Authors: I. Soszynski, A. Olechowska, M. Ratajczak et al.
First Author’s Institution: University of Warsaw
Status: Submitted to APJL [closed access]
Red giants are peculiar stars – they are old (formed over ten billion years ago), they have exhausted the fuel in their cores, and they show large fluctuations in their brightness. These brightness fluctuations occur on timescales ranging from a few days to as long as several years. Until now, astronomers have identified the physical mechanisms responsible for all of these variations, except the longest timescale ones! The authors of today’s paper demonstrate that these very long timescale variations are caused by the presence of a tiny dwarf star orbiting the red giant star.
The mysterious Long Secondary Periods in red giants
The variations in red giants are truly spectacular – they cause the brightness of the star to change by several factors; they can be periodic or irregular; and their timescales range from a single day to several years. All red giant stars exhibit brightness variations on short (days-month) timescales. The causes of these short-term variations are well understood – they are a result of pulsations of the star or convection in the envelope of the star. However, about a third of all giant stars exhibit long-term variations in addition to their short period variations. These long-term variations have periods ranging from several months to years, and are termed as Long Secondary Period (LSP) variations. The origin of LSP variations in red giants has remained a mystery.
Can LSPs be explained by binary companions?
The presence of a binary companion star is a possible explanation for LSPs. A binary companion can eclipse the light of the red giant stars as it crosses in front of it, causing the observed brightness to decrease. This gives rise to long period variations in the observed brightness of the star on a timescale that is related to the orbital period of the binary system. Typically, we would expect the brightness profile of such a system to show two dips – once when the companion is in front of the red giant and once when it is behind (example here). Interestingly, the lightcurves of LSP giant stars at optical (visual) wavelengths show only a single dip, suggesting that not much optical light is being blocked when the red giant is in front of the companion. This indicates that the companion star is very faint at optical wavelengths. On the contrary, LSP giant stars are very bright at infrared (IR) wavelengths, suggesting that they are surrounded by large clumps of dust (as dust absorbs optical light and re-emits it at infrared wavelengths). Motivated by these features, a possible explanation for LSPs is that they are caused by a tiny, dusty star such as a brown dwarf that is orbiting the red giant.
A “NEW, WISE” method to look for brown dwarfs around LSP stars
Brown dwarfs are extremely bright in the IR and faint at optical wavelengths, so we expect to see their eclipses (termed as secondary eclipses) only in IR lightcurves. The authors used IR observations from the NEOWISE mission to look for these secondary eclipses. NEOWISE is a space mission that has been mapping the entire sky at mid-IR wavelengths (3.4 and 4.6 um) repeatedly since 2013. The authors obtained mid-IR lightcurves for ~700 LSP stars from the NEOWISE mission, dating back to 2013. They also obtained optical lightcurves for their sources from the OGLE survey. As expected, no secondary eclipses are seen in the optical lightcurves. Yet, when looking at their IR lightcurves instead, the authors find that about half of these stars show secondary minima (Figure 1)! This is direct evidence for a dusty low-mass binary companion, such as a brown dwarf, to these stars!
How do you get a brown dwarf around a giant star?
The authors suggest that the brown dwarf around the giant star is formed when the star dumps large amounts of dust onto an orbiting planet. This converts the planet into a brown dwarf. This hypothesis is supported by two facts: 1) Almost all giant stars produce large amounts of dust around them, and 2) About a third of giant stars host a Jupiter-sized planet that can potentially be converted into a brown dwarf. The binary explanation of LSPs is thus compatible with observational as well as theoretical arguments.
The authors thus provide compelling evidence that a large fraction of LSP giants have a brown dwarf companion. This is an important step towards solving a long-standing mystery in the field of stellar variability.