Principal Investigator: Augusto Damineli VSS Coordinator: Mark Blackford

Eta Carinae is now so bright that its 400-year long history of photometry risks being interrupted. Ironically, this could happen in a phase when we finally can see the central stars almost un-obscured. Very few CCD observers today are able to continue this historical monitoring since the star saturates with 1 sec exposure time even using small aperture telescopes.

Continued visual estimations are strongly encouraged to monitor long term trends; however, Augusto Damineli and other professional astronomers also require precision photometry data for their models.

Scientific background

Eta Carinae (star + Homunculus) has been brightening at a rate ΔV ~0.02 mag/yr since 2000 (Figure 1). In 2010 the central star brightness overcame that of the reflection nebula, which remained almost constant (V~5.46) – see Damineli et al. 2019 MNRAS (https://arxiv.org/abs/1901.00531). This is due to the fact that the central star is obscured by a natural coronagraph mask which is dissipating, a process that will end in 2032 (+/-4 yr).

Figure 1 – Red: V-band light-curve, Green: the same light-curve shifted by 1 cycle and displaced by -0.25 magnitudes. The zoom shows the present ongoing short-lived event and the corresponding spectroscopic changes.

The second major feature in the eta’s light-curve is the Orbital Modulation. It has an amplitude of ΔV~0.3 and is strictly periodic (P=2023d or 5.538yr, the orbital period of the companion star). It is caused by the distorted shape of the primary’s stellar wind as it rotates. The distortion is shaped by the cavity produced by the wind-wind collision (see Figure 2).

Observing the system up to the next maximum of the Orbital Modulation (early 2021) is crucial to obtain data for a 3D model of such an interesting mechanism.

Figure 2 – Density profile of the primary’s star wind at different phases. Observers see a continuously changing cross section. This effect is color invariant, as observed.

The brightness in 2019.0 is inside a local minimum and is expected to rise during 2019 and 2020, when it will reach a new maximum.

Example Observation (Figure 3)

• Observer = Mark Blackford
• Eta Carinae is in the center of the FOV
• Telescope = 80mm f6 refractor, stopped down to 50mm, 2x tele extender and field flattener
• CCD= Atik One 6.0
• Filter = V
• Binning = 1×1
• Exposure = 2 secs x30 images
• Calibration = bias, dark and flat field
• Aligned and average stacked
• Plate scale = 0.948”/pixel
• JD = 2458540.951
• Measured V = 4.464 +/-0.001
• Photometry extraction parameters (using IRAF):
• Aperture radius = 12”
• Sky annulus: internal radius= 13” width: annulus=5”

Figure 3 – Finder chart and aperture for photometry extraction (center crop of stacked image).

Figure 4 – Relative intensity of eta Car and its comparison star.

Observing requirements

• Method: differential photometry
• V-Filter (B-filter would also be useful, but not critical)
• Image scale about 1 arcsec/pixel or more expanded
• Reasonably sharp focus
• Exposure such that the brightest pixel in eta Car is ~50% of the CCD saturation to ensure the comparison star is reasonably well exposed (typically ~1 second, see note below regarding illumination artefacts due to short exposure times)
• Record 10 to 50 individual images to reduce effect of scintillation
• Calibrate each individual image (bias, dark and flat field)
• Align and average stack the individual calibrated images
• Copy the average stacked image from each night into the Project Archive folder so the PI can use IRAF to perform photometry
• Cadence once per week before 1 December 2019, once per day from 1 December 2019 to 1 August 2020, then once per week from 1 August 2020 to 1 February 2021

Ways to reduce the flux

• Eta Carinae is too bright for typical amateur photometry setups. It will be necessary to reduce the flux per pixel to allow exposures of a second or longer. Some possible solutions are listed below:
• Focal expansion to spread light over more pixels -> very good alternative
• Mask in front of the objective. For large apertures, use a 4 holes mask – this keeps a better PSF and avoids, to some extent, the scintillation effects
• Use y-filter (Strömgren) rather than broad V-band
• Avoid defocusing as it takes time to refocus again for other observations and risks throwing light outside the extraction aperture
• Neutral density filter
• The new CMOS cameras allow very short exposures and high frame rates; stack several hundred frames to minimize scintillation

CCD illumination artefacts

Many CCD cameras have shutters that open circularly causing the illumination to decrease radially. The non-uniformity is negligible for exposures >5 secs. The effect can be minimized by pointing the telescope to a coordinate in between the variable and comparison stars so they are affected by the same amount. Augusto suggests:

• RA (2000.0) = 10:45:05
• DEC (2000.0) = -59:40:32

If you wish to participate in this campaign, please contact Mark Blackford (markgblackford@outlook.com).