatic evolution of the donor for $P_{\mathrm{orb}} < 30$ minutes so that its mass-radius relation depends primarily on its initial entropy. As the donor reaches $ M_{\mathrm{He}} \approx 0.02-0.03 \, M_{\odot}$ at $P_{\mathrm{orb}} \simeq 30 $ minutes, it becomes fully convective and could lose entropy and expand much less than expected under further mass loss. However, we show that the lack of reliable opacities for the donor's surface inhibit a secure prediction for this possible cooling. Our calculations capture the core heating that occurs during the first $\approx 10^7$ years of accretion and continue the evolution into the phase of WD cooling that follows. When compared to existing data for accreting WDs, as seen by Cheng and collaborators for isolated WDs, we also find that the accreting WDs are not as cool as we would expect given the amount of time they have had to cool.

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arXiv2021

Year 1 of the ZTF high-cadence Galactic Plane Survey: Strategy, goals, and early results on new single-mode hot subdwarf B-star pulsators

We present the goals, strategy and first results of the high-cadence Galactic plane survey using the Zwicky Transient Facility (ZTF). The goal of the survey is to unveil the Galactic population of short-period variable stars, including short period binaries and stellar pulsators with periods less than a few hours. Between June 2018 and January 2019, we observed 64 ZTF fields resulting in 2990 deg$^2$ of high stellar density in ZTF-$r$ band along the Galactic Plane. Each field was observed continuously for 1.5 to 6 hrs with a cadence of 40 sec. Most fields have between 200 and 400 observations obtained over 2-3 continuous nights. As part of this survey we extract a total of $\approx$230 million individual objects with at least 80 epochs obtained during the high-cadence Galactic Plane survey reaching an average depth of ZTF-$r$ $\approx$20.5 mag. For four selected fields with 2 million to 10 million individual objects per field we calculate different variability statistics and find that $\approx$1-2% of the objects are astrophysically variable over the observed period. We present a progress report on recent discoveries, including a new class of compact pulsators, the first members of a new class of Roche Lobe filling hot subdwarf binaries as well as new ultracompact double white dwarfs and flaring stars. Finally we present a sample of 12 new single-mode hot subdwarf B-star pulsators with pulsation amplitudes between ZTF-$r$ = 20-76 mmag and pulsation periods between $P$ = 5.8-16 min with a strong cluster of systems with periods $\approx$ 6 min. All of the data have now been released in either ZTF Data Release 3 or data release 4.

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arXiv2020

An Outburst by AM CVn binary SDSS J113732.32+405458.3

We report the discovery of a one magnitude increase in the optical brightness of the 59.63 minute orbital period AM CVn binary SDSS J113732.32+405458.3. Public $g$, $r$, and $i$ band data from the Zwicky Transient Facility (ZTF) exhibit a decline over a 300 day period, while a few data points from commissioning show that the peak was likely seen. Such an outburst is likely due to a change in the state of the accretion disk, making this the longest period AM CVn binary to reveal an unstable accretion disk. The object is now back to its previously observed (by SDSS and PS-1) quiescent brightness that is likely set by the accreting white dwarf. Prior observations of this object also imply that the recurrence times for such outbursts are likely more than 12 years.

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arXiv2020

Multi-Gigayear White Dwarf Cooling Delays from Clustering-Enhanced Gravitational Sedimentation

Cooling white dwarfs (WDs) can yield accurate ages when theoretical cooling models fully account for the physics of the dense plasma of WD interiors. We use MESA to investigate cooling models for a set of massive and ultra-massive WDs (0.9-1.3 $M_\odot$) for which previous models fail to match kinematic age indicators based on Gaia DR2. We find that the WDs in this population can be explained as C/O cores experiencing unexpectedly rapid $^{22}$Ne sedimentation in the strongly liquid interior just prior to crystallization. We propose that this rapid sedimentation is due to the formation of solid clusters of $^{22}$Ne in the liquid C/O background plasma. We show that these heavier solid clusters sink faster than individual $^{22}$Ne ions and enhance the sedimentation heating rate enough to dramatically slow WD cooling. MESA models including our prescription for cluster formation and sedimentation experience cooling delays of $\approx$4 Gyr on the WD Q branch, alleviating tension between cooling ages and kinematic ages. This same model then predicts cooling delays coinciding with crystallization of 6 Gyr or more in lower mass WDs (0.6-0.8 $M_\odot$). Such delays are compatible with, and perhaps required by, observations of WD populations in the local 100 pc WD sample and the open cluster NGC 6791. These results motivate new investigations of the physics of strongly coupled C/O/Ne plasma mixtures in the strongly liquid state near crystallization and tests through comparisons with observed WD cooling.

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arXiv2020

Convectively Driven Three Dimensional Turbulence in Massive Star Envelopes: I. A 1D Implementation of Diffusive Radiative Transport

Massive ($M >30\,$M$_{\odot}$) stars exhibit luminosities that are near the Eddington-limit for electron scattering causing the increase in opacity associated with iron at $T\approx180,000\,$K to trigger supersonic convection in their outer envelopes. Three dimensional radiative hydrodynamics simulations by Jiang and collaborators with the Athena++ computational tool have found order of magnitude density and radiative flux fluctuations in these convective regions, even at optical depths $\gg100$. We show here that radiation can diffuse out of a parcel during the timescale of convection in these optically thick parts of the star, motivating our use of a "pseudo" Mach number to characterize both the fluctuation amplitudes and their correlations. In this first paper, we derive the impact of these fluctuations on the radiative pressure gradient needed to carry a given radiative luminosity. This implementation leads to a remarkable improvement between 1D and 3D radiative pressure gradients, and builds confidence in our path to an eventual 1D implementation of these intrinsically 3D envelopes. However, simply reducing the radiation pressure gradient is not enough to implement a new 1D model. Rather, we must also account for the impact of two other aspects of turbulent convection: the substantial pressure, and the ability to transport an appreciable fraction of the luminosity, which will be addressed in upcoming works. This turbulent convection also arises in other instances where the stellar luminosity approaches the Eddington luminosity. Hence, our effort should apply to other astrophysical situations where an opacity peak arises in a near Eddington limited, radiation pressure dominated plasma.

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arXiv2020

A new class of Roche lobe-filling hot subdwarf binaries

We present the discovery of the second binary with a Roche lobe-filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of $T_{\rm eff}=33,700\pm1000$ K and a surface gravity of $log(g)=5.54\pm0.11$. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of $M_{\rm sdOB}=0.41\pm0.04$ M$_\odot$ and a WD mass of $M_{\rm WD}=0.68\pm0.05$ M$_\odot$. The weak eclipses imply a WD black-body temperature of $63,000\pm10,000$ K and a radius $R_{\rm WD}=0.0148\pm0.0020$ M$_\odot$ as expected for a WD of such high temperature. The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for $\approx1$ Myrs at a rate of $10^{-9} M_\odot {\rm yr}^{-1}$ which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in $\approx30$ Myrs. We suggest that Galactic reddening could bias discoveries towards preferentially finding Roche lobe-filling systems during the short-lived shell burning phase. Studies using reddening corrected samples should reveal a large population of helium core-burning hot subdwarfs with $T_{\rm eff}\approx25,000$ K in binaries of 60-90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational wave emission and explode as a sub-luminous thermonuclear supernova or evolve into a massive single WD.

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arXiv2020

Comparing Moment-Based and Monte Carlo Methods of Radiation Transport Modeling for Type II-Plateau Supernova Light Curves

Time-dependent electromagnetic signatures from core-collapse supernovae are the result of detailed transport of the shock-deposited and radioactively-powered radiation through the stellar ejecta. Due to the complexity of the underlying radiative processes, considerable approximations are made to simplify key aspects of the radiation transport problem. We present a systematic comparison of the moment-based radiation hydrodynamical code STELLA and the Monte Carlo radiation transport code Sedona in the 1D modeling of Type II-Plateau supernovae. Based on explosion models generated from the Modules for Experiments in Stellar Astrophysics (MESA) instrument, we find remarkable agreements in the modeled light curves and the ejecta structure thermal evolution, affirming the fidelity of both radiation transport modeling approaches. The radiative moments computed directly by the Monte Carlo scheme in Sedona also verify the accuracy of the moment-based scheme. We find that the coarse resolutions of the opacity tables and the numerical approximations in STELLA have insignificant impact on the resulting bolometric light curves, making it an efficient tool for the specific task of optical light curve modeling.

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arXiv2020

The Value of Progenitor Radius Measurements for Explosion Modeling of Type II-Plateau Supernovae

Using Modules for Experiments in Stellar Astrophysics (MESA)+STELLA, we show that very different physical models can adequately reproduce a specific observed Type II-Plateau Supernova (SN). We consider SN2004A, SN2004et, SN2009ib, SN2017eaw, and SN2017gmr, Nickel-rich ($M_\mathrm{Ni}>0.03M_\odot$) events with bolometric lightcurves and a well-sampled decline from the plateau. These events also have constraints on the progenitor radius, via a progenitor image, or, in the case of SN2017gmr, a radius from fitting shock-cooling models. In general, many explosions spanning the parameter space of progenitors can yield excellent lightcurve and Fe line velocity agreement, demonstrating the success of scaling laws in motivating models which match plateau properties for a given radius and highlighting the degeneracy between plateau luminosity and velocity in models and observed events, which can span over 50% in ejecta mass, radius, and explosion energy. This can help explain disagreements in explosion properties reported for the same event using different model calculations. Our calculations yield explosion properties when combined with pre-explosion progenitor radius measurements or a robust understanding of the outermost $<0.1\,M_\odot$ of material that quantifies the progenitor radius from SN observations a few days after explosion.

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arXiv2020

The Zwicky Transient Facility Census of the Local Universe I: Systematic search for Calcium rich gap transients reveal three related spectroscopic sub-classes

(Abridged) Using the Zwicky Transient Facility alert stream, we are conducting a large campaign to spectroscopically classify all transients occurring in galaxies in the Census of the Local Universe (CLU) catalog. The aim of the experiment is to construct a spectroscopically complete, volume-limited sample of transients coincident within 100" of CLU galaxies out to 200 Mpc, and to a depth of 20 mag. We describe the survey design and spectroscopic completeness from the first 16 months of operations. We present results from a systematic search for Calcium rich gap transients in the sample of 22 low luminosity (peak absolute magnitude $M > -17$), hydrogen poor events found in the experiment (out of 754 spectroscopically classified SNe). We report the detection of eight Calcium rich gap transients, and constrain their volumetric rate to be at least $\approx 15\pm5$% of the SN Ia rate. Combining this sample with ten events from the literature, we find a likely continuum of spectroscopic properties ranging from events with SN Ia-like features (Ca-Ia objects) to SN Ib/c-like features (Ca-Ib/c objects) at peak light. Within the Ca-Ib/c events, we find two populations of events distinguished by their red ($g - r \approx 1.5$ mag) or green ($g - r \approx 0.5$ mag) spectral colors at $r$-band peak, wherein redder events show strong line blanketing signatures, slower light curves, weaker He lines and lower [Ca II]/[O I] in the nebular phase. Together, we find that the spectroscopic continuum, volumetric rates and striking old environments are consistent with the explosive burning of He shells on low mass white dwarfs. We posit that Ca-Ia and red Ca-Ib/c objects are consistent with the double detonation of He shells with high He burning efficiency, while green Ca-Ib/c objects could arise from less efficient He burning scenarios such as detonations in low density He shells or He shell deflagrations.

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arXiv2020

Variability of massive stars in M31 from the Palomar Transient Factory

Using data from the (intermediate) Palomar Transient Factory (iPTF), we characterize the time variability of ~500 massive stars in M31. Our sample is those stars which are spectrally typed by Massey and collaborators, including Luminous Blue Variables, Wolf-Rayets, and warm and cool supergiants. We use the high-cadence, long-baseline (~5 years) data from the iPTF survey, coupled with data-processing tools that model complex features in the light curves. We find widespread photometric (R-band) variability in the upper Hertzsprung Russell diagram (or CMD) with an increasing prevalence of variability with later spectral types. Red stars (V-I>1.5) exhibit larger amplitude fluctuations than their bluer counterparts. We extract a characteristic variability timescale, tch, via wavelet transformations that are sensitive to both continuous and localized fluctuations. Cool supergiants are characterized by longer timescales (>100 days) than the hotter stars. The latter have typical timescales of tens of days but cover a wider range, from our resolution limit of a few days to longer than 100 days timescales. Using a 60-night block of data straddling two nights with a cadence of around 2 minutes, we extracted tch in the range 0.1--10 days with amplitudes of a few percent for 13 stars. Though there is broad agreement between the observed variability characteristics in the different parts of the upper CMD with theoretical predictions, detailed comparison requires models with a more comprehensive treatment of the various physical processes operating in these stars such as pulsation, subsurface convection, and the effect of binary companions.

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