ABSTRACT
Evolutionary models have shown the substantial effect that strong mass-loss rates ($\dot{M}$s) can have on the fate of massive stars. Red supergiant (RSG) mass-loss is poorly understood ...theoretically, and so stellar models rely on purely empirical $\dot{M}$–luminosity relations to calculate evolution. Empirical prescriptions usually scale with luminosity and effective temperature, but $\dot{M}$ should also depend on the current mass and hence the surface gravity of the star, yielding more than one possible $\dot{M}$ for the same position on the Hertzsprung–Russell diagram. One can solve this degeneracy by measuring $\dot{M}$ for RSGs that reside in clusters, where age and initial mass (Minit) are known. In this paper we derive $\dot{M}$ values and luminosities for RSGs in two clusters, NGC 2004 and RSGC1. Using newly derived Minit measurements, we combine the results with those of clusters with a range of ages and derive an Minit-dependent $\dot{M}$ prescription. When comparing this new prescription to the treatment of mass-loss currently implemented in evolutionary models, we find models drastically overpredict the total mass-loss, by up to a factor of 20. Importantly, the most massive RSGs experience the largest downward revision in their mass-loss rates, drastically changing the impact of wind mass-loss on their evolution. Our results suggest that for most initial masses of RSG progenitors, quiescent mass-loss during the RSG phase is not effective at removing a significant fraction of the H-envelope prior to core-collapse, and we discuss the implications of this for stellar evolution and observations of SNe and SN progenitors.
Abstract
We present a systematic study of the most luminous (
M
IR
Vega magnitudes brighter than −14) infrared (IR) transients discovered by the
SPitzer
InfraRed Intensive Transients Survey (SPIRITS) ...between 2014 and 2018 in nearby galaxies (
D
< 35 Mpc). The sample consists of nine events that span peak IR luminosities of
M
4.5,peak
between −14 and −18.2, show IR colors between 0.2 < (3.6–4.5) < 3.0, and fade on timescales between 55 days <
t
fade
< 480 days. The two reddest events (
A
V
> 12) show multiple, luminous IR outbursts over several years and have directly detected, massive progenitors in archival imaging. With analyses of extensive, multiwavelength follow-up, we suggest the following possible classifications: five obscured core-collapse supernovae (CCSNe), two erupting massive stars, one luminous red nova, and one intermediate-luminosity red transient. We define a control sample of all optically discovered transients recovered in SPIRITS galaxies and satisfying the same selection criteria. The control sample consists of eight CCSNe and one Type Iax SN. We find that 7 of the 13 CCSNe in the SPIRITS sample have lower bounds on their extinction of 2 <
A
V
< 8. We estimate a nominal fraction of CCSNe in nearby galaxies that are missed by optical surveys as high as
(90% confidence). This study suggests that a significant fraction of CCSNe may be heavily obscured by dust and therefore undercounted in the census of nearby CCSNe from optical searches.
We present infrared (IR) photometry and spectroscopy of the Type II-P SN 2017eaw and its progenitor in the nearby galaxy NGC 6946. Progenitor observations in the Ks band in four epochs from 1 yr to 1 ...day before the explosion reveal no significant variability in the progenitor star greater than 6% that lasts longer than 200 days. SN 2017eaw is a typical SN II-P with near-IR and mid-IR photometric evolution similar to those of SNe 2002hh and 2004et, other normal SNe II-P in the same galaxy. Spectroscopic monitoring during the plateau phase reveals a possible high-velocity He i 1.083 m absorption line, indicative of a shock interaction with the circumstellar medium. Spectra between 389 and 480 days postexplosion reveal a strong CO first overtone emission at 389 days, with a line profile matching that of SN 1987A from the same epoch, indicating ∼10−3 M of CO at 1800 K. From the 389 days epoch until the most recent observation at 566 days, the first overtone feature fades while the 4.5 m excess, likely from the CO fundamental band, remains. This behavior indicates that the CO has not been destroyed, but that the gas has cooled enough that the levels responsible for first overtone emissions are no longer populated. Finally, the evolution of Spitzer 3.6 m photometry shows evidence for dust formation in SN 2017eaw, with a dust mass of 10−6 or 10−4 M assuming carbonaceous or silicate grains, respectively.
We report 4.5 km luminosities for 27 nearby (D 5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 km luminosity-metallicity (L-Z) relation ...for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 km L-Z relation is 12 + log(O/H) = (5.78 c 0.21) + (-0.122c 0.012)M sub(4.5), where M sub(4.5) is the absolute magnitude at 4.5 km. The dispersion in the near-infrared L-Z relation is smaller than the corresponding dispersion in the optical L-Z relation. The subsequently derived stellar mass-metallicity (M*-Z) relation is 12 + log(O/H) = (5.65 c 0.23) + (0.298 c 0.030) log M*, and extends the SDSS M*-Z relation to lower mass by about 2.5 dex. We find that the dispersion in the M*-Z relation is similar over 5 orders of magnitude in stellar mass, and that the relationship between stellar mass and interstellar medium metallicity is similarly tight from high-mass to low-mass systems. We find a larger scatter at low mass in the relation between effective yield and total baryonic mass. In fact, there are a few dwarf galaxies with large yields, which is difficult to explain if galactic winds are ubiquitous in dwarf galaxies. The low scatter in the L-Z and M*-Z relationships are difficult to understand if galactic superwinds or blowout are responsible for the low metallicities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed.
The Age of Westerlund 1 Revisited Beasor, Emma R.; Davies, Ben; Smith, Nathan ...
Astrophysical journal/The Astrophysical journal,
05/2021, Letnik:
912, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
The cluster Westerlund 1 (Wd1) is host to a large variety of post-main-sequence (MS) massive stars. The simultaneous presence of these stars can only be explained by stellar models if the ...cluster has a finely tuned age of 4–5 Myr, with several published studies independently claiming ages within this range. At this age, stellar models predict that the cool supergiants (CSGs) should have luminosities of
log
(
L
/
L
⊙
)
≈
5.5
, close to the empirical luminosity limit. Here, we test that prediction using archival data and new photometry from Stratospheric Observatory for Infrared Astronomy to estimate bolometric luminosities for the CSGs. We find that these stars are on average 0.4 dex too faint to be 5 Myr old, regardless of which stellar evolutionary model is used, and instead are indicative of a much older age of
10.4
−
1.2
+
1.3
Myr. We argue that neither systematic uncertainties in the extinction law nor stellar variability can explain this discrepancy. In reviewing various independent age estimates of Wd1 in the literature, we first show that those based on stellar diversity are unreliable. Second, we reanalyze Wd1's pre-MS stars employing the Damineli extinction law, finding an age of
7.2
−
2.3
+
1.1
Myr; older than that of previous studies, but which is vulnerable to systematic errors that could push the age close to 10 Myr. However, there remains significant tension between the CSG age and that inferred from the eclipsing binary W13. We conclude that stellar evolutionary models cannot explain Wd1 under the single-age paradigm. Instead, we propose that the stars in the Wd1 region formed over a period of several megayears.
Luminous red novae (LRNe) are astrophysical transients associated with the partial ejection of a binary system’s common envelope shortly before its merger. Here we present the results of our ...photometric and spectroscopic follow-up campaign of AT 2018bwo (DLT 18x), a LRN discovered in NGC 45, and investigate its progenitor system using binary stellar-evolution models. The transient reached a peak magnitude of
M
r
= −10.97 ± 0.11 and maintained this brightness during its optical plateau of
t
p
= 41 ± 5 days. During this phase, it showed a rather stable photospheric temperature of ∼3300 K and a luminosity of ∼10
40
erg s
−1
. Although the luminosity and duration of AT 2018bwo is comparable to the LRNe V838 Mon and M31-2015LRN, its photosphere at early times appears larger and cooler, likely due to an extended mass-loss episode before the merger. Toward the end of the plateau, optical spectra showed a reddened continuum with strong molecular absorption bands. The IR spectrum at +103 days after discovery was comparable to that of a M8.5 II type star, analogous to an extended AGB star. The reprocessed emission by the cooling dust was also detected in the mid-infrared bands ∼1.5 years after the outburst. Archival
Spitzer
and
Hubble
Space Telescope data taken 10−14 yrs before the transient event suggest a progenitor star with
T
prog
∼ 6500 K,
R
prog
∼ 100
R
⊙
, and
L
prog
= 2 × 10
4
L
⊙
, and an upper limit for optically thin warm (1000 K) dust mass of
M
d
< 10
−6
M
⊙
. Using stellar binary-evolution models, we determined the properties of binary systems consistent with the progenitor parameter space. For AT 2018bwo, we infer a primary mass of 12–16
M
⊙
, which is 9–45% larger than the ∼11
M
⊙
obtained using single-star evolution models. The system, consistent with a yellow-supergiant primary, was likely in a stable mass-transfer regime with −2.4 ≤ log(
Ṁ
/
M
⊙
yr
−1
) ≤ −1.2 a decade before the main instability occurred. During the dynamical merger, the system would have ejected 0.15–0.5
M
⊙
with a velocity of ∼500 km s
−1
.
Supernova (SN) 2014C is unique: a seemingly typical hydrogen-poor SN that started to interact with a dense, hydrogen-rich circumstellar medium (CSM) ∼100 days post-explosion. The delayed interaction ...suggests a detached CSM shell, unlike in a typical SN IIn where the CSM is much closer and the interaction commences earlier post-explosion, indicating a different mass-loss history. We present infrared observations of SN 2014C 1-5 yr post-explosion, including uncommon 9.7 m imaging with COMICS on the Subaru telescope. Spectroscopy shows the intermediate-width He I 1.083 m emission from the interacting region up to the latest epoch 1639 days post-explosion. The last Spitzer/IRAC photometry at 1920 days confirms ongoing CSM interaction. The 1-10 m spectral energy distributions (SEDs) can be explained by a dust model with a mixture of 62% carbonaceous and 38% silicate dust, pointing to a chemically inhomogeneous CSM. The inference of silicate dust is the first among interacting SNe. An SED model with purely carbonaceous CSM dust, while possible, requires more than 0.22 M of dust, an order of magnitude larger than what has been observed in any SNe at this epoch. The light curve beyond 500 days is well fit by an interaction model with a wind-driven CSM and a mass-loss rate of ∼10−3 M yr−1, which presents an additional CSM density component exterior to the constant-density shell reported previously in the literature. SN 2014C could originate in a binary system, similar to RY Scuti, which would explain the observed chemical and density profile inhomogeneity in the CSM.
We present the discovery by the SPitzer InfraRed Intensive Transients Survey (SPIRITS) of a likely supernova (SN) in NGC 3556 (M108) at only 8.8 Mpc that was not detected by optical searches. A ...luminous infrared (IR) transient at M4.5 = −16.7 mag (Vega), SPIRITS 16tn is coincident with a dust lane in the inclined, star-forming disk of the host. Using observations in the IR, optical, and radio, we attempt to determine the nature of this event. We estimate AV 8-9 mag of extinction, placing it among the three most highly obscured IR-discovered SNe. The 4.5 light curve declined at a rate of 0.013 mag day−1, and the 3.6-4.5 color increased from 0.7 to 1.0 mag by 184.7 days post discovery. Optical/IR spectroscopy shows a red continuum but no clearly discernible features, preventing a definitive spectroscopic classification. Radio observations constrain the radio luminosity of SPIRITS 16tn to L 1024 erg s−1 Hz−1 between 3 and 15 GHz, excluding many varieties of core-collapse SNe. An SN Ia is ruled out by the observed IR color and lack of spectroscopic features from Fe-peak elements. SPIRITS 16tn was fainter at 4.5 than typical stripped-envelope SNe by 1 mag. Comparison of the spectral energy distribution to SNe II suggests that SPIRITS 16tn was both highly obscured and intrinsically dim, possibly akin to the low-luminosity SN 2005cs. We infer the presence of an IR dust echo powered by an initial peak luminosity of the transient of 5 × 1040 erg s−1 Lpeak 4 × 1043 erg s−1, consistent with the observed range for SNe II. This discovery illustrates the power of IR surveys to overcome the compounding effects of visible extinction and optically subluminous events in completing the inventory of nearby SNe.
We report the first results of imaging the Carina nebula (NGC 3372) with the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope, providing a photometry catalogue of over 44 000 point ...sources as well as a catalogue of over 900 candidate young stellar objects (YSOs) based on fits to their spectral energy distributions (SEDs). We discuss several aspects of the extended emission, including the structure of dozens of dust pillars that result when a clumpy molecular cloud is shredded by feedback from massive stars. There are surprisingly few of the ‘extended green objects’ (EGOs) that are normally taken as signposts of outflow activity in Spitzer data, and not one of the dozens of Herbig-Haro jets detected optically are seen as EGOs. EGOs are apparently poor tracers of outflow activity in strongly irradiated environments, due to the effects of massive star feedback. A population of ‘extended red objects’ tends to be found around late O-type and early B-type stars, some with clear bow-shock morphology. These are dusty shocks where stellar winds collide with photoevaporative flows off nearby clouds. Finally, the relative distributions of O-type stars, small star clusters and subclusters of YSOs as compared to the dust pillars show that while some YSOs are located within dust pillars, many more stars and YSOs reside just outside pillar heads. We suggest that pillars are transient phenomena, part of a continuous outwardly propagating wave of star formation driven by feedback from massive stars. As the pillars are destroyed, they leave newly formed stars in their wake, and these are then subsumed into the young OB association. The YSOs are found predominantly in the cavity between pillars and massive stars, arguing that their formation was in fact triggered. Altogether, the current generation of YSOs shows no strong deviation from a normal initial mass function (IMF). The number of YSOs is consistent with a roughly constant star-formation rate over the past ∼3 Myr, implying that propagating star formation in pillars constitutes an important mechanism to construct unbound OB associations. These accelerated pillars may give birth to massive O-type stars that, after several million years, could appear to have formed in isolation.
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